ZipSlip.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>

<overview>
<p>Extracting files from a malicious zip archive without validating that the destination file path
is within the destination directory can cause files outside the destination directory to be
overwritten, due to the possible presence of directory traversal elements (<code>..</code>) in
archive paths.</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 determine an output file to write the contents of the archive item to, then
the file may be written to 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 check that
<code>".."</code> does not occur in the path.
</p>

</recommendation>

<example>
<p>
In this example an archive is extracted without validating file paths.
If <code>archive.zip</code> contained relative paths (for
instance, if it were created by something like <code>zip archive.zip
../file.txt</code>) then executing this code could write to locations
outside the destination directory.
</p>

<sample src="ZipSlipBad.js" />

<p>To fix this vulnerability, we need to check that the path does not
contain any <code>".."</code> elements in it.
</p>

<sample src="ZipSlipGood.js" />

</example>
<references>

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

</references>
</qhelp>