REST APIs fuzzer and negative testing tool. Run thousands of self-healing API tests within minutes with no coding effort!

• Comprehensive: tests are generated automatically based on a large number scenarios and cover every field and header
• Intelligent: tests are generated based on data types and constraints; each Fuzzer have specific expectations depending on the scenario under test
• Highly Configurable: high amount of customization: you can exclude specific Fuzzers, HTTP response codes, provide business context and a lot more
• Self-Healing: as tests are generated, any OpenAPI spec change is picked up automatically
• Simple to Learn: flat learning curve, with intuitive configuration and syntax
• Fast: automatic process for write, run and report tests which covers thousands of scenarios within minutes

By using a simple and minimal syntax, with a flat learning curve, CATS (Contract Auto-generated Tests for Swagger) enables you to generate thousands of API tests within minutes with no coding effort. All tests are generated, run and reported automatically based on a pre-defined set of 84 Fuzzers. The Fuzzers cover a wide range of input data from fully random large Unicode values to well crafted, context dependant values based on the request data types and constraints. Even more, you can leverage the fact that CATS generates request payloads dynamically and write simple end-to-end functional tests.

This is a list of articles with step-by-step guides on how to use CATS:

• Testing the GitHub API with CATS
• How to write self-healing functional tests with no coding effort

• hashicorp/vault#13274
• hashicorp/vault#13273
• hashicorp/vault#13225
• hashicorp/vault#13232

CATS is bundled both as an executable JAR or a native binary. The native binaries do not need Java installed.

After downloading your OS native binary, you can add it in classpath so that you can execute it as any other command line tool:

sudo cp cats-macos /usr/local/bin/cats

You can also get autocomplete by downloading the cats_autocomplete script and do:

source cats_autocomplete

To get persistent autocomplete, add the above line in ~/.zshrc or ./bashrc, but make sure you put the fully qualified path for the cats_autocomplete script.

You can also check the cats_autocomplete source for alternative setup.

There is no native binary for Windows, but you can use the uberjar version. This requires Java 11+ to be installed.

You can run it as java -jar cats.jar.

Head to the releases page to download the latest versions: https://github.com/Endava/cats/releases.

You can build CATS from sources on you local box. You need Java 11+. Maven is already bundled.

Before running the first build, please make sure you do a ./mvnw clean. CATS uses a fork ok OKHttpClient which will install locally under the 4.9.1-CATS version, so don't worry about overriding the official versions.

You can use the following Maven command to build the project:

./mvnw package -Dquarkus.package.type=uber-jar

cp target/

You will end up with a cats.jar in the current folder. You can run it wih java -jar cats.jar ....

You can also build native images using a GraalVM Java version.

./mvnw package -Pnative

Note: You will need to configure Maven with a Github PAT with read-packages scope to get some dependencies for the build.

You may see some ERROR log messages while running the Unit Tests. Those are expected behaviour for testing the negative scenarios of the Fuzzers.

Blackbox mode means that CATS doesn't need any specific context. You just need to provide the service URL, the OpenAPI spec and most probably authentication headers.

cats --contract=openapy.yaml --server=http://localhost:8080 --headers=headers.yml --blackbox

In blackbox mode CATS will only report ERRORs if the received HTTP response code is a 5XX. Any other mismatch between what the Fuzzer expects vs what the service returns (for example service returns 400 and service returns 200) will be ignored.

The blackbox mode is similar to a smoke test. It will quickly tell you if the application has major bugs that must be addressed immediately.

The real power of CATS relies on running it in a non-blackbox mode also called context mode. Each Fuzzer has an expected HTTP response code based on the scenario under test and will also check if the response is matching the schema defined in the OpenAPI spec specific to that response code. This will allow you to tweak either your OpenAPI spec or service behaviour in order to create good quality APIs and documentation and also to avoid possible serious bugs.

Running CATS in context mode usually implies providing it a --refData file with resource identifiers specific to the business logic. CATS cannot create data on its own (yet), so it's important that any request field or query param that requires pre-existence of those entities/resources to be created in advance and added to the reference data file.

cats --contract=openapy.yaml --server=http://localhost:8080 --headers=headers.yml --refData=referenceData.yml

You may notice a significant number of tests marked as skipped. CATS will try to apply all Fuzzers to all fields, but this is not always possible. For example the BooleanFieldsFuzzer cannot be applied to String fields. This is why that test attempt will be marked as skipped. It was an intentional decision to also report the skipped tests in order to show that CATS actually tries all the Fuzzers on all the fields/paths/endpoints.

Additionally, CATS support a lot more arguments that allows you to restrict the number of fuzzers, provide timeouts, limit the number of requests per minute and so on.

CATS generates tests based on configured Fuzzers. Each Fuzzer has a specific scenario and a specific expected result. The CATS engine will run the scenario, get the result from the service and match it with the Fuzzer expected result. Depending on the matching outcome, CATS will report as follows:

• INFO/SUCCESS is expected and documented behaviour. No need for action.
• WARN is expected but undocumented behaviour or some misalignment between the contract and the service. This will ideally be actioned.
• ERROR is abnormal/unexpected behaviour. This must be actioned.

CATS will iterate through all endpoints, all HTTP methods and all the associated requests bodies and parameters (including multiple combinations when dealing with oneOf/anyOf elements) and fuzz their values considering their defined data type and constraints. The actual fuzzing depends on the specific Fuzzer executed. Please see the list of fuzzers and their behaviour. There are also differences on how the fuzzing works depending on the HTTP method:

• for methods with request bodies like POST, PUT the fuzzing will be applied at the request body data models level
• for methods without request bodies like GET, DELETE the fuzzing will be applied at the URL parameters level

This means that for methods with request bodies (POST,PUT) that have also URL/path parameters, you need to supply the path parameters via urlParams or the referenceData file as failure to do so will result in Illegal character in path at index ... errors.

