Creating custom YARA rules

In a previous post, we created YARA rules to detect compromised CCleaner executables (YARA rules to detect compromised CCleaner executables). We will use this example as an opportunity to illustrate how the creation of these custom YARA rules was performed.

In its blog post, Talos shared 3 hashes as Indicators Of Compromise (IOCs):

1a4a5123d7b2c534cb3e3168f7032cf9ebf38b9a2a97226d0fdb7933cf6030ff
6f7840c77f99049d788155c1351e1560b62b8ad18ad0e9adda8218b9f432f0a9
36b36ee9515e0a60629d2c722b006b33e543dce1c8c2611053e0651a0bfdb2e9

Although not explicitly stated in the Talos report, these are SHA256 hashes. We can infer this from the length of the hexadecimal strings. A hexadecimal character represents 4 bits, hence  64 hexadecimal characters represent 256 bits.

Admittedly, YARA is not the best tool to scan files with hashes as IOCs. YARA is designed to use pattern matching to scan malware, and early versions of YARA did not even have the capability to use cryptographic hashes. In later versions, a hash module was introduced to provide this capability.

A simple rule to match files based on a SHA256 IOC can be implemented like this:

import "hash"</p>
<p style="text-align:justify;">rule simple_hash_rule {
condition:
hash.sha256(0, filesize) == "1a4a5123d7b2c534cb3e3168f7032cf9ebf38b9a2a97226d0fdb7933cf6030ff"
}

Line 1 imports the hash module, which we need to calculate the SHA256 value of the content of a file.

The rule simple_hash_rule has just a condition (line 5): the SHA256 value of the content of the file is calculated with method call hash.sha256(0, filesize) and then compared to the IOC as a string. Remark that string comparison is case-sensitive, and that the cryptographic hash functions of the hash module return the hash values as lowercase hexadecimal strings. Hence, we have to provide the IOC hash as a lowercase string.

Using this simple rule to scan the C: drive of a Windows workstation in search of compromised CCleaner executables will take time: the YARA engine has to completely read each file on the disk to calculate its SHA256 value.

This can be optimized by writing a rule that will not hash files that can not possibly yield the hash we are looking for. One method to achieve this, is to first check the file size of a file and only calculate the SHA256 hash if the file size corresponds to the file size of the compromised executable.

This is what we did in this rule:

import "hash"</p>
<p style="text-align:justify;">rule ccleaner_compromised_installer {
condition:
filesize == 9791816 and hash.sha256(0, filesize) == "1a4a5123d7b2c534cb3e3168f7032cf9ebf38b9a2a97226d0fdb7933cf6030ff"
}

In this rule, we first compare the filesize variable to 9791816 and then we calculate the hash. Because the and operator uses lazy evaluation, the right-side operand will only be evaluated if the left-side operator is true. With this, we achieve our goal: only if the filesize is right, will the SHA256 hash be calculated. This will speed up the scanning process by excluding files with the wrong file size.

There is one caveat though: Talos did not report the file sizes for the IOCs they published. However, we were able to obtain samples of the compromised CCleaner files, and that’s how we know the file sizes.

If you are not in a position to get a copy of the samples, you can still search the Internet for information on these samples like the file size. For example, this particular sample was analyzed by Hybrid Analysis, and the file size is mentioned in the report:

You can also find information for this sample on VirusTotal, but the analysis report does not mention the exact file size, just 9.34 MB.

This information can still be used to create a rule that is optimized for performance, by accepting a range of possible file size values, like this:

import "hash"</p>
<p style="text-align:justify;">rule filesize_and_hash {
condition:
filesize &amp;gt;= 9.33 * 1024 * 1024 and filesize &amp;lt;= 9.35 * 1024 * 1024 and hash.sha256(0, filesize) == "1a4a5123d7b2c534cb3e3168f7032cf9ebf38b9a2a97226d0fdb7933cf6030ff"
}

This rule will first check if the file size is between 9.33MB and 9.35MB (1024 * 1024 is 1 MB) and if true, then calculate the hash.

Conclusion

Writing custom YARA rules is a skill that requires practice. If you want to become proficient at writing your own YARA rules, we recommend that you embrace all opportunities to write rules, and step by step make more complex rules.

About the author

Didier Stevens is a malware expert working for NVISO. Didier is a SANS Internet Storm Center handler and Microsoft MVP, and has developed numerous popular tools to assist with malware analysis. You can find Didier on Twitter and LinkedIn.

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