ShellCheck Security Scan Skill

ShellCheck is a static analysis tool that identifies bugs, security vulnerabilities, and stylistic issues in shell scripts. It supports bash, sh, dash, and ksh scripts.

When to Use This Skill

Invoke this skill when:

•Analyzing shell scripts (.sh, .bash) for security vulnerabilities
•Checking for command injection risks in shell code
•Detecting unquoted variable expansions that could lead to code execution
•Reviewing installation scripts, CI/CD pipelines, or build scripts
•Scanning for obfuscated or suspicious shell patterns
•Auditing Dockerfiles' RUN commands containing shell code
•Checking shell scripts in package managers (npm scripts, Makefiles)
•Triaging potentially malicious or compromised scripts
•Incident response requiring rapid shell script analysis

Malicious Code Detection Priority

When scanning for potentially malicious or compromised shell scripts, use this prioritized approach:

Critical Checks (Always Enable)

Code	Pattern	Malicious Intent
SC2091	`$(curl http://...)`	Remote code execution, payload download
SC2086	`rm -rf $VAR`	Command injection, arbitrary file deletion
SC2046	`cmd $(user_input)`	Subshell injection
SC2115	`rm -rf "$DIR/"*`	Root filesystem wipe when var is empty

High-Risk Patterns (Suspicious Behavior)

Code	Pattern	Indicator
SC2211	`*.sh` as command	Arbitrary script execution
SC2216	`find \| xargs rm`	Mass file deletion attack
SC2029	SSH command injection	Remote command confusion
SC2087	Unquoted heredoc	Credential/data injection

Indicators of Malicious Intent (Manual Review)

Beyond ShellCheck findings, flag scripts containing:

•Reverse shells: bash -i, /dev/tcp/, nc -e, mkfifo
•Data exfiltration: curl -d "$(cat /etc/passwd)", encoded POST data
•Obfuscation: base64 -d | bash, eval "$(printf '\x...')", xxd -r
•Persistence: crontab manipulation, /etc/init.d/, systemd service creation
•Privilege escalation: SUID manipulation, sudo config changes
•Defense evasion: history -c, unset HISTFILE, log deletion

Prerequisites

Installation

macOS (Homebrew):

bash

brew install shellcheck

Ubuntu/Debian:

bash

apt-get install shellcheck

Fedora/RHEL:

bash

dnf install ShellCheck

Using Cabal (Haskell):

bash

cabal update
cabal install ShellCheck

Docker:

bash

docker pull koalaman/shellcheck

Verify Installation

bash

shellcheck --version

Core Commands

Basic Security Scan

bash

# Scan a single script
shellcheck script.sh

# Scan with severity threshold (error, warning, info, style)
shellcheck --severity=warning script.sh

# Scan multiple files
shellcheck *.sh

# Scan recursively
find . -name "*.sh" -exec shellcheck {} +

Output Formats

bash

# JSON output for parsing
shellcheck --format=json script.sh

# SARIF output for security tools
shellcheck --format=sarif script.sh

# GCC-compatible format
shellcheck --format=gcc script.sh

# Checkstyle XML format
shellcheck --format=checkstyle script.sh

# Quiet mode (only show errors)
shellcheck --format=quiet script.sh

Shell Dialect Selection

bash

# Specify shell dialect
shellcheck --shell=bash script.sh
shellcheck --shell=sh script.sh
shellcheck --shell=dash script.sh
shellcheck --shell=ksh script.sh

Advanced Options

bash

# Enable all optional checks
shellcheck --enable=all script.sh

# Exclude specific checks
shellcheck --exclude=SC2086,SC2046 script.sh

# Check scripts sourced from stdin
cat script.sh | shellcheck -

# Follow source statements
shellcheck --source-path=SCRIPTDIR script.sh

Deep Security Scan for Malicious Code

Comprehensive Single-File Analysis

bash

# Maximum security scan with all checks, filter critical issues
shellcheck --enable=all --severity=style --format=json script.sh | \
  jq '[.[] | select(.code | tostring | test("^20(86|46|91|29|87|68|34|64|89|90)|2115|2116|2211|2216"))]'

