CAR-2021-05-009: CertUtil With Decode Argument
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.
ATT&CK Detections
Technique | Subtechnique(s) | Tactic(s) | Level of Coverage |
---|---|---|---|
Deobfuscate/Decode Files or Information | N/A | Defense Evasion | Moderate |
D3FEND Techniques
ID | Name |
---|---|
D3-PSA | Process Spawn Analysis |
Data Model References
Object | Action | Field |
---|---|---|
process | create | exe |
process | create | command_line |
Implementations
Pseudocode – CertUtil with Decode Argument (Pseudocode, CAR native)
Pseudocode implementation of the Splunk search below
processes = search Process:Create
certutil_downloads = filter processes where (
exe =”C:\Windows\System32\certutil.exe” AND command_line = *decode* )
output certutil_downloads
Splunk code (Splunk, Endpoint)
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.
| tstats count min(_time) as firstTime max(_time) as lastTime from datamodel=Endpoint.Processes where Processes.process_name=certutil.exe Processes.process=*decode* by Processes.dest Processes.user Processes.parent_process Processes.process_name Processes.process Processes.process_id Processes.parent_process_id
Unit Tests
Test Case 1
Configurations: Using Splunk Attack Range
Replay the detection dataset using the Splunk attack range with the commands below
python attack_range.py replay -dn data_dump [--dump NAME_OF_DUMP]
Test Case 2
Configurations: Using Invoke-AtomicRedTeam
execute the atomic test T1140 against a Windows target.
Invoke-AtomicTest T1140