Detect Unix Shell in Google Chronicle
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/
YARA-L Detection Query
rule unix_shell_suspicious_execution {
meta:
author = "Argus Detection Engineering"
description = "Detects suspicious Unix shell commands indicative of reverse shells, piped remote code execution, base64 payload delivery, SUID manipulation, and privilege escalation (MITRE ATT&CK T1059.004)"
mitre_attack_technique = "T1059.004"
mitre_attack_tactic = "Execution"
severity = "HIGH"
confidence = "HIGH"
version = "1.0"
events:
$e.metadata.event_type = "PROCESS_LAUNCH"
$e.target.process.file.full_path = /\/(bash|sh|zsh|dash|ash|ksh|csh|tcsh|busybox)$/
(
$e.target.process.command_line = /\/dev\/(tcp|udp)\/[0-9]+\.[0-9]+/ or
$e.target.process.command_line = /bash\s+-i\s+>&\s+\/dev\/tcp/ or
$e.target.process.command_line = /nc(at)?\s+-e\s+\/bin\// or
$e.target.process.command_line = /socat\s+exec:/ or
$e.target.process.command_line = /curl.+\|\s*(ba)?sh/ or
$e.target.process.command_line = /wget.+\|\s*(ba)?sh/ or
$e.target.process.command_line = /base64\s+(--decode|-d)/ or
$e.target.process.command_line = /mkfifo\s+\/tmp\// or
$e.target.process.command_line = /mknod\s+\/tmp\// or
$e.target.process.command_line = /chmod\s+(\+s|4755|u\+s)/ or
$e.target.process.command_line = /useradd|usermod\s+-aG/ or
$e.target.process.command_line = /iptables\s+(-F|-P\s+ACCEPT)/ or
$e.target.process.command_line = /python3?\s+-c\s+['"]import socket/ or
$e.target.process.command_line = /perl\s+-e\s+['"]use Socket/
)
condition:
$e
}Chronicle YARA-L 2.0 rule detecting suspicious Unix shell command execution patterns matching T1059.004. Triggers on process launch events where bash, sh, zsh, dash, ash, ksh, csh, tcsh, or busybox is the executable and the command line contains indicators of reverse shell activity, piped remote code execution, base64 payload staging, named pipe creation, SUID bit manipulation, unauthorized user management, or firewall rule deletion.
Data Sources
Required Tables
False Positives & Tuning
- Automated deployment pipelines using curl | bash to pull and execute installer scripts for legitimate software such as Docker, Node.js, or Python virtual environments
- Security testing tools like Metasploit or custom red team frameworks executing staged payloads during authorized penetration tests on systems in the environment
- System administrators using netcat for legitimate network diagnostics or data transfer between trusted hosts on internal networks
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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