Windows PowerShell Process Implementing Manual Base64 Decoder
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.
Query · spl
| tstats `security_content_summariesonly`
count min(_time) as firstTime
max(_time) as lastTime
from datamodel=Endpoint.Processes where
`process_powershell`
Processes.process = "*ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/*"
Processes.process = "*.Substring(*"
Processes.process = "*.GetString(*"
Processes.process = "*.IndexOf(*"
Processes.process IN ("*-shl*", *-shr*, "*-bxor*", "*-bor*", "*-band*")
by Processes.action Processes.dest Processes.original_file_name Processes.parent_process Processes.parent_process_exec
Processes.parent_process_guid Processes.parent_process_id Processes.parent_process_name Processes.parent_process_path
Processes.process Processes.process_exec Processes.process_guid Processes.process_hash Processes.process_id
Processes.process_integrity_level Processes.process_name Processes.process_path Processes.user Processes.user_id
Processes.vendor_product
| `drop_dm_object_name(Processes)`
| `security_content_ctime(firstTime)`
| `security_content_ctime(lastTime)`
| `windows_powershell_process_implementing_manual_base64_decoder_filter`
Implementation guide
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.
Analyst notes
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.