Command Injection Detection

When to Use

Audit any package that wraps CLI tools, runs build commands, processes files via external programs, or interfaces with git/ffmpeg/imagemagick/pandoc/etc.

CVSS is typically CRITICAL 9.8 for confirmed RCE.

Key Distinctions

Command Injection vs Argument Injection

• Command injection: Attacker breaks out of the intended command entirely (; rm -rf /)
• Argument injection: Attacker adds flags to the intended command (--upload-pack=malicious)
• Both are reportable. Command injection is CRITICAL, argument injection is HIGH.

Shell vs No-Shell Execution

• Shell execution (exec, system, os.popen): Command string passed to shell interpreter. Metacharacters (;, |, &&, backticks, $()) are interpreted. DANGEROUS.
• Direct execution (execFile, spawn without shell, subprocess with list args): Arguments passed directly to the program. No shell interpretation. SAFER but argument injection may still work.

Process

Step 1: Find Shell Execution Sinks

# JavaScript/TypeScript — look for child_process usage
grep -rn "child_process" .
grep -rn "\.exec\('" .
grep -rn "\.execSync\(" .
grep -rn "spawn.*shell.*true" .
grep -rn "shelljs" .

# Python
grep -rn "os\.system\|os\.popen" .
grep -rn "subprocess.*shell.*True" .
grep -rn "commands\.getoutput\|commands\.getstatusoutput" .

# Go
grep -rn 'exec\.Command.*"bash"\|exec\.Command.*"sh"' .

# Ruby
grep -rn "system(\|%x{" . --include="*.rb"
grep -rn "IO\.popen\|Open3" .

# PHP
grep -rn "system(\|passthru(\|shell_exec(\|popen(" .
grep -rn "proc_open\|pcntl_exec" .

Step 2: Trace User Input to Sink

For each sink:

1. What command string is constructed?
2. Is any part from user input (HTTP params, filenames, config values)?
3. Is the user input interpolated into a shell string or passed as an argument array?

Step 3: Check Sanitization

grep -rn "escapeshellarg\|escapeshellcmd\|shlex\.quote\|shellescape" .
grep -rn "sanitize\|escape\|clean\|validate" .

Verify sanitization is:

• Applied to ALL user-controlled parts (not just some)
• Using the right function (escapeshellarg vs escapeshellcmd)
• Not bypassable (blocklists are almost always bypassable)

Step 4: Check for Argument Injection

Even with execFile/spawn (no shell), check for:

• --flag injection: user input starts with - or --
• Git-specific: --upload-pack, -c core.fsmonitor, --config
• Arguments that accept commands: --exec, --filter, --diff-filter
• Double-dash (--) separator missing before user-controlled args

Common Vulnerable Patterns

Pattern 1: String Interpolation in Shell Execution

// VULNERABLE — shell interprets metacharacters
const cp = require('child_process');
cp.exec(`convert ${inputFile} ${outputFile}`);
// Exploit: inputFile = "; id; #"

Pattern 2: Filename-Based Injection

cp.exec(`file "${filename}"`);
// Exploit: filename = '$(id).txt' or filename = '"; id; #"'

Pattern 3: Git Argument Injection

// Even without shell, git interprets dangerous flags
cp.execFile('git', ['clone', userUrl, '--config', 'core.fsmonitor=id']);

Pattern 4: Environment Variable Injection

cp.exec(command, { env: { ...process.env, USER_INPUT: untrusted } });
// If command references $USER_INPUT or uses env vars unsafely

Pattern 5: Newline Injection

cp.execFile('program', ['--option=' + userInput]);
// Exploit: userInput = "value\n--dangerous-flag"

Grep Patterns by Vulnerability Type

Direct Shell Injection

grep -rn "exec\(.*\+" .        # String concatenation in exec
grep -rn "exec\(.*\$\{" .      # Template literal in exec
grep -rn "exec\(.*%" .         # Format string in exec (Python)

Argument Injection

grep -rn "execFile\|spawn" .
# Then check if user input is in the args array without -- separator

CVSS Guidance

• Unauthenticated RCE: CRITICAL 9.8
• Authenticated RCE (low-priv): HIGH 8.8
• Argument injection (limited impact): HIGH 7.5-8.1
• Requires specific config/setup: HIGH 7.2 (AC:H)

References

• Sinks — Shell execution sinks by language
• False Positive Indicators — When this isn't exploitable
• PoC Skeleton — Command injection PoC template