Name:Windows PowerShell Process Implementing Manual Base64 Decoder id:08d67349-0808-4f55-b431-1037269fa517 version:5 date:None author:Nasreddine Bencherchali status:production type:Anomaly Description:The following analytic identifies Windows PowerShell processes that implement a manual Base64 decoder.
Threat actors often use Base64 encoding to obfuscate malicious payloads or commands within PowerShell scripts.
By manually decoding Base64 strings, attackers can evade detection mechanisms that look for standard decoding functions like using the "-enc" flag or the "FromBase64String" function.
This detection focuses on PowerShell processes that exhibit characteristics of manual Base64 decoding, such as the presence of specific string manipulation methods and bitwise operations.
Security teams should investigate any instances of such activity, especially if found in conjunction with other suspicious behaviors or on systems that should not be using PowerShell for such tasks.
Data_source:
-Sysmon EventID 1
-Windows Event Log Security 4688
-CrowdStrike ProcessRollup2
search:| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time) as lastTime
how_to_implement:The detection is based on data that originates from Endpoint Detection
and Response (EDR) agents. These agents are designed to provide security-related
telemetry from the endpoints where the agent is installed. To implement this search,
you must ingest logs that contain the process GUID, process name, and parent process.
Additionally, you must ingest complete command-line executions. These logs must
be processed using the appropriate Splunk Technology Add-ons that are specific to
the EDR product. The logs must also be mapped to the `Processes` node of the `Endpoint`
data model. Use the Splunk Common Information Model (CIM) to normalize the field
names and speed up the data modeling process.
known_false_positives:Legitimate use of PowerShell for decoding Base64 strings by administrators
or power users may trigger this detection. It is essential to review the context
of the PowerShell execution to determine if it is malicious, suspicious or even authorized activity.
References: -https://www.virustotal.com/gui/file/4b3ab4d9f2332da6b6cd8d9d0f4910a5eb85ac8c969108acb3ad49631812f998/behavior drilldown_searches: name:'View the detection results for - "$dest$" and "$user$"' search:'%original_detection_search% | search dest = "$dest$" user = "$user$"' earliest_offset:'$info_min_time$' latest_offset:'$info_max_time$' name:'View risk events for the last 7 days for - "$dest$" and "$user$"' search:'| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$dest$", "$user$") | stats count min(_time) as firstTime max(_time) as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk Message" values(analyticstories) as "Analytic Stories" values(annotations._all) as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics" by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`' earliest_offset:'7d' latest_offset:'0' analytic_story:['Compromised Windows Host', 'Deobfuscate-Decode Files or Information']