Prioritized Critical Check Scan

bash

# Focus on high-risk injection patterns only
shellcheck --enable=all script.sh 2>&1 | \
  grep -E "SC20(86|46|91)|SC2115|SC2211|SC2216"

Project-Wide Malicious Code Hunt

bash

# Scan all shell scripts with security focus, exclude vendor
find . -type f \( -name "*.sh" -o -name "*.bash" \) \
  ! -path "./vendor/*" ! -path "./node_modules/*" \
  -exec shellcheck --enable=all --severity=warning --format=gcc {} + 2>&1 | \
  sort -t: -k4 | uniq -c | sort -rn

CI/CD Pipeline Script Audit

bash

# Extract and scan shell code from GitHub Actions workflows
find .github -name "*.yml" -exec grep -l "run:" {} + | \
  xargs -I{} sh -c 'grep -A5 "run:" "{}" | grep -v "^--$"' | \
  shellcheck --shell=bash -

Detecting Obfuscated Malicious Payloads

ShellCheck's static analysis may miss sophisticated obfuscation. Combine with pattern detection:

Base64 Payloads

bash

# Find base64 decode + execute patterns
grep -rn --include="*.sh" -E "(base64\s+(-d|--decode)|decode.*base64).*\|\s*(ba)?sh" .

Hex/Octal Encoding

bash

# Find printf-based obfuscation
grep -rn --include="*.sh" -E "printf.*\\\\x[0-9a-fA-F]{2}" .
grep -rn --include="*.sh" -E '\$\(printf.*\\[0-7]{3}' .

Reverse Shell Signatures

bash

# Common reverse shell patterns
grep -rn --include="*.sh" -E "(bash\s+-i|/dev/tcp/|nc\s+(-e|-c)|mkfifo|0<&[0-9])" .

Eval-Based Execution

bash

# Dangerous eval patterns
grep -rn --include="*.sh" -E "eval\s+['\"]?\\\$\(" .

These patterns should trigger manual review even if ShellCheck passes.

Security-Relevant Checks

Code	Severity	Description	Security Impact
SC2086	Warning	Double quote to prevent globbing and word splitting	Command injection, arbitrary file access
SC2046	Warning	Quote to prevent word splitting on command substitution	Command injection
SC2091	Warning	Remove surrounding $() to avoid executing output	Arbitrary code execution
SC2155	Warning	Declare and assign separately to avoid masking return values	Logic bypass
SC2012	Warning	Use find instead of ls to better handle non-alphanumeric filenames	Path traversal
SC2029	Warning	Commands run on client, not server	Unintended local execution
SC2034	Warning	Variable appears unused	Dead code, possible data leak
SC2064	Warning	Use single quotes for trap commands	Unexpected expansion timing
SC2068	Warning	Double quote array expansions	Argument injection
SC2087	Warning	Quote heredoc to prevent variable expansion	Data injection
SC2089	Warning	Quotes/backslashes in variables don't work	Escape bypass
SC2090	Warning	Quotes/backslashes will be treated literally	Command injection
SC2116	Warning	Useless echo? Instead of `echo $(cmd)`, use `cmd`	Unnecessary subshell
SC2129	Style	Consider using redirections instead of pipes	Efficiency
SC2145	Warning	Argument mixes string and array	Unexpected behavior
SC2148	Warning	Tips depend on target shell	Wrong shell execution
SC2154	Warning	Variable is referenced but not assigned	Undefined behavior
SC2162	Warning	read without -r mangles backslashes	Input manipulation
SC2174	Warning	mkdir -m only applies to the deepest directory	Permission bypass
SC2206	Warning	Quote to prevent word splitting	Array injection
SC2211	Warning	Glob used as command name	Arbitrary command execution
SC2215	Warning	Flag appears after filename	Argument confusion
SC2216	Warning	Piping to rm is unsafe	Arbitrary file deletion
SC2220	Warning	Invalid flags	Unexpected behavior
SC2222	Warning	Remove invalid flags	Unexpected behavior
SC2223	Warning	Quote to prevent empty command	Logic errors
SC2224	Warning	Numeric comparison on non-number	Logic bypass
SC2225	Warning	Source outside of subroutine	Scope leakage
SC2226	Warning	Use -exec or -exec + instead of -execdir	Path injection

