The following analytic utilizes a Windows Event Log - CAPI2 - or CryptoAPI 2, to identify failed trust validation. Typically, this event log is meant for diagnosing PKI issues, however is a great source to identify failed trust validation. Note that this event log is noisy as it captures common PKI requests from many different processes. EventID 81 is generated anytime a trust validation fails. The description for EventID 81 is "The digital signature of the object did not verify." STRT tested this analytic using Mimikatz binary.
- Type: Anomaly
Product: Splunk Enterprise, Splunk Enterprise Security, Splunk Cloud
- Last Updated: 2023-10-10
- Author: Michael Haag, Splunk
- ID: 6ffc7f88-415b-4278-a80d-b957d6539e1a
Kill Chain Phase
- CIS 10
1 2 3 4 5 `capi2_operational` EventID=81 "The digital signature of the object did not verify." | xmlkv UserData_Xml | stats count min(_time) as firstTime max(_time) as lastTime by Computer, UserData_Xml | rename Computer as dest | `windows_sip_winverifytrust_failed_trust_validation_filter`
The SPL above uses the following Macros:
windows_sip_winverifytrust_failed_trust_validation_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.
List of fields required to use this analytic.
How To Implement
To implement this analytic, one will need to enable the Microsoft-Windows-CAPI2/Operational log within the Windows Event Log. Note this is a debug log for many purposes, and the analytic only focuses in on EventID 81. Review the following gist for additional enabling information.
Known False Positives
False positives may be present in some instances of legitimate binaries with invalid signatures. Filter as needed.
Associated Analytic Story
|64.0||80||80||Failed trust validation via the CryptoAPI 2 on $dest$ for a binary.|
The Risk Score is calculated by the following formula: Risk Score = (Impact * Confidence/100). Initial Confidence and Impact is set by the analytic author.
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