ZipSlip.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>Extracting files from a malicious zip file, or similar type of archive,
is at risk of directory traversal attacks if filenames from the archive are
not properly validated.</p>

<p>Zip archives contain archive entries representing each file in the archive. These entries
include a file path for the entry, but these file paths are not restricted and may contain
unexpected special elements such as the directory traversal element (<code>..</code>). If these
file paths are used to create a filesystem path, then a file operation may happen in an 
unexpected location. This can result in sensitive information being
revealed or deleted, or an attacker being able to influence behavior by modifying unexpected
files.</p>

<p>For example, if a zip file contains a file entry <code>..\sneaky-file</code>, and the zip file
is extracted to the directory <code>c:\output</code>, then naively combining the paths would result
in an output file path of <code>c:\output\..\sneaky-file</code>, which would cause the file to be
written to <code>c:\sneaky-file</code>.</p>

</overview>
<recommendation>

<p>Ensure that output paths constructed from zip archive entries are validated to prevent writing
files to unexpected locations.</p>

<p>The recommended way of writing an output file from a zip archive entry is to:</p>

<ol>
<li>Use <code>Path.Combine(destinationDirectory, archiveEntry.FullName)</code> to determine the raw
output path.</li>
<li>Use <code>Path.GetFullPath(..)</code> on the raw output path to resolve any directory traversal
elements.</li>
<li>Use <code>Path.GetFullPath(destinationDirectory + Path.DirectorySeparatorChar)</code> to
determine the fully resolved path of the destination directory.</li>
<li>Validate that the resolved output path <code>StartsWith</code> the resolved destination
directory, aborting if this is not true.</li>
</ol>

<p>Another alternative is to validate archive entries against a whitelist of expected files.</p>

</recommendation>
<example>

<p>In this example, a file path taken from a zip archive item entry is combined with a
destination directory. The result is used as the destination file path without verifying that
the result is within the destination directory. If provided with a zip file containing an archive
path like <code>..\sneaky-file</code>, then this file would be written outside the destination
directory.</p>

<sample src="examples/ZipSlipBad.cs" />

<p>To fix this vulnerability, we need to make three changes. Firstly, we need to resolve any
directory traversal or other special characters in the path by using <code>Path.GetFullPath</code>.
Secondly, we need to identify the destination output directory, again using
<code>Path.GetFullPath</code>, this time on the output directory. Finally, we need to ensure that
the resolved output starts with the resolved destination directory, and throw an exception if this
is not the case.</p>

<sample src="examples/ZipSlipGood.cs" />

</example>
<references>

<li>
Snyk:
<a href="https://snyk.io/research/zip-slip-vulnerability">Zip Slip Vulnerability</a>.
</li>
<li>
OWASP:
<a href="https://owasp.org/www-community/attacks/Path_Traversal">Path Traversal</a>.
</li>

</references>
</qhelp>