skill-scanner – Agents & Automation skill for AI agents

Bundled upstream support

Supporting files are included at skill-relative paths. See [UPSTREAM-NOTICE.md](UPSTREAM-NOTICE.md) for their pinned sources, licenses, hashes, and import changes.

Skill Security Scanner

Scan agent skills for security issues before adoption. Detects prompt injection, malicious code, excessive permissions, secret exposure, and supply chain risks.

Resolve this installed skill directory explicitly. Run the bundled helper with an already reviewed Python environment containing PyYAML; no repository-root working directory or implicit dependency download is required.

When to Use

• You need to evaluate a skill for prompt injection, malicious code, over-broad permissions, or supply-chain risk before adopting it.
• You want a static scan plus manual review workflow for a skill directory.
• The task is to decide whether a skill is safe enough to trust in an agent environment.

Bundled Script

scripts/scan_skill.py

Static analysis scanner that detects deterministic patterns. Outputs structured JSON (skill-scanner.locators.v2). It reports fixed finding descriptions, local file/line locators, counts, recognized tools and metadata field presence/types. It omits raw metadata values, source excerpts, decoded payloads, commands, symlink targets and complete URLs. Inspect those privately at the reported location. Relative filenames, the selected directory identifier and URL hostnames remain visible identifiers; the report is not an anonymizer for sensitive names. Unsupported inputs remain incomplete, not safe.

python3 "<installed-skill-dir>/scripts/scan_skill.py" "<authorized-inert-skill-directory>"

Returns JSON with findings, URL host/line locators, structure info, and severity counts. The script catches patterns mechanically — your job is to evaluate intent and filter false positives.

Scanner Coverage and Report Privacy

Use a private inert snapshot: this helper is static inspection, not an execution sandbox. It refuses final symlinks/non-regular files and bounds a file to 1 MiB, a tree to 1,000 files / 16 MiB. Hosts without guarded no-follow file opens report incomplete rather than silently following links. Nested regular text files are included; binary/unreadable inputs remain explicit coverage gaps. A scan error or incomplete coverage exits nonzero, not a clean pass.

Source payloads are omitted from the report rather than relying on recognition of every secret format. Visible filenames and hostnames can still be confidential identifiers; inspect reports locally before sharing. No target script, lifecycle hook, test or external URL is executed. Domain familiarity and lexical overlap do not satisfy semantic review or merge authority.

Workflow

Phase 1: Input & Discovery

Determine the scan target:

• If the user provides a skill directory path, use it directly
• If the user names a skill, look for it under plugins/*/skills/<name>/ or .claude/skills/<name>/
• If the user says "scan all skills", discover all */SKILL.md files and scan each

Validate the target contains a SKILL.md file. List the skill structure:

ls -la <skill-directory>/
ls <skill-directory>/references/ 2>/dev/null
ls <skill-directory>/scripts/ 2>/dev/null

Phase 2: Automated Static Scan

Run the bundled scanner:

python3 "<installed-skill-dir>/scripts/scan_skill.py" "<authorized-inert-skill-directory>"

Parse the JSON output. The script produces findings with severity levels, URL analysis, and structure information. Use these as leads for deeper analysis.

Fallback: If the script fails, proceed with manual analysis using Grep patterns from the reference files.

Phase 3: Frontmatter Validation

Read the SKILL.md and check:

• Required fields: name and description must be present
• Name consistency: name field should match the directory name
• Tool assessment: Review allowed-tools — is Bash justified? Are tools unrestricted (*)?
• Model override: Is a specific model forced? Why?
• Description quality: Does the description accurately represent what the skill does?

Phase 4: Prompt Injection Analysis

Load the installed skill's references/prompt-injection-patterns.md for context.

