fp-check
An agent skill by trailofbits, from trailofbits/skills. Tags: analysis, debugging, evaluation, security, validation.
What it does
Systematically verifies suspected security bugs to eliminate false positives, producing a TRUE POSITIVE or FALSE POSITIVE verdict with documented evidence for each. Use when asked whether a specific finding is real, exploitable, or a false positive, or to verify or validate a suspected vulnerability — not for hunting or discovering new bugs.
Install
With the skills CLI, which installs into Claude Code, Codex, Cursor and other agents:
npx skills add trailofbits/skills --skill fp-checkOr copy the skill folder into Claude Code's skills directory by hand (~/.claude/skills for every project, or .claude/skills inside one):
git clone --depth 1 https://github.com/trailofbits/skills
cp -r skills/plugins/fp-check/skills/fp-check ~/.claude/skills/fp-checkSafety box score
Grade A, minimal risk: 7 of 100 risk points, rated 2026-10-20. The grade rates what the skill and its scripts can reach on the machine of whoever installs it, across eight categories from shell execution to secrets access. How the score works.
This skill is a structured vulnerability-verification workflow for code repositories. Its main blast radius is limited to reading project files and, during verification, potentially running Bash-based checks and creating workspace artifacts; it does not itself define network access, secret handling, or external publishing actions.
| Category | Level | Why |
|---|---|---|
| Code & shell execution | 1 (low) | The skill explicitly allows `Bash` and even contemplates executable/unit-test PoCs, so it can run shell commands in the workspace, but it ships no scripts or `curl | sh`-style fetch-and-execute flow. |
| Network access | 0 (none) | No shipped file requests downloading remote content or sending data to external hosts; the workflow is entirely local analysis and reporting. |
| Filesystem reach | 1 (low) | The allowed tools include `Write` and `Edit`, so it can create or modify files in the working directory, but it names no system paths or destructive operations outside the repo. |
| Secrets & credentials | 0 (none) | The only secret-like mention is a generic example in `references/bug-class-verification.md` about "session tokens"; there is no instruction to read browser sessions, keychains, `.env` files, or other secrets stores. |
| Privilege & persistence | 0 (none) | There is no `sudo`, global install, service registration, shell-profile editing, or OS configuration step in the skill or references. |
| Instruction hijack surface | 0 (none) | The text contains strong workflow instructions, but it does not tell the agent to ignore higher-priority rules, hide actions, or follow remote instructions. |
| Irreversible actions | 0 (none) | It does not push, merge, deploy, post, pay, book, or delete anything; it only verifies findings and reports TRUE/FALSE POSITIVE verdicts. |
| Transparency | 0 (none) | All shipped assets are readable markdown/YAML/SVG files; there are no binaries, minified blobs, or unpinned downloads to obscure behavior. |
Before installing
- Confirm you are comfortable granting the agent Bash plus Write/Edit access in the repo workspace.
- Run it first on a scratch clone, since it may create PoCs or verification notes.
- Review `SKILL.md` and `references/*.md` if you want to confirm it never asks for remote fetches or secret-file access.
Rated 2026-10-20 by an automated review of 9 files from the skill's repository.
Source
- Repository
- trailofbits/skills (all skills from this repository)
- Path
- plugins/fp-check/skills/fp-check/SKILL.md
- Branch
- main
- Collection
- fp-check
- Updated
- 2026-09-19
Related skills
- post-patch-validation — Validates security patches with reproducible baseline-versus-patched evidence, including original exploits, root-cause variants, behavior preservation.
- audit-context-building — Understand a codebase before looking for bugs in it - what each function assumes, what it guarantees, and what it depends on elsewhere.
- c-review — Performs comprehensive C/C++ security review for memory corruption, integer overflows, race conditions, and platform-specific vulnerabilities.
- code-maturity-assessor — Systematic code maturity assessment using Trail of Bits' 9-category framework.
- constant-time-analysis — Detects timing side-channel vulnerabilities in cryptographic code.
- differential-review — Performs security-focused differential review of code changes.