CATS produces an execution report in a folder called cats-report/TIMESTAMP or cats-report depending on the --timestampReports argument. The folder will be created inside the current folder (if it doesn't exist) and for each run a new subfolder will be created with the TIMESTAMP value when the run started. This allows you to have a history of the runs. The report itself is in the index.html file, where you can:

• filter test runs based on the result: All, Success, Warn and Error
• filter based on the Fuzzer so that you can only see the runs for that specific Fuzzer
• see summary with all the tests with their corresponding path against they were run, and the result
• have ability to click on any tests and get details about the Scenario being executed, Expected Result, Actual result as well as request/response details

Understanding the Result Reason values:

• Unexpected Exception - reported as error; this might indicate a possible bug in the service or a corner case that is not handled correctly by CATS
• Not Matching Response Schema - reported as a warn; this indicates that the service returns an expected response code and a response body, but the response body does not match the schema defined in the contract
• Undocumented Response Code - reported as a warn; this indicates that the service returns an expected response code, but the response code is not documented in the contract
• Unexpected Response Code - reported as an error; this indicates a possible bug in the service - the response code is documented, but is not expected for this scenario
• Unexpected Behaviour - reported as an error; this indicates a possible bug in the service - the response code is neither documented nor expected for this scenario
• Not Found - reported as an error in order to force providing more context; this indicates that CATS needs additional business context in order to run successfully - you can do this using the --refData and/or --urlParams arguments

This is the summary page:

And this is what you get when you click on a specific test:

CATS has a significant number of Fuzzers. Currently, 84 and growing. Some of the Fuzzers are executing multiple tests for every given field within the request. For example the ControlCharsOnlyInFieldsFuzzer has 63 control chars values that will be tried for each request field. If a request has 15 fields for example, this will result in 1020 tests. Considering that there are additional Fuzzers with the same magnitude of tests being generated, you can easily get to 20k tests being executed on a typical run. This will result in huge reports and long run times (i.e. minutes, rather than seconds).

Below are some recommended strategies on how you can separate the tests in chunks which can be executed as stages in a deployment pipeline, one after the other.

You can use the --paths=PATH argument to run CATS sequentially for each path.

You can use the --checkXXX arguments to run CATS only with specific Fuzzers like: --checkHttp, -checkFields, etc.

You can use various arguments like --fuzers=Fuzzer1,Fuzzer2 or -skipFuzzers=Fuzzer1,Fuzzer2 to either include or exclude specific Fuzzers. For example, you can run all Fuzzers except for the ControlChars and Whitespaces ones like this: --skipFuzzers=ControlChars,Whitesspaces. This will skip all Fuzzers containing these strings in their name. After, you can create an additional run only with these Fuzzers: --fuzzers=ControlChars,Whitespaces.

These are just some recommendations on how you can split the types of tests cases. Depending on how complex your API is, you might go with a combination of the above or with even more granular splits.

Please note that due to the fact that ControlChars, Emojis and Whitespaces generate huge number of tests even for small OpenAPI contracts, they are disabled by default. You can enable them using the --includeControlChars, --includeWhitespaces and/or --includeEmojis arguments. The recommendation is to run them in separate runs so that you get manageable reports and optimal running times.

By default, CATS will report WARNs and ERRORs according to the specific behaviour of each Fuzzer. There are cases though when you might want to focus only on critical bugs. You can use the --ignoreResponseCodes argument to supply a list of result codes that should be ignored as issues (overriding the Fuzzer behaviour) and report those cases as success instead or WARN or ERROR. For example, if you want CATS to report ERRORs only when there is an Exception or the service returns a 500, you can use this: --ignoreResultCodes="2xx,4xx".

You can also choose to ignore checks done by the Fuzzers. By default, each Fuzzer has an expected response code, based on the scenario under test and will report and WARN the service returns the expected response code, but the response code is not documented inside the contract. You can make CATS ignore the undocumented response code checks (i.e. checking expected response code inside the contract) using the --ignoreResponseCodeUndocumentedCheck argument. CATS with now report these cases as SUCCESS instead of WARN.

Additionally, you can also choose to ignore the response body checks. By default, on top of checking the expected response code, each Fuzzer will check if the response body matches what is defined in the contract and will report an WARN if not matching. You can make CATS ignore the response body checks using the --ingoreResponseBodyCheck argument. CATS with now report these cases as SUCCESS instead of WARN.

When CATS runs, for each test, it will export both an HTML file that will be linked in the final report and individual JSON files. The JSON files can be used to replay that test. When replaying a test (or a list of tests), CATS won't produce any report. The output will be solely available in the console. This is useful when you want to see the exact behaviour of the specific test or attach it in a bug report for example.

The syntax for replaying tests is the following:

cats replay "Test1,Test233,Test15.json,dir/Test19.json"

Some notes on the above example:

• test names can be separated by comma ,
• if you provide a json extension to a test name, that file will be search as a path i.e. it will search for Test15.json in the current folder and Test19.json in the dir folder
• if you don't provide a json extension to a test name, it will search for that test in the cats-report folder i.e. cats-report/Test1.json and cats-report/Test233.json

To list all available commands, run CATS with no arguments: cats -h

Other ways to get help from the CATS command are as follows:

• cats help or cats -h will list all available options

• cats --version will display the current CATS version

• cats list --fuzzers will list all the existing fuzzers, grouped on categories

• cats list --fieldsFuzzingStrategy will list all the available fields fuzzing strategies

• cats list --paths --contract=CONTRACT will list all the paths available within the contract

