majiayu000 Variant Analysis Skill
Purpose
When you find one vulnerability, systematically hunt for every variant of the same root cause across the entire codebase. Variant analysis is the difference between finding 1 bug and finding 5–15 bugs from a single discovery. Elite auditors use this skill to multiply their finding count and provide comprehensive coverage.
Why Variant Analysis Matters
| Situation | Without Variant Analysis | With Variant Analysis |
|---|---|---|
Found unchecked return in withdraw() |
Report 1 finding | Search all external calls → find 4 more instances → 5 findings |
Found missing access control on setFee() |
Report 1 finding | Check all admin functions → find setPause(), setOracle() unprotected → 3 findings |
Found reentrancy in claim() |
Report 1 finding | Check all state-changing functions with external calls → find liquidate() also vulnerable → 2 findings |
Found rounding error in deposit() |
Report 1 finding | Check all division operations → find withdraw(), getExchangeRate() → 3 findings |
Real-world data: Analysis of Code4rena and Sherlock contest results shows that top auditors consistently find 2–5x more instances of a bug class than median auditors, primarily through systematic variant analysis.
Core Concept
Found Bug → Abstract to Pattern → Search Entire Codebase → Validate Each Match → Report All Variants
The Abstraction Ladder
Moving from a specific bug to a general pattern:
Level 0 (Specific): "withdraw() doesn't check IERC20.transfer return value"
Level 1 (Function): "unchecked return value on token transfer"
Level 2 (Category): "unchecked external call return value"
Level 3 (Root Cause): "missing validation of external interaction result"
Level 4 (Universal): "missing input/output validation"
Optimal search level: Level 2–3. Too specific (Level 0–1) misses variants. Too broad (Level 4) produces too many false positives.
Variant Dimensions
A single root cause can manifest across multiple dimensions:
1. Same Function, Different Contracts
Found: Missing slippage check in PoolA.swap()
Search: All contracts with swap() functions
Result: PoolB.swap(), PoolC.swap() also missing slippage check
2. Same Root Cause, Different Functions
Found: Reentrancy in withdraw()
Search: All functions that make external calls before state updates
Result: claim(), liquidate(), flashLoan() also have CEI violations
3. Same Pattern, Different Manifestation
Found: Oracle price can be manipulated via flash loan
Search: All places where spot prices are used for critical calculations
Result: Liquidation threshold uses spot price too → same attack vector
4. Cross-Contract Variants
Found: Access control missing on Module A's admin function
Search: ALL modules in the protocol for admin functions
Result: Module B and Module C have same missing guard
5. Cross-Protocol Variants
Found: ERC4626 first-depositor attack in Vault A
Search: Historical data for same pattern in other protocols
Result: 28+ findings in Solodit database with same root cause
Root Cause Taxonomy
| Root Cause Category | Common Variants | Search Strategy |
|---|---|---|
| Missing validation | Unchecked return, missing bounds, no zero-check | Grep all external interactions |
| Incorrect ordering | CEI violation, pre-state read, TOCTOU | Trace state reads vs external calls |
| Access control gap | Missing modifier, wrong role, unprotected init | Grep all public/external functions |
| Arithmetic error | Wrong rounding, precision loss, overflow | Grep all division/multiplication |
| State inconsistency | Stale cache, cross-function read, reentrancy | Map state dependencies |
| Integration mismatch | Fee-on-transfer, non-standard ERC20, decimals | List all token interactions |
| Oracle dependency | Stale price, spot manipulation, decimal mismatch | Find all price reads |
| Timing dependency | Block timestamp, deadline, frontrunning | Grep all time-based logic |
Process Overview
┌─────────────┐ ┌─────────────┐ ┌─────────────┐
│ 1. FIND │────►│ 2. ABSTRACT │────►│ 3. SEARCH │
│ Initial │ │ Root cause │ │ Grep / AST │
│ Bug │ │ pattern │ │ Full cbase │
└─────────────┘ └─────────────┘ └──────┬──────┘
│
┌─────────────┐ ┌─────────────┐ ┌──────▼──────┐
│ 6. REPORT │◄────│ 5. EXPAND │◄────│ 4. VALIDATE │
│ All grouped│ │ Related │ │ Each match │
│ by root │ │ patterns │ │ exploitable│
└─────────────┘ └─────────────┘ └─────────────┘
- Find: Discover initial vulnerability during audit
- Abstract: Reduce to root cause pattern at Level 2–3
- Search: Grep/regex/AST search entire codebase for the pattern
- Validate: Confirm each match is actually reachable and exploitable
- Expand: Check related patterns (same root cause, different manifestation)
- Report: Group all variants under single root cause with per-instance severity
Variant Sources
| Source | Use Case | Example |
|---|---|---|
| Current audit findings | Found 1, find all | Reentrancy in one function → check all |
| Cyfrin/Solodit database | Historical pattern matching | "This oracle pattern has 145+ findings" |
| DeFiHackLabs | Real exploit reproduction | "Euler hack used same reentrancy variant" |
| Code4rena / Sherlock reports | Cross-protocol patterns | "Compound fork X had this — check Y" |
| Known Solidity compiler bugs | Version-specific issues | "Solidity <0.8.15 has ABI encoder bug" |
| OpenZeppelin advisories | Dependency vulnerabilities | "OZ v4.7.3 has governance vulnerability" |
Resources
- Variant Patterns — Catalog of 12 major variant pattern classes with search strategies
- Variant Hunt Workflow — Step-by-step methodology for executing a variant hunt
Integration with Other Skills
| Skill | Integration |
|---|---|
cyfrin-findings/ |
Query historical findings for the same root cause pattern |
patterns/ |
Cross-reference with vulnerability pattern catalog |
anti-patterns/ |
Anti-pattern signatures can seed variant searches |
exploit-forensics/ |
Real-world exploits reveal pattern classes to hunt for |
static-analysis/ |
Automate variant detection with custom Slither detectors |
checklists/ |
Each variant class maps to checklist items |