fp-check

name	fp-check
description	Systematic false positive verification for security findings. Provides structured methodology to confirm or dismiss scanner results, manual audit findings, and automated alerts. Adapted from Trail of Bits. Use when triaging security scan results or verifying audit findings.

False Positive Verification

Not every finding is real. But dismissing a real finding as "false positive" is worse than investigating a false one. This skill provides a systematic approach to verify findings without bias.

Verification Process

Step 1: Reproduce the Claim

Before dismissing anything, attempt to confirm:

FINDING: SQL injection in /api/users
CLAIM: User input reaches database query unsanitized

VERIFICATION:
1. Read the actual code at the reported location
2. Trace the data flow from input to sink
3. Check for sanitization/validation between input and sink
4. Check for framework-level protections (ORM, parameterized queries)
5. Attempt to construct an exploit payload

Step 2: Evidence-Based Triage

Verdict	Criteria	Evidence Required
TRUE POSITIVE	Vulnerability exists and is exploitable	Code path + exploit scenario
TRUE POSITIVE (mitigated)	Vulnerability exists but other controls prevent exploitation	Code path + mitigation proof
FALSE POSITIVE (provable)	Finding is wrong due to tool limitation	Specific reason why tool was wrong
FALSE POSITIVE (contextual)	Code is technically flagged but context makes it safe	Context documentation
NEEDS INVESTIGATION	Cannot determine without more analysis	What additional info is needed

Step 3: Document the Decision

FINDING: [scanner/auditor finding description]
SOURCE: [which tool/person reported it]
LOCATION: file.ts:42

VERDICT: [TRUE POSITIVE | FALSE POSITIVE | NEEDS INVESTIGATION]

EVIDENCE:
  - [What you checked]
  - [What you found]
  - [Why you reached this conclusion]

REASONING:
  [Detailed explanation of why this is/isn't a real finding]

CONFIDENCE: [HIGH | MEDIUM | LOW]
  [If LOW, explain what would increase confidence]

Common False Positive Patterns

1. Scanner Doesn't Understand Context

Scanner says: "Hardcoded password detected"
Actual code: const DEFAULT_LABEL = "password"
Verdict: FALSE POSITIVE -- it's a UI label, not a credential
Evidence: Variable is used only in form field label rendering

2. Framework Protection Not Recognized

Scanner says: "SQL injection in query"
Actual code: db.query("SELECT * FROM users WHERE id = $1", [userId])
Verdict: FALSE POSITIVE -- parameterized query prevents injection
Evidence: $1 is a parameter placeholder, userId is bound safely

3. Dead Code / Unreachable Path

Scanner says: "XSS in renderUserInput()"
Actual code: renderUserInput() exists but is never called
Verdict: FALSE POSITIVE -- function is dead code
Evidence: grep shows no callers; function should be removed anyway
WARNING: Verify it's truly unreachable, not just unused currently

4. Test Code Flagged

Scanner says: "Hardcoded API key"
Actual code: const TEST_KEY = "test-key-123" in test/fixtures.ts
Verdict: FALSE POSITIVE -- test fixture, not production code
Evidence: File is in test directory, key is clearly a test value
WARNING: Verify the key isn't a real key used in test environment

5. Intentional Behavior

Scanner says: "Insecure random number generation"
Actual code: Math.random() used for UI animation timing
Verdict: FALSE POSITIVE -- not used for security purposes
Evidence: Used only for visual jitter in animation, no security impact

Red Flags: When "False Positive" Is Actually Real

Do NOT dismiss if:

Red Flag	Why It Matters
"It's behind a VPN"	VPNs get compromised, zero trust is the standard
"Only admins can reach it"	Admin accounts get compromised
"The input is from our other service"	Services can be compromised too
"We sanitize it elsewhere"	Verify the "elsewhere" actually runs
"It's just a low severity"	Low severity findings chain into high impact
"The scanner is always wrong about this"	Verify EACH instance independently
"We've never been exploited"	Survivorship bias

Verification Techniques

1. Data Flow Tracing

Follow the data from source to sink:

Source (user input) -> [validation?] -> [transformation?] -> [sanitization?] -> Sink (dangerous operation)

If ANY step is missing or bypassable, it's a TRUE POSITIVE.

2. Control Flow Analysis

Check all paths to the vulnerable code:

Can the code be reached without authentication?
Can the code be reached with different parameters?
Can the code be reached through an alternative route?

3. Exploit Attempt

Construct a minimal proof:

Input: [specific malicious input]
Expected: [what should happen if vulnerable]
Actual: [what actually happens]
Blocked by: [what prevents exploitation, if anything]

4. Historical Check

# Has this code had real vulnerabilities before?
git log --grep="fix\|vuln\|security\|CVE" -- <file>

# Has the scanner been wrong about this pattern before?
# Check past triage decisions for this rule

Batch Triage Template

For large scan results:

# Security Scan Triage - [Date]

Scanner: [tool name and version]
Scan target: [repo/branch/commit]
Total findings: [N]

## Summary
| Verdict | Count |
|---------|-------|
| True Positive | X |
| True Positive (mitigated) | X |
| False Positive | X |
| Needs Investigation | X |

## True Positives (Action Required)
1. [SEVERITY] file.ts:42 -- [description] -- [recommended fix]

## False Positives (Documented)
1. file.ts:88 -- [reason it's false positive]

## Needs Investigation
1. file.ts:120 -- [what additional info is needed]

Integration with vibecosystem

• security-reviewer agent: Use fp-check after running security scans
• sast-scanner agent: Triage Semgrep results with this methodology
• code-reviewer agent: When flagging potential issues, verify first
• verifier agent: Include false positive check in quality gate

Inspired by Trail of Bits fp-check plugin.