• cats replay "test1,test2" will replay the given tests test1 and test2

• --contract=LOCATION_OF_THE_CONTRACT supplies the location of the OpenApi or Swagger contract.
• --server=URL supplies the URL of the service implementing the contract.
• --basicauth=USR:PWD supplies a username:password pair, in case the service uses basic auth.
• --fuzzers=LIST_OF_FUZZERS supplies a comma separated list of fuzzers. The supplied list of Fuzzers can be partial names, not full Fuzzer names. CATS which check for all Fuzzers containing the supplied strings. If the argument is not supplied, all fuzzers will be run.
• --log=PACKAGE:LEVEL can configure custom log level for a given package. You can provide a comma separated list of packages and levels. This is helpful when you want to see full HTTP traffic: --log=org.apache.http.wire:debug or suppress CATS logging: --log=com.endava.cats:warn
• --paths=PATH_LIST supplies a comma separated list of OpenApi paths to be tested. If no path is supplied, all paths will be considered.
• --skipPaths=PATH_LIST a comma separated list of paths to ignore. If no path is supplied, no path will be ignored
• --fieldsFuzzingStrategy=STRATEGY specifies which strategy will be used for field fuzzing. Available strategies are ONEBYONE, SIZE and POWERSET. More information on field fuzzing can be found in the sections below.
• --maxFieldsToRemove=NUMBER specifies the maximum number of fields to be removed when using the SIZE fields fuzzing strategy.
• --refData=FILE specifies the file containing static reference data which must be fixed in order to have valid business requests. This is a YAML file. It is explained further in the sections below.
• --headers=FILE specifies a file containing headers that will be added when sending payloads to the endpoints. You can use this option to add oauth/JWT tokens for example.
• --edgeSpacesStrategy=STRATEGY specifies how to expect the server to behave when sending trailing and prefix spaces within fields. Possible values are trimAndValidate and validateAndTrim.
• --sanitizationStrategy=STRATEGY specifies how to expect the server to behave when sending Unicode Control Chars and Unicode Other Symbols within the fields. Possible values are sanitizeAndValidate and validateAndSanitize
• --urlParams A comma separated list of 'name:value' pairs of parameters to be replaced inside the URLs. This is useful when you have static parameters in URLs (like 'version' for example).
• --customFuzzerFile a file used by the CustomFuzzer that will be used to create user-supplied payloads.
• --skipFuzzers=LIST_OF_FIZZERs a comma separated list of fuzzers that will be skipped for all paths. You can either provide full Fuzzer names (for example: --skippedFuzzers=VeryLargeStringsFuzzer) or partial Fuzzer names (for example: --skipFuzzers=VeryLarge). CATS will check if the Fuzzer names contains the string you provide in the arguments value.
• --skipFields=field1,field2#subField1 a comma separated list of fields that will be skipped by replacement Fuzzers like EmptyStringsInFields, NullValuesInFields, etc.
• --httpMethods=PUT,POST,etc a comma separated list of HTTP methods that will be used to filter which http methods will be executed for each path within the contract
• --securityFuzzerFile A file used by the SecurityFuzzer that will be used to inject special strings in order to exploit possible vulnerabilities
• --printExecutionStatistics If supplied (no value needed), prints a summary of execution times for each endpoint and HTTP method. By default this will print a summary for each endpoint: max, min and average. If you want detailed reports you must supply --printExecutionStatistics=detailed
• --timestampReports If supplied (no value needed), it will output the report still inside the cats-report folder, but in a sub-folder with the current timestamp
• --reportFormat=FORMAT Specifies the format of the CATS report. You can use htmlOnly if you want the report to not contain any Javascript. This is useful in CI environments due to Javascript content security policies. Default is htmlJs which is the original CATS single page report format.
• --useExamples If true (default value when not supplied) then CATS will use examples supplied in the OpenAPI contact. If false CATS will rely only on generated values
• --checkFields If supplied (no value needed), it will only run the Field Fuzzers
• --checkHeaders If supplied (no value needed), it will only run the Header Fuzzers
• --checkHttp If supplied (no value needed), it will only run the HTTP Fuzzers
• --checkContract If supplied (no value needed), it will only run the ContractInfo Fuzzers
• --includeWhitespaces If supplied (no value needed), it will include the Whitespaces Fuzzers
• --includeEmojis If supplied (no value needed), it will include the Emojis Fuzzers
• --includeControlChars If supplied (no value needed), it will include the ControlChars Fuzzers
• --sslKeystore Location of the JKS keystore holding certificates used when authenticating calls using one-way or two-way SSL
• --sslKeystorePwd The password of the sslKeystore
• --sslKeyPwd The password of the private key from the sslKeystore
• --proxyHost The proxy server's host name (if running behind proxy)
• --proxyPort The proxy server's port number (if running behind proxy)
• --maxRequestsPerMinute Maximum number of requests per minute; this is useful when APIs have rate limiting implemented; default is 10000
• --connectionTimeout Time period in seconds which CATS should establish a connection with the server; default is 10 seconds
• --writeTimeout Maximum time of inactivity in seconds between two data packets when sending the request to the server; default is 10 seconds
• --readTimeout Maximum time of inactivity in seconds between two data packets when waiting for the server's response; default is 10 seconds
• --dryRun If provided, it will simulate a run of the service with the supplied configuration. The run won't produce a report, but will show how many tests will be generated and run for each OpenAPI endpoint
• --ignoreResponseCodes HTTP_CODES_LIST a comma separated list of HTTP response codes that will be considered as SUCCESS, even if the Fuzzer will typically report it as WARN or ERROR. You can use response code families as 2xx, 4xx, etc. If provided, all Contract Fuzzers will be skipped.
• --tests TESTS_LIST a comma separated list of executed tests in JSON format from the cats-report folder. If you supply the list without the .json extension CATS will search the test in the cats-report folder
• --ignoreResponseCodeUndocumentedCheck If supplied (not value needed) it won't check if the response code received from the service matches the value expected by the fuzzer and will return the test result as SUCCESS instead of WARN
• --ignoreResponseBodyCheck If supplied (not value needed) it won't check if the response body received from the service matches the schema supplied inside the contract and will return the test result as SUCCESS instead of WARN
• --blackbox If supplied (not value needed) it will ignore all response codes expect for 5XX which will be returned as ERROR. This is similar to --ignoreResponseCodes="2xx,4xx"
• --contentType A custom mime type if the OpenAPI spec uses content type negotiation versioning.

cats --contract=my.yml --server=https://locathost:8080 --checkHeaders

This will run CATS against http://localhost:8080 using my.yml as an API spec and will only run the HTTP headers Fuzzers.

