Detection: Windows CertUtil Decode File
Description
CertUtil.exe may be used to encode and decode a file, including PE and script code. Encoding will convert a file to base64 with -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- tags. Malicious usage will include decoding a encoded file that was downloaded. Once decoded, it will be loaded by a parallel process. Note that there are two additional command switches that may be used - encodehex and decodehex. Similarly, the file will be encoded in HEX and later decoded for further execution. During triage, identify the source of the file being decoded. Review its contents or execution behavior for further analysis.
Annotations
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Implementation
To successfully implement this search you need to be ingesting information on process that include the name of the process responsible for the changes from your endpoints into the Endpoint datamodel in the Processes node. In addition, confirm the latest CIM App 4.20 or higher is installed and the latest TA for the endpoint product.
Known False Positives
Typically seen used to encode files, but it is possible to see legitimate use of decode. Filter based on parent-child relationship, file paths, endpoint or user.
Associated Analytic Story
Risk Based Analytics (RBA)
| Risk Message | Risk Score | Impact | Confidence |
|---|---|---|---|
| An instance of $parent_process_name$ spawning $process_name$ was identified on endpoint $dest_device_id$ by user $dest_user_id$ attempting to decode a file on disk. | 40 | 50 | 80 |
References
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https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1140/T1140.md
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https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/certutil
Version: 4