Review scanner findings in the "Prompt Injection" category. For each finding:

1. Read the surrounding context in the file

2. Determine if the pattern is performing injection (malicious) or discussing/detecting injection (legitimate)

3. Skills about security, testing, or education commonly reference injection patterns — this is expected

Critical distinction: A security review skill that lists injection patterns in its references is documenting threats, not attacking. Only flag patterns that would execute against the agent running the skill.

Phase 5: Behavioral Analysis

This phase is agent-only — no pattern matching. Read the full SKILL.md instructions and evaluate:

Description vs. instructions alignment:

• Does the description match what the instructions actually tell the agent to do?
• A skill described as "code formatter" that instructs the agent to read ~/.ssh is misaligned

Config/memory poisoning:

• Instructions to modify CLAUDE.md, MEMORY.md, settings.json, .mcp.json, or hook configurations
• Instructions to add itself to allowlists or auto-approve permissions
• Writing to ~/.claude/ or any agent configuration directory

Scope creep:

• Instructions that exceed the skill's stated purpose
• Unnecessary data gathering (reading files unrelated to the skill's function)
• Instructions to install other skills, plugins, or dependencies not mentioned in the description

Information gathering:

• Reading environment variables beyond what's needed
• Listing directory contents outside the skill's scope
• Accessing git history, credentials, or user data unnecessarily

Phase 6: Script Analysis

If the skill has a scripts/ directory:

1. Load the installed skill's references/dangerous-code-patterns.md for context

2. Read each script file fully (do not skip any)

3. Check scanner findings in the "Malicious Code" category

4. For each finding, evaluate:

• Data exfiltration: Does the script send data to external URLs? What data?
• Reverse shells: Socket connections with redirected I/O
• Credential theft: Reading SSH keys, .env files, tokens from environment
• Dangerous execution: eval/exec with dynamic input, shell=True with interpolation
• Config modification: Writing to agent settings, shell configs, git hooks

5. Check PEP 723 dependencies — are they legitimate, well-known packages?

6. Verify the script's behavior matches the SKILL.md description of what it does

Legitimate patterns: gh CLI calls, git commands, reading project files, JSON output to stdout are normal for skill scripts.

Phase 7: Supply Chain Assessment

Review URL domains and file/line locators from the scanner output; inspect complete URLs locally in the source:

• Familiar domains: GitHub, PyPI and documentation sites are discovery context, not proof that a particular repository/package is trustworthy
• Untrusted domains: Unknown domains, personal sites, URL shorteners — flag for review
• Remote instruction loading: Any URL that fetches content to be executed or interpreted as instructions is high risk
• Dependency downloads: Scripts that download and execute binaries or code at runtime
• Unverifiable sources: References to packages or tools not on standard registries

Phase 8: Permission Analysis

Load the installed skill's references/permission-analysis.md for the tool risk matrix.

Evaluate:

• Least privilege: Are all granted tools actually used in the skill instructions?
• Tool justification: Does the skill body reference operations that require each tool?
• Risk level: Rate the overall permission profile using the tier system from the reference

Example assessments:

• Read Grep Glob — Low risk, read-only analysis skill
• Read Grep Glob Bash — Medium risk, needs Bash justification (e.g., running bundled scripts)
• Read Grep Glob Bash Write Edit WebFetch Task — High risk, near-full access

Confidence Levels

| Level | Criteria | Action |

|-------|----------|--------|

| HIGH | Pattern confirmed + malicious intent evident | Report with severity |

| MEDIUM | Suspicious pattern, intent unclear | Note as "Needs verification" |

| LOW | Theoretical, best practice only | Do not report |

False positive awareness is critical. The biggest risk is flagging legitimate security skills as malicious because they reference attack patterns. Always evaluate intent before reporting.

Output Format

## Skill Security Scan: [Skill Name]

### Summary
- **Findings**: X (Y Critical, Z High, ...)
- **Risk Level**: Critical / High / Medium / Low / No pattern matches
- **Skill Structure**: SKILL.md only / +references / +scripts / full

### Fin

… (full content is in the download package)