To get a list of fuzzers run cats list --fuzzers. A list of all available fuzzers will be returned, along with a short description for each.

There are multiple categories of Fuzzers available:

• Field Fuzzers which target request body fields or path parameters
• Header Fuzzers which target HTTP headers
• HTTP Fuzzers which target just the interaction with the service (without fuzzing fields or headers)

Additional checks which are not actually using any fuzzing, but leverage the CATS internal model of running the tests as Fuzzers:

• ContractInfo Fuzzers which checks the contract for API good practices
• Special Fuzzers a special category which need further configuration and are focused on more complex activities like functional flow or security testing

CATS has currently 41 registered Field Fuzzers:

• BooleanFieldsFuzzer - iterate through each Boolean field and send random strings in the targeted field
• DecimalFieldsLeftBoundaryFuzzer - iterate through each Number field (either float or double) and send requests with outside the range values on the left side in the targeted field
• DecimalFieldsRightBoundaryFuzzer - iterate through each Number field (either float or double) and send requests with outside the range values on the right side in the targeted field
• DecimalValuesInIntegerFieldsFuzzer - iterate through each Integer field and send requests with decimal values in the targeted field
• EmptyStringValuesInFieldsFuzzer - iterate through each field and send requests with empty String values in the targeted field
• ExtremeNegativeValueDecimalFieldsFuzzer - iterate through each Number field and send requests with the lowest value possible (-999999999999999999999999999999999999999999.99999999999 for no format, -3.4028235E38 for float and -1.7976931348623157E308 for double) in the targeted field
• ExtremeNegativeValueIntegerFieldsFuzzer - iterate through each Integer field and send requests with the lowest value possible (-9223372036854775808 for int32 and -18446744073709551616 for int64) in the targeted field
• ExtremePositiveValueDecimalFieldsFuzzer - iterate through each Number field and send requests with the highest value possible (999999999999999999999999999999999999999999.99999999999 for no format, 3.4028235E38 for float and 1.7976931348623157E308 for double) in the targeted field
• ExtremePositiveValueInIntegerFieldsFuzzer - iterate through each Integer field and send requests with the highest value possible (9223372036854775807 for int32 and 18446744073709551614 for int64) in the targeted field
• IntegerFieldsLeftBoundaryFuzzer - iterate through each Integer field and send requests with outside the range values on the left side in the targeted field
• IntegerFieldsRightBoundaryFuzzer - iterate through each Integer field and send requests with outside the range values on the right side in the targeted field
• InvalidValuesInEnumsFieldsFuzzer - iterate through each ENUM field and send invalid values
• LeadingWhitespacesInFieldsTrimValidateFuzzer - iterate through each field and send requests with Unicode whitespaces and invisible separators prefixing the current value in the targeted field
• LeadingControlCharsInFieldsTrimValidateFuzzer - iterate through each field and send requests with Unicode control chars prefixing the current value in the targeted field
• LeadingSingleCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values prefixed with single code points emojis
• LeadingMultiCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values prefixed with multi code points emojis
• MaxLengthExactValuesInStringFieldsFuzzer - iterate through each String fields that have maxLength declared and send requests with values matching the maxLength size/value in the targeted field
• MaximumExactValuesInNumericFieldsFuzzer - iterate through each Number and Integer fields that have maximum declared and send requests with values matching the maximum size/value in the targeted field
• MinLengthExactValuesInStringFieldsFuzzer - iterate through each String fields that have minLength declared and send requests with values matching the minLength size/value in the targeted field
• MinimumExactValuesInNumericFieldsFuzzer - iterate through each Number and Integer fields that have minimum declared and send requests with values matching the minimum size/value in the targeted field
• NewFieldsFuzzer - send a 'happy' flow request and add a new field inside the request called 'catsFuzzyField'
• NullValuesInFieldsFuzzer - iterate through each field and send requests with null values in the targeted field
• OnlyControlCharsInFieldsTrimValidateFuzzer - iterate through each field and send values with control chars only
• OnlyWhitespacesInFieldsTrimValidateFuzzer - iterate through each field and send values with unicode separators only
• OnlySingleCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values with single code point emojis only
• OnlyMultiCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values with multi code point emojis only
• RemoveFieldsFuzzer - iterate through each request fields and remove certain fields according to the supplied 'fieldsFuzzingStrategy'
• StringFieldsLeftBoundaryFuzzer - iterate through each String field and send requests with outside the range values on the left side in the targeted field
• StringFieldsRightBoundaryFuzzer - iterate through each String field and send requests with outside the range values on the right side in the targeted field
• StringFormatAlmostValidValuesFuzzer - iterate through each String field and get its 'format' value (i.e. email, ip, uuid, date, datetime, etc); send requests with values which are almost valid (i.e. email@yhoo. for email, 888.1.1. for ip, etc) in the targeted field
• StringFormatTotallyWrongValuesFuzzer - iterate through each String field and get its 'format' value (i.e. email, ip, uuid, date, datetime, etc); send requests with values which are totally wrong (i.e. abcd for email, 1244. for ip, etc) in the targeted field
• StringsInNumericFieldsFuzzer - iterate through each Integer (int, long) and Number field (float, double) and send requests having the fuzz string value in the targeted field
• TrailingWhitespacesInFieldsTrimValidateFuzzer - iterate through each field and send requests with trailing with Unicode whitespaces and invisible separators in the targeted field
• TrailingControlCharsInFieldsTrimValidateFuzzer - iterate through each field and send requests with trailing with Unicode control chars in the targeted field
• TrailingSingleCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values trailed with single code point emojis
• TrailingMultiCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values trailed with multi code point emojis
• VeryLargeStringsFuzzer - iterate through each String field and send requests with very large values (40000 characters) in the targeted field
• WithinControlCharsInFieldsSanitizeValidateFuzzer - iterate through each field and send values containing unicode control chars
• WithinSingleCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values containing single code point emojis
• WithinMultiCodePointEmojisInFieldsTrimValidateFuzzer - iterate through each field and send values containing multi code point emojis

You can run only these Fuzzers by supplying the --checkFields argument.

