gabrielmoreira Detecting Command Injection Patterns
Overview
Command injection (CWE-78, OWASP A03:2021) shows up wherever an
application shells out to a binary. Image conversion (convert),
archive extraction (tar, unzip), video processing (ffmpeg),
DNS lookup (dig), and "we just need to call this CLI tool once"
are the common origins.
The vulnerability shape is universal: a string is built including
user input, then handed to a shell interpreter. The shell parses
the string with normal shell semantics — including ;, |, &,
$(), backticks. Any of those in the user-controlled portion
becomes shell-executable.
When the skill produces findings
| Finding | Severity | Threshold | Affected control |
|---|---|---|---|
Python subprocess.run(..., shell=True) with interpolation |
CRITICAL | f-string / concat / format argument with shell=True |
CWE-78 |
Python os.system(...) with interpolation |
CRITICAL | non-literal argument | CWE-78 |
Python os.popen(...) with interpolation |
CRITICAL | non-literal argument | CWE-78 |
Node child_process.exec(...) with template literal |
CRITICAL | ${...} in the command string |
CWE-78 |
Node child_process.execSync(...) with template |
CRITICAL | same | CWE-78 |
| Ruby backticks with interpolation | CRITICAL | `cmd #{var}` |
CWE-78 |
Ruby Kernel#system(string) with interpolation |
CRITICAL | system("cmd #{var}") |
CWE-78 |
Go exec.Command("sh", "-c", ...) with interpolation |
HIGH | shell wrapper with var | CWE-78 |
PHP system / exec / passthru / shell_exec with $-interp |
CRITICAL | system("cmd $var") |
CWE-78 |
Java Runtime.exec(String) with concat |
HIGH | single-string form (vs array) with var | CWE-78 |
Prerequisites
- Python 3.9+
- Target source tree on local filesystem
Instructions
Step 1 — Run the scanner
python3 ${CLAUDE_PLUGIN_ROOT}/skills/detecting-command-injection-patterns/scripts/scan_cmdi.py /path/to/repo
Options:
Usage: scan_cmdi.py PATH [OPTIONS]
Options:
--output FILE Write findings to FILE
--format FMT json | jsonl | markdown (default: markdown)
--min-severity SEV (default: info)
--include-tests Include test directories (default: excluded)
--languages LIST Comma-separated subset to scan
Step 2 — Interpret findings
CRITICAL = direct user-input → shell construction. Fix immediately.
HIGH = pattern where the shell layer exists but user-input reachability needs verification.
Step 3 — Remediation
The universal fix: pass arguments as a list (array), not a single string. Most APIs have a list form that bypasses shell entirely.
See references/PLAYBOOK.md for per-language patterns.
Examples
Example 1 — Pre-commit on a media-processing service
python3 ${CLAUDE_PLUGIN_ROOT}/skills/detecting-command-injection-patterns/scripts/scan_cmdi.py \
--min-severity high $(git diff --name-only main...HEAD | tr '\n' ' ')
Example 2 — CI gate
- name: Command-injection scan
run: |
python3 plugins/security/penetration-tester/skills/detecting-command-injection-patterns/scripts/scan_cmdi.py \
. --min-severity high
Output
JSON / JSONL / Markdown. Exit codes: 0 clean, 1 high/critical, 2 error.
Error Handling
False positives common in build scripts that interpolate fixed build constants. Verify each finding by reading whether the interpolated value is user-reachable.
Resources
references/THEORY.md— Why shell=True is the default footgun, per-language shell-out idioms, argument-vector vs command-string semanticsreferences/PLAYBOOK.md— Per-language safe-shellout patterns (Python subprocess list-args, Node spawn, Ruby Open3.capture3, Go exec.Command list-args, Java ProcessBuilder, PHP escapeshellarg)