
False Positive Check
FreeSystematically verify security bugs to eliminate false positives.
Free · Opens the source repo
What False Positive Check does
The False Positive Check skill is designed for developers and security engineers who need to validate suspected security vulnerabilities in their code. This skill systematically evaluates claims of security bugs, providing a thorough analysis to determine whether a finding is a true positive or a false positive. It emphasizes a structured approach to verification, ensuring that each claim is meticulously documented and assessed based on its context and specifics.
Users initiate the skill when they encounter a suspected security bug and need to confirm its legitimacy. The skill guides users through a series of steps that involve restating the vulnerability claim, identifying the root cause, and understanding the execution context. By following a detailed verification path—either standard or deep—the skill ensures that no stone is left unturned in the analysis process. This is particularly useful for teams that prioritize security and need to maintain a high standard of code integrity.
The False Positive Check skill is not intended for discovering new vulnerabilities or conducting general code reviews. Instead, it focuses on verifying existing claims, making it an essential tool for security audits and assessments. It is particularly beneficial for organizations that handle sensitive data or operate in regulated industries where the accuracy of vulnerability claims is critical. By eliminating false positives, teams can allocate their resources more effectively and focus on genuine security threats.
In summary, this skill is a valuable addition for any development team that requires a reliable method for validating security vulnerabilities. It helps streamline the verification process, reduces the risk of overlooking critical issues, and ultimately contributes to a more secure codebase.
When to use it
Use this skill when you need to verify the legitimacy of a suspected security vulnerability or determine if a finding is exploitable.
When not to use it
Do not use this skill for discovering new bugs or conducting general code reviews; it is strictly for verification of existing claims.
What you can build with it
Validating a Suspected Vulnerability
When a developer suspects a security bug, they can use this skill to verify its legitimacy and determine if it poses a real threat.
Assessing Multiple Findings
In scenarios where a team has several suspected vulnerabilities, this skill can help triage and verify each finding systematically.
Compliance Audits
During security audits, organizations can utilize this skill to ensure that all reported vulnerabilities are accurately assessed and documented.
How to install False Positive Check
View source1. Install with the skills CLI
npx skills add trailofbits/skills/fp-check --agent claude-code2. Or install it manually
Download the skill folder and drop it into ~/.claude/skills/ for all projects, or .claude/skills/ to scope it to one repo. Restart Claude Code so it picks up the new skill.
Anthropic's agentic coding CLI, and the reference implementation of Agent Skills. Drop a skill folder into ~/.claude/skills and Claude Code loads it automatically whenever a task matches the skill's description. Claude Code docs
Inside SKILL.md
Written by trailofbitsFalse 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.
| Rationalization | Why It's Wrong | Required Action |
|---|---|---|
| "Rapid analysis of remaining bugs" | Every bug gets full verification | Return to task list, verify next bug through all phases |
| "This pattern looks dangerous, so it's a vulnerability" | Pattern recognition is not analysis | Complete data flow tracing before any conclusion |
| "Skipping full verification for efficiency" | No partial analysis allowed | Execute all steps per the chosen verification path |
| "The code looks unsafe, reporting without tracing data flow" | Unsafe-looking code may have upstream validation | Trace the complete path from source to sink |
| "Similar code was vulnerable elsewhere" | Each context has different validation, callers, and protections | Verify this specific instance independently |
| "This is clearly critical" | LLMs are biased toward seeing bugs and overrating severity | Complete 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()whencontent_lengthexceeds 4096") - What is the alleged root cause? (e.g., "missing bounds check before
memcpyat 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:
- Run Step 0 for all bugs first — restating each claim often collapses obvious false positives immediately
- Route each bug independently (some may be standard, others deep)
- Process all standard-routed bugs first, then deep-routed bugs
- 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:
- Counts: X TRUE POSITIVES, Y FALSE POSITIVES
- TRUE POSITIVE list: Each with brief vulnerability description
- 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
Frequently asked questions about False Positive Check
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