web3-bug-classes

BUG CLASSES — DeFi Smart Contract Vulnerabilities

10 bug classes. Each one with root cause, vulnerable code, fix, grep patterns, and real paid examples.

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1. ACCOUNTING STATE DESYNCHRONIZATION

#1 Critical bug class — 28% of all Criticals on Immunefi. Real protocols: Yeet, Alchemix V3, Folks Finance, ResupplyFi, MetaPool

What It Is

Two state variables are supposed to stay in sync. One code path updates variable A but forgets variable B. Later code reads both and makes decisions based on the stale B.

Real Value = A - B
If A is updated but B isn't → Real Value appears larger than it is → phantom value

Root Cause Pattern

// BEFORE (correct state):
// aToken.balanceOf(this) = 1000  (principal + yield)
// totalSupply = 1000              (only principal)
// yield = 1000 - 1000 = 0        ✓ correct

// Attacker triggers startUnstake:
totalSupply -= amount;  // decremented BEFORE transfer
// totalSupply = 900 now
// aToken.balanceOf still = 1000
// yield appears = 1000 - 900 = 100 (PHANTOM)

// Now harvest():
yieldAmount = aToken.balanceOf(this) - totalSupply;
// = 1000 - 900 = 100 (phantom yield — no real yield was earned)
// Protocol harvests 100 of principal and distributes as "yield"

Variants

Variant 1: Phantom Yield — totalSupply decremented before transfer

// Yeet protocol (35 duplicate reports):
function startUnstake(uint256 amount) external {
    totalSupply -= amount;  // decremented here, transfer happens later
    // balanceOf(this) - totalSupply now shows phantom yield
}

Variant 2: Fast Path Skips State Update — early return bypasses critical updates

// Alchemix V3 claimRedemption:
function claimRedemption(uint256 tokenId) external {
    if (transmuter.balance >= amount) {
        transmuter.transfer(user, amount);
        _burn(tokenId);
        return;  // EARLY RETURN — cumulativeEarmarked, _redemptionWeight, totalDebt never updated
    }
    // SLOW PATH: updates all state vars correctly
    alchemist.redeem(...);
}