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Python: Initial version and help of tar-slip (CWE-022) query.
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‎python/ql/src/Security/CWE-022/TarSlip.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>Extracting files from a malicious tar archive without validating that the destination file path
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is within the destination directory can cause files outside the destination directory to be
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overwritten, due to the possible presence of directory traversal elements (<code>..</code>) in
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archive paths.</p>
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<p>Tar archives contain archive entries representing each file in the archive. These entries
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include a file path for the entry, but these file paths are not restricted and may contain
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unexpected special elements such as the directory traversal element (<code>..</code>). If these
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file paths are used to determine an output file to write the contents of the archive item to, then
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the file may be written to an unexpected location. This can result in sensitive information being
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revealed or deleted, or an attacker being able to influence behavior by modifying unexpected
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files.</p>
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<p>For example, if a tar archive contains a file entry <code>..\sneaky-file</code>, and the tar archive
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is extracted to the directory <code>c:\output</code>, then naively combining the paths would result
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in an output file path of <code>c:\output\..\sneaky-file</code>, which would cause the file to be
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written to <code>c:\sneaky-file</code>.</p>
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</overview>
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<recommendation>
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<p>Ensure that output paths constructed from tar archive entries are validated
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to prevent writing files to unexpected locations.</p>
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<p>The recommended way of writing an output file from a tar archive entry is to check that
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<code>".."</code> does not occur in the path.
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</p>
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</recommendation>
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<example>
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<p>
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In this example an archive is extracted without validating file paths.
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If <code>archive.tar</code> contained relative paths (for
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instance, if it were created by something like <code>tar -cf archive.tar
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../file.txt</code>) then executing this code could write to locations
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outside the destination directory.
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</p>
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<sample src="examples/tarslip_bad.py" />
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<p>To fix this vulnerability, we need to check that the path does not
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contain any <code>".."</code> elements in it.
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</p>
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<sample src="examples/tarslip_good.py" />
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</example>
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<references>
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<li>
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Snyk:
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<a href="https://snyk.io/research/zip-slip-vulnerability">Zip Slip Vulnerability</a>.
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</li>
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<li>
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OWASP:
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<a href="https://www.owasp.org/index.php/Path_traversal">Path Traversal</a>.
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</li>
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<li>
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Python Library Reference:
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< href="https://docs.python.org/3/library/tarfile.html#tarfile.TarFile.extract">TarFile.extract</a>.
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</li>
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<li>
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Python Library Reference:
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< href="https://docs.python.org/3/library/tarfile.html#tarfile.TarFile.extractall">TarFile.extractall</a>.
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</li>
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</references>
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</qhelp>

‎python/ql/src/Security/CWE-022/TarSlip.ql‎

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/**
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* @name Arbitrary file write during tarfile extraction
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* @description Extracting files from a malicious tar archive without validating that the
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* destination file path is within the destination directory can cause files outside
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* the destination directory to be overwritten.
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* @kind path-problem
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* @id py/tarslip
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* @problem.severity error
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* @precision medium
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* @tags security
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* external/cwe/cwe-022
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*/
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import python
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import semmle.python.security.Paths
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import semmle.python.security.TaintTracking
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/** A TaintKind to represent open tarfile objects. That is, the result of calling `tarfile.open(...)` */
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class OpenTarFile extends TaintKind {
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OpenTarFile() {
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this = "tarfile.open"
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}
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}
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/** The source of open tarfile objects. That is, any call to `tarfile.open(...)` */
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class TarfileOpen extends TaintSource {
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TarfileOpen() {
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Module::named("tarfile").attr("open").getACall() = this
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and
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/* If argument refers to a string object, then it's a hardcoded path and
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* this tarfile is safe.
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*/
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not this.(CallNode).getAnArg().refersTo(any(StringObject str))
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}
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override predicate isSourceOf(TaintKind kind) {
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kind instanceof OpenTarFile
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}
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}
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/* Any call to an extract method */
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class ExtractionSink extends TaintSink {
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CallNode call;
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ExtractionSink() {
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this = call.getFunction().(AttrNode).getObject(extract())
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}
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override predicate sinks(TaintKind kind) {
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kind instanceof OpenTarFile
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}
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}
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private string extract() {
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result = "extract" or result = "extractall"
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}
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//evil = [e for e in members if os.path.relpath(e).startswith(('/', '..'))]
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class TarSlipConfiguration extends TaintTracking::Configuration {
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TarSlipConfiguration() { this = "TarSlip configuration" }
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override predicate isSource(TaintTracking::Source source) { source instanceof TarfileOpen }
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override predicate isSink(TaintTracking::Sink sink) { sink instanceof ExtractionSink }
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}
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from TarSlipConfiguration config, TaintedPathSource src, TaintedPathSink sink
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where config.hasFlowPath(src, sink)
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select sink.getSink(), src, sink, "Extraction of tarfile from $@", src.getSource(), "a potentially untrusted source"
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‎python/ql/src/Security/CWE-022/examples/tarslip_bad.py‎

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import tarfile
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with tarfile.open('archive.zip') as tar:
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#BAD : This could write any file on the filesystem.
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for entry in tar:
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tar.extract(entry, "/tmp/unpack/")

‎python/ql/src/Security/CWE-022/examples/tarslip_good.py‎

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import tarfile
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import os.path
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with tarfile.open('archive.zip') as tar:
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for entry in tar:
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#GOOD: Check that entry is safe
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if os.path.isabs(entry.name) or ".." in entry.name:
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raise ValueError("Illegal tar archive entry")
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tar.extract(entry, "/tmp/unpack/")

‎python/ql/test/query-tests/Security/CWE-022/TarSlip.expected‎

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edges
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| tarslip.py:12:7:12:39 | tarfile.open | tarslip.py:13:1:13:3 | tarfile.open |
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| tarslip.py:12:7:12:39 | tarfile.open | tarslip.py:14:1:14:3 | tarfile.open |
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| tarslip.py:16:7:16:39 | tarfile.open | tarslip.py:17:14:17:16 | tarfile.open |
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| tarslip.py:16:7:16:39 | tarfile.open | tarslip.py:18:5:18:7 | tarfile.open |
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parents
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#select
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| tarslip.py:13:1:13:3 | Taint sink | tarslip.py:12:7:12:39 | tarfile.open | tarslip.py:13:1:13:3 | tarfile.open | Extraction of tarfile from $@ | tarslip.py:12:7:12:39 | Taint source | a potentially untrusted source |
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| tarslip.py:18:5:18:7 | Taint sink | tarslip.py:16:7:16:39 | tarfile.open | tarslip.py:18:5:18:7 | tarfile.open | Extraction of tarfile from $@ | tarslip.py:16:7:16:39 | Taint source | a potentially untrusted source |

‎python/ql/test/query-tests/Security/CWE-022/TarSlip.qlref‎

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Security/CWE-022/TarSlip.ql

‎python/ql/test/query-tests/Security/CWE-022/tarslip.py‎

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#!/usr/bin/python
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import tarfile
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unsafe_filename_tar = sys.argv[1]
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safe_filename_tar = "safe_path.tar"
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tar = tarfile.open(safe_filename_tar)
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for entry in tar:
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tar.extract(entry)
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tar = tarfile.open(unsafe_filename_tar)
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tar.extractall()
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tar.close()
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tar = tarfile.open(unsafe_filename_tar)
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for entry in tar:
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tar.extract(entry)
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tar = tarfile.open(safe_filename_tar)
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tar.extractall()
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tar.close()

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