CATS has currently 26 registered Header Fuzzers:

• CheckSecurityHeadersFuzzer - check all responses for good practices around Security related headers like: [{name=Cache-Control, value=no-store}, {name=X-XSS-Protection, value=1; mode=block}, {name=X-Content-Type-Options, value=nosniff}, {name=X-Frame-Options, value=DENY}]
• DummyAcceptHeadersFuzzer - send a request with a dummy Accept header and expect to get 406 code
• DummyContentTypeHeadersFuzzer - send a request with a dummy Content-Type header and expect to get 415 code
• DuplicateHeaderFuzzer - send a 'happy' flow request and duplicate an existing header
• EmptyStringValuesInHeadersFuzzer - iterate through each header and send requests with empty String values in the targeted header
• ExtraHeaderFuzzer - send a 'happy' flow request and add an extra field inside the request called 'Cats-Fuzzy-Header'
• LargeValuesInHeadersFuzzer - iterate through each header and send requests with large values in the targeted header
• LeadingControlCharsInHeadersFuzzer - iterate through each header and prefix values with control chars
• LeadingWhitespacesInHeadersFuzzer - iterate through each header and prefix value with unicode separators
• LeadingSpacesInHeadersFuzzer - iterate through each header and send requests with spaces prefixing the value in the targeted header
• RemoveHeadersFuzzer - iterate through each header and remove different combinations of them
• OnlyControlCharsInHeadersFuzzer - iterate through each header and replace value with control chars
• OnlySpacesInHeadersFuzzer - iterate through each header and replace value with spaces
• OnlyWhitespacesInHeadersFuzzer - iterate through each header and replace value with unicode separators
• TrailingSpacesInHeadersFuzzer - iterate through each header and send requests with trailing spaces in the targeted header \
• TrailingControlCharsInHeadersFuzzer - iterate through each header and trail values with control chars
• TrailingWhitespacesInHeadersFuzzer - iterate through each header and trail values with unicode separators
• UnsupportedAcceptHeadersFuzzer - send a request with an unsupported Accept header and expect to get 406 code
• UnsupportedContentTypesHeadersFuzzer - send a request with an unsupported Content-Type header and expect to get 415 code

You can run only these Fuzzers by supplying the --checkHeaders argument.

CATS has currently 6 registered HTTP Fuzzers:

• BypassAuthenticationFuzzer - check if an authentication header is supplied; if yes try to make requests without it
• DummyRequestFuzzer - send a dummy json request {'cats': 'cats'}
• HappyFuzzer - send a request with all fields and headers populated
• HttpMethodsFuzzer - iterate through each undocumented HTTP method and send an empty request
• MalformedJsonFuzzer - send a malformed json request which has the String 'bla' at the end
• NonRestHttpMethodsFuzzer - iterate through a list of HTTP method specific to the WebDav protocol that are not expected to be implemented by REST APIs

You can run only these Fuzzers by supplying the --checkHttp argument.

Usually a good OpenAPI contract must follow several good practices in order to make it easy digestible by the service clients and act as much as possible as self-sufficient documentation:

• follow good practices around naming the contract elements like paths, requests, responses
• always use plural for the path names, separate paths words through hyphens/underscores, use camelCase or snake_case for any json types and properties
• provide tags for all operations in order to avoid breaking code generation on some languages and have a logical grouping of the API operations
• provide good description for all paths, methods and request/response elements
• provide meaningful responses for POST, PATCH and PUT requests
• provide examples for all requests/response elements
• provide structural constraints for (ideally) all request/response properties (min, max, regex)
• heaver some sort of CorrelationIds/TraceIds within headers
• have at least a security schema in place
• avoid having the API version part of the paths
• document response codes for both "happy" and "unhappy" flows
• avoid using xml payload unless there is a really good reason (like documenting an old API for example)
• json types and properties do not use the same naming (like having a Pet with a property named pet)

CATS has currently 9 registered ContractInfo Fuzzers:

• HttpStatusCodeInValidRangeFuzzer - verifies that all HTTP response codes are within the range of 100 to 599
• NamingsContractInfoFuzzer - verifies that all OpenAPI contract elements follow REST API naming good practices
• PathTagsContractInfoFuzzer - verifies that all OpenAPI paths contain tags elements and checks if the tags elements match the ones declared at the top level
• RecommendedHeadersContractInfoFuzzer - verifies that all OpenAPI contract paths contain recommended headers like: CorrelationId/TraceId, etc.
• RecommendedHttpCodesContractInfoFuzzer - verifies that the current path contains all recommended HTTP response codes for all operations
• SecuritySchemesContractInfoFuzzer - verifies if the OpenApi contract contains valid security schemas for all paths, either globally configured or per path
• TopLevelElementsContractInfoFuzzer - verifies that all OpenAPI contract level elements are present and provide meaningful information: API description, documentation, title, version, etc.
• VersionsContractInfoFuzzer - verifies that a given path doesn't contain versioning information
• XmlContentTypeContractInfoFuzzer - verifies that all OpenAPI contract paths responses and requests does not offer application/xml as a Content-Type

You can run only these Fuzzers by supplying the --checkContract argument.

