chore(agents): Add skill-scanner skill (#19608)

Uses `dotagents` to add the `skill-scanner` skill from
`getsentry/skills` for scanning agent skills for security issues such as
prompt injection, malicious scripts and supply chain risks.

Closes #19609 (added automatically)

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: chargome

.agents/skills/skill-scanner/SKILL.md

@@ -0,0 +1,208 @@
+---
+name: skill-scanner
+description: Scan agent skills for security issues. Use when asked to "scan a skill",
+  "audit a skill", "review skill security", "check skill for injection", "validate SKILL.md",
+  or assess whether an agent skill is safe to install. Checks for prompt injection,
+  malicious scripts, excessive permissions, secret exposure, and supply chain risks.
+allowed-tools: Read, Grep, Glob, Bash
+---
+
+# Skill Security Scanner
+
+Scan agent skills for security issues before adoption. Detects prompt injection, malicious code, excessive permissions, secret exposure, and supply chain risks.
+
+**Important**: Run all scripts from the repository root using the full path via `${CLAUDE_SKILL_ROOT}`.
+
+## Bundled Script
+
+### `scripts/scan_skill.py`
+
+Static analysis scanner that detects deterministic patterns. Outputs structured JSON.
+
+```bash
+uv run ${CLAUDE_SKILL_ROOT}/scripts/scan_skill.py <skill-directory>
+```
+
+Returns JSON with findings, URLs, structure info, and severity counts. The script catches patterns mechanically — your job is to evaluate intent and filter false positives.
+
+## 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:
+
+```bash
+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:
+
+```bash
+uv run ${CLAUDE_SKILL_ROOT}/scripts/scan_skill.py <skill-directory>
+```
+
+Parse the JSON output. The script produces findings with severity levels, URL analysis, and structure information. Use these as leads for deeper analysis.
+
+**Fallback**: If the script fails, proceed with manual analysis using Grep patterns from the reference files.
+
+### Phase 3: Frontmatter Validation
+
+Read the SKILL.md and check:
+
+- **Required fields**: `name` and `description` must be present
+- **Name consistency**: `name` field should match the directory name
+- **Tool assessment**: Review `allowed-tools` — is Bash justified? Are tools unrestricted (`*`)?
+- **Model override**: Is a specific model forced? Why?
+- **Description quality**: Does the description accurately represent what the skill does?
+
+### Phase 4: Prompt Injection Analysis
+
+Load `${CLAUDE_SKILL_ROOT}/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 `${CLAUDE_SKILL_ROOT}/references/dangerous-code-patterns.md` for context
+2. Read each script file fully (do not skip any)
+3. Check scanner findings in the "Malicious Code" category
+4. For each finding, evaluate:
+   - **Data exfiltration**: Does the script send data to external URLs? What data?
+   - **Reverse shells**: Socket connections with redirected I/O
+   - **Credential theft**: Reading SSH keys, .env files, tokens from environment
+   - **Dangerous execution**: eval/exec with dynamic input, shell=True with interpolation
+   - **Config modification**: Writing to agent settings, shell configs, git hooks
+5. Check PEP 723 `dependencies` — are they legitimate, well-known packages?
+6. Verify the script's behavior matches the SKILL.md description of what it does
+
+**Legitimate patterns**: `gh` CLI calls, `git` commands, reading project files, JSON output to stdout are normal for skill scripts.
+
+### Phase 7: Supply Chain Assessment
+
+Review URLs from the scanner output and any additional URLs found in scripts:
+
+- **Trusted domains**: GitHub, PyPI, official docs — normal
+- **Untrusted domains**: Unknown domains, personal sites, URL shorteners — flag for review
+- **Remote instruction loading**: Any URL that fetches content to be executed or interpreted as instructions is high risk
+- **Dependency downloads**: Scripts that download and execute binaries or code at runtime
+- **Unverifiable sources**: References to packages or tools not on standard registries
+
+### Phase 8: Permission Analysis
+
+Load `${CLAUDE_SKILL_ROOT}/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
+
+```markdown
+## Skill Security Scan: [Skill Name]
+
+### Summary
+
+- **Findings**: X (Y Critical, Z High, ...)
+- **Risk Level**: Critical / High / Medium / Low / Clean
+- **Skill Structure**: SKILL.md only / +references / +scripts / full
+
+### Findings
+
+#### [SKILL-SEC-001] [Finding Type] (Severity)
+
+- **Location**: `SKILL.md:42` or `scripts/tool.py:15`
+- **Confidence**: High
+- **Category**: Prompt Injection / Malicious Code / Excessive Permissions / Secret Exposure / Supply Chain / Validation
+- **Issue**: [What was found]
+- **Evidence**: [code snippet]
+- **Risk**: [What could happen]
+- **Remediation**: [How to fix]
+
+### Needs Verification
+
+[Medium-confidence items needing human review]
+
+### Assessment
+
+[Safe to install / Install with caution / Do not install]
+[Brief justification for the assessment]
+```
+
+**Risk level determination**:
+
+- **Critical**: Any high-confidence critical finding (prompt injection, credential theft, data exfiltration)
+- **High**: High-confidence high-severity findings or multiple medium findings
+- **Medium**: Medium-confidence findings or minor permission concerns
+- **Low**: Only best-practice suggestions
+- **Clean**: No findings after thorough analysis
+
+## Reference Files
+
+| File                                      | Purpose                                                                        |
+| ----------------------------------------- | ------------------------------------------------------------------------------ |
+| `references/prompt-injection-patterns.md` | Injection patterns, jailbreaks, obfuscation techniques, false positive guide   |
+| `references/dangerous-code-patterns.md`   | Script security patterns: exfiltration, shells, credential theft, eval/exec    |
+| `references/permission-analysis.md`       | Tool risk tiers, least privilege methodology, common skill permission profiles |