MITRE ATT&CK Mappings

Technique ID	Technique Name	Relevant Checks
T1059.004	Command and Scripting Interpreter: Unix Shell	SC2086, SC2046, SC2091
T1027	Obfuscated Files or Information	SC2089, SC2090
T1105	Ingress Tool Transfer	SC2029 (curl/wget patterns)
T1222.002	File and Directory Permissions Modification	SC2174
T1070.004	Indicator Removal: File Deletion	SC2216
T1552.001	Unsecured Credentials: Credentials in Files	SC2034 (unused sensitive vars)

Security Triage Workflow

Step 1: Quick Risk Assessment

bash

# Count security-relevant findings by severity
shellcheck --enable=all --format=json script.sh | \
  jq -r '.[] | "\(.level): \(.code)"' | sort | uniq -c | sort -rn

Step 2: Critical Issue Identification

bash

# Extract only critical security issues
shellcheck --enable=all --format=json script.sh | \
  jq -r '.[] | select(.code == 2086 or .code == 2046 or .code == 2091 or .code == 2115) | 
    "[\(.level)] Line \(.line): \(.message)"'

Step 3: Context Analysis

For each critical finding, assess exploitability:

•SC2086 in rm/curl/wget: Likely exploitable → HIGH PRIORITY
•SC2086 in echo/printf: Lower risk → MEDIUM PRIORITY
•SC2091: Always critical → IMMEDIATE REVIEW
•SC2115: Empty variable + rm = catastrophic → CRITICAL

Step 4: Malicious Intent Assessment

bash

# Look for network activity + execution patterns together
grep -E "(curl|wget|nc).*\|.*(ba)?sh" script.sh && echo "⚠️  SUSPICIOUS: Download-and-execute pattern"

# Check for data exfiltration patterns
grep -E "curl.*-d.*\$|wget.*--post-data" script.sh && echo "⚠️  SUSPICIOUS: Data exfiltration pattern"

# Check for persistence mechanisms
grep -E "crontab|/etc/cron|systemctl.*enable|update-rc.d" script.sh && echo "⚠️  CHECK: Persistence mechanism"

Step 5: Generate Security Report

bash

# Create SARIF report for security tools/SIEM integration
shellcheck --format=sarif --enable=all script.sh > shellcheck-security.sarif

# Or create a summary report
echo "=== ShellCheck Security Report ==="
echo "File: script.sh"
echo "Date: $(date -Iseconds)"
shellcheck --enable=all --format=gcc script.sh 2>&1 | \
  awk -F: '{print $4}' | sort | uniq -c | sort -rn

Interpreting Results

Output Format

code

In script.sh line 5:
rm -rf $DIR
       ^--^ SC2086: Double quote to prevent globbing and word splitting.

