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Merge pull request #6112 from jorgectf/jorgectf/python/deserialization
Python: Port and extend XXE modeling
2 parents bbb2847 + 0e9da4a commit 2f4a22c

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‎python/ql/src/experimental/Security/CWE-611/SimpleXmlRpcServer.ql‎

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/**
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* @name SimpleXMLRPCServer DoS vulnerability
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* @description SimpleXMLRPCServer is vulnerable to DoS attacks from untrusted user input
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* @kind problem
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* @problem.severity warning
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* @precision high
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* @id py/simple-xml-rpc-server-dos
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* @tags security
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* external/cwe/cwe-776
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*/
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private import python
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private import experimental.semmle.python.Concepts
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private import semmle.python.ApiGraphs
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from DataFlow::CallCfgNode call, string kinds
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where
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call = API::moduleImport("xmlrpc").getMember("server").getMember("SimpleXMLRPCServer").getACall() and
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kinds =
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strictconcat(ExperimentalXML::XMLVulnerabilityKind kind |
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kind.isBillionLaughs() or kind.isQuadraticBlowup()
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|
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kind, ", "
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)
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select call, "SimpleXMLRPCServer is vulnerable to: " + kinds + "."

‎python/ql/src/experimental/Security/CWE-611/XXE.xml‎

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<?xml version="1.0"?>
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<!DOCTYPE dt [
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<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
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<test>&xxe;</test>

‎python/ql/src/experimental/Security/CWE-611/XmlEntityInjection.py‎

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from flask import request, Flask
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import lxml.etree
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import xml.etree.ElementTree
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app = Flask(__name__)
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# BAD
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@app.route("/bad")
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def bad():
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xml_content = request.args['xml_content']
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parser = lxml.etree.XMLParser()
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parsed_xml = xml.etree.ElementTree.fromstring(xml_content, parser=parser)
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return parsed_xml.text
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# GOOD
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@app.route("/good")
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def good():
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xml_content = request.args['xml_content']
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parser = lxml.etree.XMLParser(resolve_entities=False)
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parsed_xml = xml.etree.ElementTree.fromstring(xml_content, parser=parser)
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return parsed_xml.text

‎python/ql/src/experimental/Security/CWE-611/XmlEntityInjection.qhelp‎

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<!DOCTYPE qhelp PUBLIC
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"-//Semmle//qhelp//EN"
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"qhelp.dtd">
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<qhelp>
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<overview>
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<p>
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Parsing untrusted XML files with a weakly configured XML parser may lead to attacks such as XML External Entity (XXE),
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Billion Laughs, Quadratic Blowup and DTD retrieval.
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This type of attack uses external entity references to access arbitrary files on a system, carry out denial of
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service, or server side request forgery. Even when the result of parsing is not returned to the user, out-of-band
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data retrieval techniques may allow attackers to steal sensitive data. Denial of services can also be carried out
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in this situation.
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</p>
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</overview>
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<recommendation>
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<p>
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Use <a href="https://pypi.org/project/defusedxml/">defusedxml</a>, a Python package aimed
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to prevent any potentially malicious operation.
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</p>
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</recommendation>
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<example>
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<p>
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The following example calls <code>xml.etree.ElementTree.fromstring</code> using a parser (<code>lxml.etree.XMLParser</code>)
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that is not safely configured on untrusted data, and is therefore inherently unsafe.
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</p>
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<sample src="XmlEntityInjection.py"/>
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<p>
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Providing an input (<code>xml_content</code>) like the following XML content against /bad, the request response would contain the contents of
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<code>/etc/passwd</code>.
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</p>
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<sample src="XXE.xml"/>
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</example>
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<references>
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<li>Python 3 <a href="https://docs.python.org/3/library/xml.html#xml-vulnerabilities">XML Vulnerabilities</a>.</li>
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<li>Python 2 <a href="https://docs.python.org/2/library/xml.html#xml-vulnerabilities">XML Vulnerabilities</a>.</li>
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<li>Python <a href="https://www.edureka.co/blog/python-xml-parser-tutorial/">XML Parsing</a>.</li>
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<li>OWASP vulnerability description: <a href="https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Processing">XML External Entity (XXE) Processing</a>.</li>
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<li>OWASP guidance on parsing xml files: <a href="https://cheatsheetseries.owasp.org/cheatsheets/XML_External_Entity_Prevention_Cheat_Sheet.html#python">XXE Prevention Cheat Sheet</a>.</li>
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<li>Paper by Timothy Morgen: <a href="https://research.nccgroup.com/2014/05/19/xml-schema-dtd-and-entity-attacks-a-compendium-of-known-techniques/">XML Schema, DTD, and Entity Attacks</a></li>
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<li>Out-of-band data retrieval: Timur Yunusov &amp; Alexey Osipov, Black hat EU 2013: <a href="https://www.slideshare.net/qqlan/bh-ready-v4">XML Out-Of-Band Data Retrieval</a>.</li>
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<li>Denial of service attack (Billion laughs): <a href="https://en.wikipedia.org/wiki/Billion_laughs">Billion Laughs.</a></li>
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</references>
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</qhelp>

