SKILL·23EAAE

fp-check

trailofbits
Updated 15 days ago
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About

The fp-check skill systematically verifies suspected security vulnerabilities to eliminate false positives, providing a clear TRUE POSITIVE or FALSE POSITIVE verdict with documented evidence. It's designed for validating specific findings when asked about exploitability or false positives, not for discovering new bugs. Key capabilities include using tools like Grep, LSP, and Bash to analyze code and produce evidence-backed security assessments.

Quick Install

Claude Code

Recommended
Primary
npx skills add trailofbits/skills -a claude-code
Plugin CommandAlternative
/plugin add https://github.com/trailofbits/skills
Git CloneAlternative
git clone https://github.com/trailofbits/skills.git ~/.claude/skills/fp-check

Copy and paste this command in Claude Code to install this skill

Documentation

False Positive Check

When to Use

  • "Is this bug real?" or "is this a true positive?"
  • "Is this a false positive?" or "verify this finding"
  • "Check if this vulnerability is exploitable"
  • Any request to verify or validate a specific suspected bug

When NOT to Use

  • Finding or hunting for bugs ("find bugs", "security analysis", "audit code")
  • General code review for style, performance, or maintainability
  • Feature development, refactoring, or non-security tasks
  • When the user explicitly asks for a quick scan without verification

Rationalizations to Reject

If you catch yourself thinking any of these, STOP.

RationalizationWhy It's WrongRequired Action
"Rapid analysis of remaining bugs"Every bug gets full verificationReturn to task list, verify next bug through all phases
"This pattern looks dangerous, so it's a vulnerability"Pattern recognition is not analysisComplete data flow tracing before any conclusion
"Skipping full verification for efficiency"No partial analysis allowedExecute all steps per the chosen verification path
"The code looks unsafe, reporting without tracing data flow"Unsafe-looking code may have upstream validationTrace the complete path from source to sink
"Similar code was vulnerable elsewhere"Each context has different validation, callers, and protectionsVerify this specific instance independently
"This is clearly critical"LLMs are biased toward seeing bugs and overrating severityComplete devil's advocate review; prove it with evidence

Step 0: Understand the Claim and Context

Before any analysis, restate the bug in your own words. If you cannot do this clearly, ask the user for clarification. Half of false positives collapse at this step — the claim doesn't make coherent sense when restated precisely.

Document:

  • What is the exact vulnerability claim? (e.g., "heap buffer overflow in parse_header() when content_length exceeds 4096")
  • What is the alleged root cause? (e.g., "missing bounds check before memcpy at line 142")
  • What is the supposed trigger? (e.g., "attacker sends HTTP request with oversized Content-Length header")
  • What is the claimed impact? (e.g., "remote code execution via controlled heap corruption")
  • What is the threat model? What privilege level does this code run at? Is it sandboxed? What can the attacker already do before triggering this bug? (e.g., "unauthenticated remote attacker vs privileged local user"; "runs inside Chrome renderer sandbox" vs "runs as root with no sandbox")
  • What is the bug class? Classify the bug and consult bug-class-verification.md for class-specific verification requirements that supplement the generic phases below.
  • Execution context: When and how is this code path reached during normal execution?
  • Caller analysis: What functions call this code and what input constraints do they impose?
  • Architectural context: Is this part of a larger security system with multiple protection layers?
  • Historical context: Any recent changes, known issues, or previous security reviews of this code area?

Route: Standard vs Deep Verification

After Step 0, choose a verification path.

Standard Verification

Use when ALL of these hold:

  • Clear, specific vulnerability claim (not vague or ambiguous)
  • Single component — no cross-component interaction in the bug path
  • Well-understood bug class (buffer overflow, SQL injection, XSS, integer overflow, etc.)
  • No concurrency or async involved in the trigger
  • Straightforward data flow from source to sink

Follow standard-verification.md. No task tracking — work through the linear checklist sequentially, documenting findings inline.

Deep Verification

Use when ANY of these hold:

  • Ambiguous claim that could be interpreted multiple ways
  • Cross-component bug path (data flows through 3+ modules or services)
  • Race conditions, TOCTOU, or concurrency in the trigger mechanism
  • Logic bugs without a clear spec to verify against
  • Standard verification was inconclusive or escalated
  • User explicitly requests full verification

Follow deep-verification.md. Track each phase as a task with explicit dependencies, and execute the phases using the plugin's analysis agents.

Default

Start with standard. Standard verification has two built-in escalation checkpoints that route to deep when complexity exceeds the linear checklist.

Batch Triage

When verifying multiple bugs at once:

  1. Run Step 0 for all bugs first — restating each claim often collapses obvious false positives immediately
  2. Route each bug independently (some may be standard, others deep)
  3. Process all standard-routed bugs first, then deep-routed bugs
  4. After all bugs are verified, check for exploit chains — findings that individually failed gate review may combine to form a viable attack

Final Summary

After processing ALL suspected bugs, provide:

  1. Counts: X TRUE POSITIVES, Y FALSE POSITIVES
  2. TRUE POSITIVE list: Each with brief vulnerability description
  3. FALSE POSITIVE list: Each with brief reason for rejection

References

  • Standard Verification — Linear single-pass checklist for straightforward bugs
  • Deep Verification — Full task-based orchestration for complex bugs
  • Gate Reviews — Six mandatory gates and verdict format
  • Bug-Class Verification — Class-specific verification requirements for memory corruption, logic bugs, race conditions, integer issues, crypto, injection, info disclosure, DoS, and deserialization
  • False Positive Patterns — 13-item checklist and red flags for common false positive patterns
  • Evidence Templates — Documentation templates for data flow, mathematical proofs, attacker control, and devil's advocate reviews

GitHub Repository

trailofbits/skills
Path: plugins/fp-check/skills/fp-check
0
agent-skills
FAQ

Frequently asked questions

What is the fp-check skill?

fp-check is a Claude Skill by trailofbits. Skills package instructions and resources that Claude loads on demand, so Claude can perform fp-check-related tasks without extra prompting.

How do I install fp-check?

Use the install commands on this page: add fp-check to Claude Code as a plugin, or clone its repository into your skills directory, then restart Claude so it picks up the skill.

What category does fp-check belong to?

fp-check is in the Documentation category.

Is fp-check free to use?

Yes. fp-check is listed on AIMCP and free to install.

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