Severity Levels

•error: Definite bugs or syntax errors
•warning: Likely bugs or problematic patterns (security-relevant)
•info: Suggestions for improvement
•style: Stylistic issues

Security Triage Priority

•Critical: SC2091 (executing output), SC2086/SC2046 with user input
•High: Unquoted variables in rm, curl, eval contexts
•Medium: General quoting issues, permission problems
•Low: Style issues, minor suggestions

Examples and Patterns

See examples/malicious-patterns.md for detailed examples of:

•Command injection vulnerabilities with ShellCheck detection
•Obfuscation patterns and their indicators
•Before/after secure code patterns
•Risk level classification for common findings

Integration with Other Security Skills

For comprehensive shell script security analysis, combine ShellCheck with other tools:

Tool	Use Case	Command
graudit	Detect secrets, backdoors, reverse shells	`graudit -d exec -d secrets script.sh`
bandit	If script calls Python code	Scan embedded Python separately
grep patterns	Obfuscation detection	See patterns in "Detecting Obfuscated Malicious Payloads"

Recommended Scan Order for Untrusted Scripts

•ShellCheck (syntax + security) → This skill
•graudit -d exec (execution patterns) → Complements static analysis
•graudit -d secrets (credential detection) → If script handles secrets
•Manual review for findings + obfuscation patterns
•Sandbox execution if script must be tested

Configuration

Inline Directives

bash

# Disable check for next line
# shellcheck disable=SC2086
rm -rf $DIR

# Disable for entire file (place at top)
# shellcheck disable=SC2086,SC2046

# Enable optional checks
# shellcheck enable=require-variable-braces

Configuration File

Create .shellcheckrc in project root:

ini

# Enable all optional checks
enable=all

# Exclude specific checks
exclude=SC2034,SC2129

# Set default shell
shell=bash

# Set severity threshold
severity=warning

Limitations and Blind Spots

Limitation	Impact	Mitigation
No runtime analysis	Can't detect dynamic payload generation	Run in sandbox if suspicious
No taint tracking	Misses data flow from untrusted sources	Manual code review
Limited obfuscation detection	Base64/hex encoding bypasses analysis	Use grep patterns + graudit
No network inspection	Can't analyze downloaded payloads	Block/intercept network in sandbox
Shell-only analysis	Embedded Python/Perl/Ruby ignored	Use language-specific tools
Shebang dependency	May miss shell dialect issues	Explicitly specify `-s shell`
False positives	Some warnings may not apply	Context-aware triage

What ShellCheck CANNOT Detect

•Obfuscated reverse shells (echo "YmFzaCAtaS4uLg==" | base64 -d | bash)
•Legitimate-looking but malicious logic (e.g., conditional backdoors)
•Time bombs or environment-triggered payloads
•Steganographic or external payload retrieval
•Encrypted or compressed payloads
•Social engineering in comments or variable names

Quick Reference: Security Scan Commands

Goal	Command
Full security scan	`shellcheck --enable=all --severity=warning script.sh`
JSON for parsing	`shellcheck --enable=all --format=json script.sh`
Critical issues only	`shellcheck script.sh 2>&1 \| grep -E "SC20(86\|46\|91)"`
Scan all scripts	`find . -name "*.sh" -exec shellcheck {} +`
SARIF report	`shellcheck --format=sarif --enable=all script.sh`
Exclude false positives	`shellcheck -e SC2034,SC2129 script.sh`
Specify shell dialect	`shellcheck -s bash script.sh`
Scan from stdin	`cat script.sh \| shellcheck -`

Additional Integration Examples

bash

# Combine with file discovery (exclude vendor directories)
find . -type f \( -name "*.sh" -o -name "*.bash" \) \
  ! -path "*/vendor/*" ! -path "*/node_modules/*" \
  -exec shellcheck {} +

# Check scripts in Dockerfiles
grep -h "RUN" Dockerfile* | sed 's/RUN //' | shellcheck -s bash -

# Pre-commit hook (error severity only for blocking)
shellcheck --severity=error $(git diff --cached --name-only --diff-filter=ACM | grep '\.sh$')

# Batch scan with parallel processing
find . -name "*.sh" -print0 | xargs -0 -P4 -I{} shellcheck --format=gcc {}

# Generate HTML report (requires pandoc)
shellcheck --format=json script.sh | \
  jq -r '.[] | "## Line \(.line)\n**\(.code)**: \(.message)\n"' | \
  pandoc -f markdown -t html > report.html