In some cases, the tests generated by CATS will not be sufficient for your situation. Using the CustomFuzzer you can leverage the full power if CATS on not writing any code, while also supply custom values for specific fields. The cool thing is that you can target a single field, and the rest of the information will be populated by CATS using valid data, just like a 'happy' flow request. It's important to note that reference data won't get replaced when using the CustomFuzzer. So if there are reference data fields, you must also supply those in the CustomFuzzer. The CustomFuzzer will only trigger if a valid customFuzzer.yml file is supplied. The file has the following syntax:

/path:
    testNumber:
        description: Short description of the test
        prop: value
        prop#subprop: value
        prop7:
          - value1
          - value2
          - value3
        oneOfSelection:
          element#type: "Value"
        expectedResponseCode: HTTP_CODE
        httpMethod: HTTP_NETHOD

This is a description of the elements within the customFuzzer.yml file:

• you can supply a description of the test. This will be set as the Scenario description. If you don't supply a description the testNumber will be used instead.
• you can have multiple tests under the same path: test1, test2, etc.
• expectedResponseCode is mandatory, otherwise the Fuzzer will ignore this test. The expectedResponseCode tells CATS what to expect from the service when sending this test.
• at most one of the properties can have multiple values. When this situation happens, that test will actually become a list of tests one for each of the values supplied. For example in the above example prop7 has 3 values. This will actually result in 3 tests, one for each value.
• CustomFuzzer only triggers when you supply a customFuzzer.yml-like file using the --customFuzzerFile=XXX argument.
• test within the file are executed in the declared order. This is why you can have outputs from one test act as inputs for the next one(s) (see the next section for details).
• if the supplied httpMethod doesn't exist in the OpenAPI given path, a warning will be issued and no test will be executed
• if the supplied httpMethod is not a valid HTTP method, a warning will be issued and no test will be executed
• if the request payload uses a oneOf element to allow multiple request types, you can control which of the possible types the CustomFuzzer will apply to using the oneOfSelection keyword. The value of the oneOfSelection keyword must match the fully qualified name of the discriminator.
• if no oneOfSelection is supplied, and the request payload accepts multiple oneOf elements, than a custom test will be created for each type of payload
• the file uses Json path syntax for all the properties you can supply; you can separate elements through # as in the example above instead of .

When you have request payloads which can take multiple object types, you can use the oneOfSelection keyword to specify which of the possible object types is required by the CustomFuzzer. If you don't provide this element, all combinations will be considered. If you supply a value, this must be exactly the one used in the discriminator.

As CATs mostly relies on generated data with small help from some reference data, testing complex business scenarios with the pre-defined Fuzzers is not possible. Suppose we have an endpoint that creates data (doing a POST), and we want to check its existence (via GET). We need a way to get some identifier from the POST call and send it to the GET call. This is now possible using the CustomFuzzer. The customFuzzerFile can have an output entry where you can state a variable name, and its fully qualified name from the response in order to set its value. You can then refer the variable using ${variable_name} from another test in order to use its value.

Here is an example:

/pet:
    test_1:
      description: Create a Pet
      httpMethod: POST
      name: "My Pet"
      expectedResponseCode: 200
      output:
        petId: pet#id
/pet/{id}:
    test_2:
      description: Get a Pet
      id: ${petId}
      expectedResponseCode: 200

Suppose the test_1 execution outputs:

{
  "pet": 
    { 
      "id" : 2
    }
}

When executing test_1 the value of the pet id will be stored in the petId variable (value 2). When executing test_2 the id parameter will be replaced with the petId variable (value 2) from the previous case.

Please note: variables are visible across all custom tests; please be careful with the naming as they will get overridden.

The CustomFuzzer can verify more than just the expectedResponseCode. This is achieved using the verify element. This is an extended version of the above customFuzzer.yml file.

/pet:
    test_1:
      description: Create a Pet
      httpMethod: POST
      name: "My Pet"
      expectedResponseCode: 200
      output:
        petId: pet#id
      verify:
        pet#name: "Baby"
        pet#id: "[0-9]+"
/pet/{id}:
    test_2:
      description: Get a Pet
      id: ${petId}
      expectedResponseCode: 200

Considering the above file:

• the CustomFuzzer will check if the response has the 2 elements pet#name and pet#id
• if the elements are found, it will check that the pet#name has the Baby value and that the pet#id is numeric

The following json response will pass test_1:

{
  "pet": 
    { 
      "id" : 2,
      "name": "Baby"
    }
}

But this one won't (pet#name is missing):

{
  "pet": 
    { 
      "id" : 2
    }
}

You can also refer to request fields in the verify section by using the ${request#..} qualifier. Using the above example, by having the following verify section:

/pet:
  test_1:
    description: Create a Pet
    httpMethod: POST
    name: "My Pet"
    expectedResponseCode: 200
    output:
      petId: pet#id
    verify:
      pet#name: "${request#name}"
      pet#id: "[0-9]+"

It will verify if the response contains a pet#name element and that its value equals My Pet as sent in the request.

Some notes:

• verify parameters support Java regexes as values
• you can supply more than one parameter to check (as seen above)
• if at least one of the parameters is not present in the response, CATs will report an error
• if all parameters are found and have valid values, but the response code is not matched, CATs will report a warning
• if all the parameters are found and match their values, and the response code is as expected, CATs will report a success

You can also set additionalProperties fields through the customFuzzerFile using the same syntax as for Setting additionalProperties in Reference Data.

The following keywords are reserved in CustomFuzzer tests: output, expectedResponseCode, httpMethod, description, oneOfSelection, verify, additionalProperties, topElement and mapValues.

Although CATs is not a security testing tool, you can use it to test basic security scenarios by fuzzing specific fields with different sets of nasty strings. The behaviour is similar to the CustomFuzzer. You can use the exact same elements for output variables, test correlation, verify responses and so forth, with the addition that you must also specify a targetFields and a stringsList element. A typical securityFuzzerFile will look like this:

/pet:
    test_1:
      description: Run XSS scenarios
      name: "My Pet"
      expectedResponseCode: 200
      targetFields:
        - pet#id
        - pet#description
      stringsFile: xss.txt

You can also supply output, httpMethod, oneOfSelection and/or verify (with the same behaviour as within the CustomFuzzer) if they are relevant to your case.

The file uses Json path syntax for all the properties you can supply; you can separate elements through # as in the example instead of ..

This is what the SecurityFuzzer will do after parsing the above securityFuzzerFile:

• it will add the fixed value "My Pet" to all the request for the field name
• for each field specified in the targetFields i.e. pet#id and pet#description it will create requests for each line from the xss.txt file and supply those values in each field
• if you consider the xss.txt sample file included in the CATs repo, this means that it will send 21 requests targeting pet#id and 21 requests targeting pet#description i.e. a total of 42 tests
• for each of these 42 tests, the SecurityFuzzer will expect a 200 response code. If another response code is returned, then CATs will report the test as error.

