Detect Unix Shell in Splunk
Adversaries may abuse Unix shell commands and scripts for execution. Unix shells are the primary command prompt on Linux, macOS, and ESXi systems, though many variations exist (sh, ash, bash, zsh, etc.). Unix shells can control every aspect of a system, with certain commands requiring elevated privileges. Adversaries may abuse Unix shells to execute various commands or payloads, access interactive shells through C2 channels, leverage shell scripts for persistence, or use stripped-down shells via Busybox on embedded devices and ESXi servers.
MITRE ATT&CK
- Tactic
- Execution
- Technique
- T1059 Command and Scripting Interpreter
- Sub-technique
- T1059.004 Unix Shell
- Canonical reference
- https://attack.mitre.org/techniques/T1059/004/
SPL Detection Query
index=linux OR index=syslog sourcetype="syslog" OR sourcetype="linux:auditd"
("bash" OR "sh" OR "zsh" OR "dash" OR "/bin/bash" OR "/bin/sh")
| eval ReverseShell=if(match(_raw, "(/dev/tcp/|nc\s+-e\s+/bin/|ncat\s+-e|socat\s+exec:)"), 1, 0)
| eval CurlPipe=if(match(_raw, "(curl.*\|\s*(ba)?sh|wget.*\|\s*(ba)?sh)"), 1, 0)
| eval Base64Decode=if(match(_raw, "(base64\s+(-d|--decode))"), 1, 0)
| eval PrivEsc=if(match(_raw, "(chmod\s+\+s|chmod\s+4755|useradd|usermod\s+-aG)"), 1, 0)
| eval SuspicionScore=ReverseShell*3 + CurlPipe*2 + Base64Decode + PrivEsc*2
| where SuspicionScore > 0
| table _time, host, user, _raw, ReverseShell, CurlPipe, Base64Decode, PrivEsc, SuspicionScore
| sort - SuspicionScore, - _timeDetects suspicious Unix shell activity from syslog or auditd logs. Identifies reverse shells, curl/wget pipe-to-shell execution, base64 decoding, and privilege escalation commands. Uses weighted suspicion scoring — reverse shells score highest (3) due to confirmed compromise, followed by privilege escalation (2) and curl-pipe (2).
Data Sources
Required Sourcetypes
False Positives & Tuning
- DevOps tools and CI/CD pipelines that use curl|bash patterns for software installation
- System administrators running legitimate setup scripts that decode base64-encoded configuration
- Configuration management tools (Ansible, Chef, Puppet, SaltStack) executing shell commands remotely
Other platforms for T1059.004
Testing Methodology
Validate this detection against 3 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Bash Reverse Shell via /dev/tcp
Expected signal: Auditd: EXECVE record for bash with /dev/tcp in arguments. Syslog: bash process creation. Network connection attempt to 127.0.0.1:4444 (will fail without listener). MDE DeviceProcessEvents on managed Linux endpoints.
- Test 2Curl Pipe to Bash
Expected signal: Auditd: EXECVE records for curl and bash. Process tree shows curl piped to bash. Network connection attempt to 127.0.0.1:8080 (will fail without listener). The curl failure means no content reaches bash.
- Test 3Base64 Encoded Command Execution
Expected signal: Auditd: EXECVE records for echo, base64, and bash. The decoded content 'whoami' will be executed. Syslog captures the process chain.
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