‎python/ql/src/experimental/Security/CWE-611/XmlEntityInjection.ql‎

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/**
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* @name XML Entity injection
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* @description User input should not be parsed allowing the injection of entities.
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* @kind path-problem
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* @problem.severity error
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* @id py/xml-entity-injection
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* @tags security
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* external/cwe/cwe-611
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* external/cwe/cwe-776
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* external/cwe/cwe-827
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*/
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// determine precision above
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import python
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import experimental.semmle.python.security.dataflow.XmlEntityInjection
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import DataFlow::PathGraph
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from
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XmlEntityInjection::XmlEntityInjectionConfiguration config, DataFlow::PathNode source,
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DataFlow::PathNode sink, string kinds
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where
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config.hasFlowPath(source, sink) and
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kinds =
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strictconcat(string kind |
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kind = sink.getNode().(XmlEntityInjection::Sink).getVulnerableKind()
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|
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kind, ", "
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)
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select sink.getNode(), source, sink,
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"$@ XML input is constructed from a $@ and is vulnerable to: " + kinds + ".", sink.getNode(),
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"This", source.getNode(), "user-provided value"

‎python/ql/src/experimental/semmle/python/Concepts.qll‎

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private import semmle.python.dataflow.new.TaintTracking
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private import experimental.semmle.python.Frameworks
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/**
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* Since there is both XML module in normal and experimental Concepts,
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* we have to rename the experimental module as this.
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*/
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module ExperimentalXML {
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/**
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* A kind of XML vulnerability.
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*
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* See https://pypi.org/project/defusedxml/#python-xml-libraries
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*/
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class XMLVulnerabilityKind extends string {
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XMLVulnerabilityKind() {
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this in ["Billion Laughs", "Quadratic Blowup", "XXE", "DTD retrieval"]
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}
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/** Holds for Billion Laughs vulnerability kind. */
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predicate isBillionLaughs() { this = "Billion Laughs" }
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/** Holds for Quadratic Blowup vulnerability kind. */
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predicate isQuadraticBlowup() { this = "Quadratic Blowup" }
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/** Holds for XXE vulnerability kind. */
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predicate isXxe() { this = "XXE" }
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/** Holds for DTD retrieval vulnerability kind. */
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predicate isDtdRetrieval() { this = "DTD retrieval" }
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}
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/**
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* A data-flow node that parses XML.
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*
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* Extend this class to model new APIs. If you want to refine existing API models,
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* extend `XMLParsing` instead.
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*/
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class XMLParsing extends DataFlow::Node instanceof XMLParsing::Range {
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/**
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* Gets the argument containing the content to parse.
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*/
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DataFlow::Node getAnInput() { result = super.getAnInput() }
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/**
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* Holds if this XML parsing is vulnerable to `kind`.
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*/
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predicate vulnerableTo(XMLVulnerabilityKind kind) { super.vulnerableTo(kind) }
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}
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/** Provides classes for modeling XML parsing APIs. */
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module XMLParsing {
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/**
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* A data-flow node that parses XML.
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*
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* Extend this class to model new APIs. If you want to refine existing API models,
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* extend `XMLParsing` instead.
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*/
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abstract class Range extends DataFlow::Node {
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/**
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* Gets the argument containing the content to parse.
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*/
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abstract DataFlow::Node getAnInput();
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/**
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* Holds if this XML parsing is vulnerable to `kind`.
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*/
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abstract predicate vulnerableTo(XMLVulnerabilityKind kind);
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}
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}
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}
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/** Provides classes for modeling LDAP query execution-related APIs. */
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module LdapQuery {
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/**

‎python/ql/src/experimental/semmle/python/Frameworks.qll‎

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*/
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private import experimental.semmle.python.frameworks.Stdlib
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private import experimental.semmle.python.frameworks.Xml
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private import experimental.semmle.python.frameworks.Flask
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private import experimental.semmle.python.frameworks.Django
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private import experimental.semmle.python.frameworks.Werkzeug

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