As an idea on how to create security tests, you can split the nasty strings into multiple files of interest in your particular context. You can have a sql_injection.txt, a xss.txt, a command_injection.txt and so on. For each of these files, you can create a test entry in the securityFuzzerFile where you include the fields you think are meaningful for these types of tests. (It was a deliberate choice (for now) to not include all fields by default.) The expectedResponseCode should be tweaked according to your particular context. Your service might sanitize data before validation, so might be perfectly valid to expect a 200 or might validate the fields directly, so might be perfectly valid to expect a 400. A 500 will usually mean something was not handled properly and might signal a possible bug.

You can also set additionalProperties fields through the customFuzzerFile using the same syntax as for Setting additionalProperties in Reference Data.

The following keywords are reserved in SecurityFuzzer tests: output, expectedResponseCode, httpMethod, description, verify, oneOfSelection, targetFields, stringsFile, additionalProperties, topElement and mapValues.

There are often cases where some fields need to contain relevant business values in order for a request to succeed. You can provide such values using a reference data file specified by the --refData argument. The reference data file is a YAML-format file that contains specific fixed values for different paths in the request document. The file structure is as follows:

/path/0.1/auth:
    prop#subprop: 12
    prop2: 33
    prop3#subprop1#subprop2: "test"
/path/0.1/cancel:
    prop#test: 1

For each path you can supply custom values for properties and sub-properties which will have priority over values supplied by any other Fuzzer. Consider this request payload:

{
    "address": {
        "phone": "123",
        "postCode": "408",
        "street": "cool street"    
    },
    "name": "Joe"
}

and the following reference data file file:

/path/0.1/auth:
    address#street: "My Street"
    name: "John"

This will result in any fuzzed request to the /path/0.1/auth endpoint being updated to contain the supplied fixed values:

{
    "address": {
        "phone": "123",
        "postCode": "408",
        "street": "My Street"    
    },
    "name": "John"
}

The file uses Json path syntax for all the properties you can supply; you can separate elements through # as in the example above instead of ..

You can use environment (system) variables in a ref data file using: $$VARIABLE_NAME. (notice double $$)

As additional properties are maps i.e. they don't actually have a structure, CATS cannot currently generate valid values. If the elements within such a data structure are essential for a request, you can supply them via the refData file using the following syntax:

/path/0.1/auth:
    address#street: "My Street"
    name: "John"
    additionalProperties:
      topElement: metadata
      mapValues:
        test: "value1"
        anotherTest: "value2"

The additionalProperties element must contain the actual key-value pairs to be sent within the requests and also a top element if needed. topElement is not mandatory. The above example will output the following json (considering also the above examples):

{
    "address": {
        "phone": "123",
        "postCode": "408",
        "street": "My Street"    
    },
    "name": "John",
    "metadata": {
        "test": "value1",
        "anotherTest": "value2"
    }   
}

The following keywords are reserved in a reference data file: additionalProperties, topElement and mapValues.

You can also have the ability to send the same reference data for ALL paths (just like you do with the headers). You can achieve this by using all as a key in the refData file:

all:
  address#zip: 123

This will try to replace address#zip in all requests (if the field is present).

There are (rare) cases when some fields may not make sense together. Something like: if you send firstName and lastName, you are not allowed to also send name. As OpenAPI does not have the capability to send request fields which are dependent on each other, you can use the refData file to instruct CATS to remove fields before sending a request to the service. You can achieve this by using the cats_remove_field as a value for the fields you want to remove. For the above case the refData field will look as follows:

all:
  name: "cats_remove_field"

You can leverage the fact that the CustomFuzzer can run functional flows in order to create dynamic --refData files which won't need manual setting the reference data values. The --refData file must be created with variables ${variable} instead of fixed values and those variables must be output variables in the --customFuzzer file. In order for the CustomFuzzer to properly replace the variables names with their values you must supply the --refData file as an argument when the CustomFuzzer runs.

cats -c contract.yml -s http://localhost:8080 --customFuzzerFile=customFuzzer.yml --refData=refData.yml --fuzzers=CustomFuzzer

The customFuzzer.yml file:

/pet:
  test_1:
    description: Create a Pet
    httpMethod: POST
    name: "My Pet"
    expectedResponseCode: 200
    output:
      petId: pet#id

The refData.yml file:

/pet-type:
  id: ${petId}

After running CATS using the command and the 2 files above, you will get a refData_replace.yml file where the id will get the value returned into the petId variable.

The refData_replaced.yml:

/pet-type:
  id: 123

You can now use the refData_replaced.yml as a --refData file for running CATS with the rest of the Fuzzers.

This can be used to send custom fixed headers with each payload. It is useful when you have authentication tokens you want to use to authenticate the API calls. You can use path specific headers or common headers that will be added to each call using an all element. Specific paths will take precedence over the all element. Sample headers file:

all:
    Accept: application/json
/path/0.1/auth:
    jwt: XXXXXXXXXXXXX
/path/0.2/cancel:
    jwt: YYYYYYYYYYYYY

This will add the Accept header to all calls and the jwt header to the specified paths. You can use environment (system) variables in a headers file using: $$VARIABLE_NAME. (notice double $$)

DELETE is the only HTTP verb that is intended to remove resources and executing the same DELETE request twice will result in the second one to fail as the resource is no longer available. It will be pretty heavy to supply a large list of identifiers within the --refData file and this is why the recommendation was to skip the DELETE method when running CATS.

But starting with version 7.0.2 CATS has some intelligence in dealing with DELETE. In order to have enough valid entities CATS will save the corresponding POST requests in an internal Queue, and everytime a DELETE request it will be executed it will poll data from there. In order to have this actually working, your contract must comply with common sense conventions:

• the DELETE path is actually the POST path plus an identifier: if POST is /pets, then DELETE is expected to be /pets/{petId}.
• CATS will try to match the {petId} parameter within the body returned by the POST request while doing various combinations of the petId name. It will try to search for the following entries: petId, id, pet-id, pet_id with different cases.
• If any of those entries is found within a stored POST result, it will replace the {petId} with that value

For example, suppose that a POST to /pets responds with:

{
  "pet_id": 2,
  "name": "Chuck"
}

When doing a DELETE request, CATS will discover that {petId} and pet_id are used as identifiers for the Pet resource, and will do the DELETE at /pets/2.

