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<h1 class="menu-title">ClamAV Documentation</h1>
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<h1 id="using-yara-rules-in-clamav"><a class="header" href="#using-yara-rules-in-clamav">Using YARA rules in ClamAV</a></h1>
<p>ClamAV can process YARA rules. ClamAV virus database file names ending with <code>.yar</code> or <code>.yara</code> are parsed as YARA rule files. The link to the YARA rule grammar documentation may be found at <a href="https://virustotal.github.io/yara/">https://virustotal.github.io/yara/</a>. There are currently a few limitations on using YARA rules within ClamAV:</p>
<ul>
<li>
<p>YARA modules are not yet supported by ClamAV. This includes the “import” keyword and any YARA module-specific keywords.</p>
</li>
<li>
<p>Global rules (<code>global</code> keyword) are not supported by ClamAV.</p>
</li>
<li>
<p>External variables(<code>contains</code> and <code>matches</code> keywords) are not supported.</p>
</li>
<li>
<p>YARA rules pre-compiled with the <code>yarac</code> command are not supported.</p>
</li>
<li>
<p>As in the ClamAV logical and extended signature formats, YARA strings and segments of strings separated by wild cards must represent at least two octets of data.</p>
</li>
<li>
<p>There is a maximum of 64 strings per YARA rule.</p>
</li>
<li>
<p>YARA rules in ClamAV must contain at least one literal, hexadecimal, or regular expression string.</p>
</li>
</ul>
<p>In addition, there are a few more ClamAV processing modes that may affect the outcome of YARA rules.</p>
<ul>
<li>
<p><em>File decomposition and decompression</em> - Since ClamAV uses file decomposition and decompression to find viruses within de-archived and uncompressed inner files, YARA rules executed by ClamAV will match against these files as well.</p>
</li>
<li>
<p><em>Normalization</em> - By default, ClamAV normalizes HTML, JavaScript, and ASCII text files. YARA rules in ClamAV will match against the normalized result. The effects of normalization of these file types may be captured using <code>clamscan --leave-temps --tempdir=mytempdir</code>. YARA rules may then be written using the normalized file(s) found in <code>mytempdir</code>. Alternatively, starting with ClamAV 0.100.0, <code>clamscan --normalize=no</code> will prevent normalization and only scan the raw file. To obtain similar behavior prior to 0.99.2, use <code>clamscan --scan-html=no</code>. The corresponding parameters for clamd.conf are <code>Normalize</code> and <code>ScanHTML</code>.</p>
</li>
<li>
<p><em>YARA conditions driven by string matches</em> - All YARA conditions are driven by string matches in ClamAV. This saves from executing every YARA rule on every file. Any YARA condition may be augmented with a string match clause which is always true, such as:</p>
</li>
</ul>
<pre><code class="language-perl">rule CheckFileSize
{
strings:
$abc = "abc"
condition:
($abc or not $abc) and filesize < 200KB
}
</code></pre>
<p>This will ensure that the YARA condition always performs the desired action (checking the file size in this example),</p>
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