Security Scan

Heuristic scan of skill content for prompt-injection and malicious patterns.

Overview

A skill is text your agent trusts: its SKILL.md is read as instructions and its scripts run on your machine. Public research (Snyk's ToxicSkills, 2026) found prompt injection in roughly a third of tested community skills. This scan flags the known bad shapes across every installed skill, in every scope (user, project, codex, plugin):

• Injection phrases — "ignore all previous instructions", "do not tell the user"
• Hidden instructions — imperative text in HTML comments (invisible when rendered), zero-width/bidi unicode between plain characters
• Payload smuggling — large decodable base64 blobs in markdown
• Dangerous scripts — network piped into a shell (curl … | bash), decode-and-execute, credential-store access (~/.ssh, ~/.aws, keychain), URL shorteners, plain-HTTP calls, uploads of variable data

Findings are heuristics, not proof: a RISK verdict means "read this before trusting it". Legit tools trip these rules too (e.g. an installer that pipes curl into bash) — the point is that YOU see it and decide.

Prerequisites

• Claude Code with the skills-janitor plugin installed (provides scripts/security.sh)
• bash 3.2+ (the stock macOS bash works; no external dependencies, no network access)

Instructions

Step 1: Run the scan

bash ~/.claude/skills/skills-janitor/scripts/security.sh           # all installed skills
bash ~/.claude/skills/skills-janitor/scripts/security.sh --json    # machine-readable
bash ~/.claude/skills/skills-janitor/scripts/security.sh --path ~/some/skill-dir   # one directory

Step 2: Present verdicts honestly

Per-skill verdict: RISK (any HIGH finding), REVIEW (any MEDIUM), PASS. For each flagged skill show the finding titles, the file, and the evidence snippet. Do NOT call a finding "malware" — describe what the pattern does and let the user judge intent (e.g. "media-use pipes a HeyGen installer from the network into bash — a common install pattern, but verify the URL before trusting it").

Step 3: Recommend next steps

• RISK on a skill the user doesn't recognize or need → /janitor-swipe or delete it outright
• RISK on a known/trusted tool → read the flagged file once, then move on
• Before installing something new → /janitor-discover <url> runs this same scan pre-install

Output

Summary line (Scanned: N | RISK: x | REVIEW: y | PASS: z) followed by flagged skills, each with severity-tagged findings, the file, and an evidence snippet. --json emits the full structured report.

Error Handling

1. Error: security.sh: No such file or directory

Solution: The plugin is installed under a different root — locate it with ls ~/.claude/skills or check the plugin cache.

1. Error: Everything shows PASS but the user expected a finding

Solution: The scan covers markdown and script files up to 1MB, 200 files per skill; binaries and huge files are skipped. Check the specific file manually.

1. Error: A trusted skill shows RISK

Solution: Expected for tools that legitimately use flagged patterns (installers, credential helpers). Read the evidence line — if it matches the tool's documented purpose, note it and move on. Verdicts are advisory; nothing is deleted.

Examples

Example 1: Full audit

Input: "Are my skills safe? Check for prompt injection."

Output: Run the scan, lead with the summary ("178 scanned, 2 RISK, 0 REVIEW"), then explain each flagged skill in plain language with its evidence, and close with a recommendation per skill.

Example 2: One suspicious folder

Input: "Scan ~/Downloads/cool-skill before I install it."

Output: Run with --path ~/Downloads/cool-skill and present the verdict; suggest /janitor-discover for the overlap check too.

Resources

• Scan engine (plugin-relative): {baseDir}/../../scripts/security.sh
• /janitor-discover <url> — pre-install check (overlap + this security scan on the fetched SKILL.md)
• /janitor-report — general health check (errors, duplicates, broken skills)
• /janitor-swipe — delete what you don't trust