If these conventions are followed (which also align to good REST naming practices), it is expected that DELETE and POSTrequests will be on-par for most of the entities.

Some APIs might use content negotiation versioning which implies formats like application/v11+json in the Accept header.

You can handle this in CATS as follows:

• if the OpenAPI contract defines its content as:

 requestBody:
        required: true
        content:
          application/v5+json:
            schema:
              $ref: '#/components/RequestV5'
          application/v6+json:
            schema:
              $ref: '#/components/RequestV6'

by having clear separation between versions, you can pass the --contentType argument with the version you want to test: cats ... --contentType="application/v6+json".

If the OpenAPI contract is not version aware (you already exported it specific to a version) and the content looks as:

 requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/RequestV5'

and you still need to pass the application/v5+json Accept header, you can use the --headers file to add it:

all:
  Accept: "application/v5+json"

There isn't a consensus on how you should handle situations when you trail or prefix valid values with spaces. One strategy will be to have the service trimming spaces before doing the validation, while some other services will just validate them as they are. You can control how CATS should expect such cases to be handled by the service using the --edgeSpacesStrategy argument. You can set this to trimAndValidate or validateAndTrim depending on how you expect the service to behave:

• trimAndValidate means that the service will first trim the spaces and after that run the validation
• validateAndTrim means that the service runs the validation first without any trimming of spaces

This is a global setting i.e. configured when CATS starts and all Fuzzer expects a consistent behaviour from all the service endpoints.

There are cases when certain parts of the request URL are parameterized. For example a case like: /{version}/pets. {version} is supposed to have the same value for all requests. This is why you can supply actual values to replace such parameters using the --urlParams argument. You can supply a ; separated list of name:value pairs to replace the name parameters with their corresponding value. For example supplying --urlParams=version:v1.0 will replace the version parameter from the above example with the value v1.0.

CATS also supports schemas with oneOf, allOf and anyOf composition. CATS wil consider all possible combinations when creating the fuzzed payloads.

The following configuration files: securityFuzzerFile, customFuzzerFile, refData support setting dynamic values for the inner fields. For now the support only exists for java.time.* and org.apache.commons.lang3.*, but more types of elements will come in the near future.

Let's suppose you have a date/date-time field, and you want to set it to 10 days from now. You can do this by setting this as a value T(java.time.OffsetDateTime).now().plusDays(10). This will return an ISO compliant time in UTC format.

A customFuzzerFile using this can look like:

/path:
    testNumber:
        description: Short description of the test
        prop: value
        prop#subprop: "T(java.time.OffsetDateTime).now().plusDays(10)"
        prop7:
          - value1
          - value2
          - value3
        oneOfSelection:
          element#type: "Value"
        expectedResponseCode: HTTP_CODE
        httpMethod: HTTP_NETHOD

You can also check the responses using a similar syntax and also accounting for the actual values returned in the response. This is a syntax than can test if a returned date is after the current date: T(java.time.LocalDate).now().isBefore(T(java.time.LocalDate).parse(expiry.toString())). It will check if the expiry field returned in the json response, parsed as date, is after the current date.

The syntax of dynamically setting dates is compliant with the Spring Expression Language specs.

If you need to run CATS behind a proxy, you can supply the following arguments: --proxyHost and --proxyPort. A typical run with proxy settings on localhost:8080 will look as follows:

cats --contract=YAML_FILE --server=SERVER_URL --proxyHost=localhost --proxyPort=8080

CATS supports any form of HTTP header(s) based authentication (basic auth, oauth, custom JWT, apiKey, etc) using the headers mechanism. You can supply the specific HTTP header name and value and apply to all endpoints. Additionally, basic auth is also supported using the --basicauth=USR:PWD argument.

By default, CATS trusts all server certificates and doesn't perform hostname verification.

For two-way SSL you can specify a JKS file (Java Keystore) that holds the client's private key using the following arguments:

• --sslKeystore Location of the JKS keystore holding certificates used when authenticating calls using one-way or two-way SSL
• --sslKeystorePwd The password of the sslKeystore
• --sslKeyPwd The password of the private key within the sslKeystore

For details on how to load the certificate and private key into a Java Keystore you can use this guide: https://mrkandreev.name/blog/java-two-way-ssl/.

When using the native binaries (not the uberjar) there might be issues when using dynamic values in the CATS files. This is due to the fact that GraalVM only bundles whatever can discover at compile time. The following classes are currently supported:

java.util.Base64.Encoder.class, java.util.Base64.Decoder.class, java.util.Base64.class, org.apache.commons.lang3.RandomUtils.class, org.apache.commons.lang3.RandomStringUtils.class, 
org.apache.commons.lang3.DateFormatUtils.class, org.apache.commons.lang3.DateUtils.class, 
org.apache.commons.lang3.DurationUtils.class, java.time.LocalDate.class, java.time.LocalDateTime.class, java.time.OffsetDateTime.class

At this moment, CATS only works with OpenAPI spec.

The Fuzzers has the following support for media types and HTTP methods:

• application/json media type only
• HTTP methods: POST, PUT, PATCH and GET

If a response contains a free Map specified using the additionalParameters tag CATS will issue a WARN level log message as it won't be able to validate that the response matches the schema.

CATS uses RgxGen in order to generate Strings based on regexes. This has certain limitations mostly with complex patterns.

All custom files that can be used by CATS (customFuzzerFile, headers, refData, etc) are in a YAML format. When setting or getting values to/from JSON for input and/or output variables, you must use a JsonPath syntax using either # or . as separators. You can find some selector examples here: JsonPath.

Please refer to CONTRIBUTING.md.