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https://github.com/splunk/security_content
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Eric
92 lines
4.8 KiB
YAML
92 lines
4.8 KiB
YAML
name: Linux Decode Base64 to Shell
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id: 637b603e-1799-40fd-bf87-47ecbd551b66
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version: 9
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date: '2025-05-02'
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author: Michael Haag, Splunk
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status: production
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type: TTP
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description: The following analytic detects the behavior of decoding base64-encoded
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data and passing it to a Linux shell. Additionally, it mitigates the potential damage
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and protects the organization's systems and data.The detection is made by searching
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for specific commands in the Splunk query, namely "base64 -d" and "base64 --decode",
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within the Endpoint.Processes data model. The analytic also includes a filter for
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Linux shells. The detection is important because it indicates the presence of malicious
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activity since Base64 encoding is commonly used to obfuscate malicious commands
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or payloads, and decoding it can be a step in running those commands. It suggests
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that an attacker is attempting to run malicious commands on a Linux system to gain
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unauthorized access, for data exfiltration, or perform other malicious actions.
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data_source:
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- Sysmon for Linux EventID 1
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search: '| tstats `security_content_summariesonly` count min(_time) as firstTime max(_time)
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as lastTime from datamodel=Endpoint.Processes where Processes.process="*|*" `linux_shells`
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by Processes.action Processes.dest Processes.original_file_name Processes.parent_process
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Processes.parent_process_exec Processes.parent_process_guid Processes.parent_process_id
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Processes.parent_process_name Processes.parent_process_path Processes.process Processes.process_exec
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Processes.process_guid Processes.process_hash Processes.process_id Processes.process_integrity_level
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Processes.process_name Processes.process_path Processes.user Processes.user_id Processes.vendor_product
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| `drop_dm_object_name(Processes)` | rex field=process "base64\s+(?<decode_flag>-{1,2}d\w*)"
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| where isnotnull(decode_flag) | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`
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| `linux_decode_base64_to_shell_filter`'
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how_to_implement: The detection is based on data that originates from Endpoint Detection
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and Response (EDR) agents. These agents are designed to provide security-related
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telemetry from the endpoints where the agent is installed. To implement this search,
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you must ingest logs that contain the process GUID, process name, and parent process.
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Additionally, you must ingest complete command-line executions. These logs must
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be processed using the appropriate Splunk Technology Add-ons that are specific to
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the EDR product. The logs must also be mapped to the `Processes` node of the `Endpoint`
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data model. Use the Splunk Common Information Model (CIM) to normalize the field
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names and speed up the data modeling process.
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known_false_positives: False positives may be present based on legitimate software
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being utilized. Filter as needed.
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references:
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- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1027/T1027.md#atomic-test-1---decode-base64-data-into-script
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- https://redcanary.com/blog/lateral-movement-with-secure-shell/
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- https://linux.die.net/man/1/base64
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drilldown_searches:
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- name: View the detection results for - "$user$" and "$dest$"
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search: '%original_detection_search% | search user = "$user$" dest = "$dest$"'
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earliest_offset: $info_min_time$
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latest_offset: $info_max_time$
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- name: View risk events for the last 7 days for - "$user$" and "$dest$"
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search: '| from datamodel Risk.All_Risk | search normalized_risk_object IN ("$user$",
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"$dest$") starthoursago=168 | stats count min(_time) as firstTime max(_time)
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as lastTime values(search_name) as "Search Name" values(risk_message) as "Risk
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Message" values(analyticstories) as "Analytic Stories" values(annotations._all)
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as "Annotations" values(annotations.mitre_attack.mitre_tactic) as "ATT&CK Tactics"
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by normalized_risk_object | `security_content_ctime(firstTime)` | `security_content_ctime(lastTime)`'
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earliest_offset: $info_min_time$
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latest_offset: $info_max_time$
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rba:
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message: An instance of $parent_process_name$ spawning $process_name$ was identified
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on endpoint $dest$ by user $user$ decoding base64 and passing it to a shell.
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risk_objects:
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- field: user
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type: user
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score: 25
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- field: dest
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type: system
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score: 25
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threat_objects:
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- field: parent_process_name
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type: parent_process_name
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- field: process_name
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type: process_name
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tags:
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analytic_story:
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- Linux Living Off The Land
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asset_type: Endpoint
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mitre_attack_id:
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- T1027
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- T1059.004
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product:
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- Splunk Enterprise
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- Splunk Enterprise Security
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- Splunk Cloud
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security_domain: endpoint
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tests:
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- name: True Positive Test
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attack_data:
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- data: https://media.githubusercontent.com/media/splunk/attack_data/master/datasets/attack_techniques/T1027/atomic_red_team/linux-sysmon.log
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source: Syslog:Linux-Sysmon/Operational
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sourcetype: sysmon:linux
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