Variant Analysis

When you find a bug, the same mistake almost certainly exists elsewhere. Variant analysis systematically hunts for siblings of a known vulnerability.

Process

Step 1: Characterize the Original Bug

Before searching, understand what makes this bug a bug:

ORIGINAL BUG:
  File: src/api/users.ts:42
  Type: Missing input validation
  Pattern: req.params.id used directly in DB query without sanitization
  Root cause: Developer assumed framework sanitizes params
  Trigger: Untrusted input reaches database query

Extract the abstract pattern -- not the specific code, but the class of mistake:

• Missing validation at a trust boundary
• Incorrect error handling in auth path
• Race condition between check and use
• Hardcoded secret in source
• SQL injection via string concatenation

Step 2: Generate Search Queries

For each bug class, create multiple search strategies:

Grep/Ripgrep (Fast, broad)

bash
# Example: SQL injection via concatenation
rg "query\(.*\+.*\)" --type ts
rg "execute\(.*\$\{" --type ts
rg "\.raw\(.*\+" --type ts

# Example: Missing auth middleware
rg "router\.(get|post|put|delete)\(" --type ts -l | \
  xargs rg -L "authenticate|authorize|requireAuth"

# Example: Hardcoded secrets
rg "(password|secret|key|token)\s*[=:]\s*['\"][^'\"]{8,}" --type ts

Semgrep (AST-aware, precise)

yaml
# Example: SQL injection
rules:
  - id: sql-injection-concatenation
    patterns:
      - pattern: $DB.query($X + ...)
      - pattern-not: $DB.query($X, [...])
    message: "Potential SQL injection via string concatenation"
    severity: ERROR

# Example: Missing null check before use
rules:
  - id: null-deref-after-find
    patterns:
      - pattern: |
          const $X = await $DB.findOne(...)
          ...
          $X.$PROP
      - pattern-not: |
          const $X = await $DB.findOne(...)
          ...
          if ($X) { ... }
    message: "Using findOne result without null check"
    severity: WARNING

CodeQL (Deep analysis)

ql
// Example: Tainted data reaching SQL
import javascript

from CallExpr call, DataFlow::Node source, DataFlow::Node sink
where
  source = DataFlow::parameterNode(any(Function f).getAParameter()) and
  sink = call.getArgument(0) and
  call.getCalleeName() = "query" and
  DataFlow::localFlow(source, sink)
select sink, "Untrusted input flows to SQL query"

Step 3: Triage Results

For each match:

| Status | Meaning | Action | |--------|---------|--------| | CONFIRMED | Same bug pattern, exploitable | File as finding | | LIKELY | Same pattern, needs deeper analysis | Investigate further | | MITIGATED | Pattern present but other controls prevent exploitation | Document as defense-in-depth gap | | FALSE POSITIVE | Pattern matches but context makes it safe | Document why it's safe |

Step 4: Report

## Variant Analysis Report

**Original Finding**: [reference to original bug]
**Pattern**: [abstract description of the vulnerability class]
**Search Method**: [grep/semgrep/codeql/manual]

### Confirmed Variants

1. **[SEVERITY]** file.ts:42 -- [description]
2. **[SEVERITY]** other.ts:88 -- [description]

### Likely Variants (Need Investigation)

3. file2.ts:15 -- [why it might be vulnerable]

### Mitigated Instances

4. safe.ts:30 -- Same pattern but [mitigation] prevents exploitation

### Statistics

- Files scanned: X
- Matches found: Y
- Confirmed: Z
- False positives: W

Common Variant Patterns

Input Validation Variants

If one endpoint lacks validation, check ALL endpoints:

bash
# Find all route handlers
rg "router\.(get|post|put|delete|patch)\(" --type ts -n

# Check each for validation middleware
# Missing validation = variant

Auth/Authz Variants

If one route lacks auth, check all routes:

bash
# Find routes without auth middleware
rg "app\.(get|post)\(['\"]" --type ts | grep -v "auth\|protect\|require"

Error Handling Variants

If one catch block leaks info, check all catch blocks:

bash
rg "catch.*\{" -A 3 --type ts | grep -E "res\.(send|json).*err"

Crypto Variants

If one place uses weak crypto, check all crypto usage:

bash
rg "createHash\(|createCipher\(|randomBytes\(" --type ts
rg "MD5\|SHA1\|DES\|RC4" --type ts

Race Condition Variants

If one TOCTOU exists, check similar check-then-act patterns:

bash
rg "if.*await.*find" -A 5 --type ts | grep -E "await.*(update|delete|create)"

Automation Integration

With coroner agent (post-mortem)

After fixing a bug, coroner should:

1. Call variant-analysis with the bug pattern
2. Check all confirmed variants
3. Create tasks for each variant fix

With security-reviewer agent

During review, if a finding is discovered:

1. Pause the linear review
2. Run variant analysis for the finding class
3. Include all variants in the review report

With code-reviewer agent

When a fix is reviewed:

1. Check if the fix addresses all known variants
2. Verify the fix pattern is applied consistently

Rationalizations to Reject

| Rationalization | Why It's Wrong | Required Action | |----------------|---------------|-----------------| | "It's just one instance" | Bugs travel in packs | Run variant analysis | | "The other code is different" | Same pattern, different syntax | Abstract the pattern | | "We already fixed this area" | Fix might be incomplete | Verify with search | | "Semgrep didn't find anything" | Rules might be too specific | Try multiple search methods | | "It's too many results" | Volume doesn't mean false positive | Triage each result |

Inspired by Trail of Bits variant-analysis plugin.