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.
- Type: TTP
-
Product: Splunk Behavioral Analytics
- Last Updated: 2022-02-16
- Author: Michael Haag, Splunk
- ID: b06983f4-8f72-11ec-ab50-acde48001122
Annotations
Kill Chain Phase
- Exploitation
NIST
- DE.CM
CIS20
- CIS 10
CVE
Search
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$main = from source | eval timestamp = time | eval metadata_uid = metadata.uid | eval process_pid = process.pid | eval process_file = process.file | eval process_file_path = process_file.path | eval process_file_name = lower(process_file.name) | eval process_cmd_line = process.cmd_line | eval actor_user = actor.user | eval actor_user_name = actor_user.name | eval actor_process = actor.process | eval actor_process_pid = actor_process.pid | eval actor_process_file = actor_process.file | eval actor_process_file_path = actor_process_file.path | eval actor_process_file_name = actor_process_file.name | eval device_hostname = device.hostname | where process_cmd_line LIKE "%decode%" AND process_file_name="certutil.exe" --finding_report--
Macros
The SPL above uses the following Macros:
windows_certutil_decode_file_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.
windows_certutil_decode_file_filter is a empty macro by default. It allows the user to filter out any results (false positives) without editing the SPL.Required fields
List of fields required to use this analytic.
- process.pid
- process.file.path
- process.file.name
- process.cmd_line
- actor.user.name
- actor.process.pid
- actor.process.file.path
- actor.process.file.name
- device.hostname
How To Implement
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
RBA
| Risk Score | Impact | Confidence | Message |
|---|---|---|---|
| 40.0 | 50 | 80 | 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. |
Reference
- https://attack.mitre.org/techniques/T1140/
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1140/T1140.md
- https://docs.microsoft.com/en-us/windows-server/administration/windows-commands/certutil
- https://www.bleepingcomputer.com/news/security/certutilexe-could-allow-attackers-to-download-malware-while-bypassing-av/
Test Dataset
Replay any dataset to Splunk Enterprise by using our replay.py tool or the UI.Alternatively you can replay a dataset into a Splunk Attack Range
source | version: 4
