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When Zero-Knowledge Proofs Break: How Groth16 Verification Key Misconfigs Drained $3M+ From DeFi

#security #solidity #web3 #defi

When Zero-Knowledge Proofs Break: How Groth16 Verification Key Misconfigs Are Draining DeFi Protocols

A deep dive into the $3M+ in losses from zkSNARK deployment failures — and the 5-point audit checklist every ZK protocol needs.

Zero-knowledge proofs are supposed to be the gold standard of trustless verification. You prove you know something without revealing what you know. Beautiful in theory, catastrophic when misconfigured.

In February 2026, FOOMCASH — an Ethereum-based ZK-proof lottery protocol — lost $2.26 million because two elliptic curve constants in their Groth16 verifier were set to the same value. No flash loans. No reentrancy. No complex DeFi mechanics. Just a single line of cryptographic misconfiguration that let an attacker forge valid proofs and drain funds at will.

The worst part? This exact vulnerability had already been exploited before, in the Veil Protocol hack. It was a known class of bug. And it will happen again — unless ZK protocol teams and auditors learn what to look for.

How Groth16 Verification Actually Works

Groth16 is the most widely deployed zkSNARK proof system in DeFi. It's used in privacy protocols (Tornado Cash derivatives), ZK bridges, and now ZK-based lotteries and governance systems.

The verification boils down to a bilinear pairing check:

e(A, B) = e(alpha, beta) · e(vk_x, gamma) · e(C, delta)
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Where:

  • e() is a bilinear pairing over the BN254 curve (Ethereum precompile 0x08)
  • A, B, C are the proof elements provided by the prover
  • alpha, beta are fixed constants from the trusted setup
  • vk_x is computed from public inputs and verification key coefficients
  • gamma binds the proof to public inputs
  • delta binds the proof to the private witness

The critical security property: gamma and delta MUST be different, independently sampled curve points. They create an algebraic separation between what's public (inputs) and what's private (witness). Without this separation, the proof system's soundness collapses.

The FOOMCASH Exploit: Anatomy of a $2.26M Drain

The Bug

In the deployed Groth16 verifier contract (0xc043865fb4D542E2bc5ed5Ed9A2F0939965671A6), the verification key constants gamma and delta were set to identical elliptic curve points.

This collapsed the verification equation:

e(A, B) = e(alpha, beta) · e(vk_x, gamma) · e(C, gamma)
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Which simplifies to:

e(A, B) = e(alpha, beta) · e(vk_x + C, gamma)
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Since vk_x depends only on public inputs (which the attacker controls), setting C = -vk_x satisfies:

e(A, B) = e(alpha, beta) · e(O, gamma)
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Where O is the point at infinity. The attacker just needs e(A, B) = e(alpha, beta), trivially satisfied by setting A = alpha, B = beta.

The Attack Flow 

┌─────────────────────────────────────────────────┐
│ 1. Attacker picks arbitrary nullifierHash       │
│ 2. Computes vk_x from public inputs             │
│ 3. Sets C = -vk_x (algebraic cancellation)      │
│ 4. Sets A = alpha, B = beta (public constants)   │
│ 5. Calls collect() with forged proof → ✅ passes │
│ 6. Increment nullifierHash, repeat              │
│ 7. $2.26M drained across Ethereum + Base        │
└─────────────────────────────────────────────────┘
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The attacker deployed a helper contract to automate repeated collect() calls, each time incrementing _nullifierHash and recomputing the forged proof. No private witness was ever needed.

Proving the Forgery

You can verify this yourself using Foundry:

cast call 0xc043865fb4D542E2bc5ed5Ed9A2F0939965671A6 \
  "verifyProof(uint256[2],uint256[2][2],uint256[2],uint256[7])(bool)" \
  "[16428432848801857252194528405604668803277877773566238944394625302971855135431,\
    16846502678714586896801519656441059708016666274385668027902869494772365009666]" \
  "[[3182164110458002340215786955198810119980427837186618912744689678939861918171,\
    16348171800823588416173124589066524623406261996681292662100840445103873053252],\
   [4920802715848186258981584729175884379674325733638798907835771393452862684714,\
    19687132236965066906216944365591810874384658708175106803089633851114028275753]]" \
  "[3580187279101084069751145503379205138644462599730770566826054966809782222536,\
    9034737010478553175860989655199489479494794665577903575343123912300687026525]" \
  "[7781146065974985251383064797749046122697630175951491630464801665880519130734,\
    99999990000,7,404000638890875347995779038680777447236549340018,0,0,0]" \
  --rpc-url https://eth.llamarpc.com
# Output: true ← forged proof accepted
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The Silver Lining

White-hat hacker Duha and security firm Decurity rescued ~$1.84M (81% of stolen funds). FOOMCASH awarded a $320K bounty and $100K security fee. But $420K remained lost — and the protocol hasn't posted updates since November 2025.

This Wasn't New: The Veil Protocol Precedent

The FOOMCASH exploit was a copycat of the earlier Veil Protocol hack, which used the exact same gamma == delta misconfiguration. In Veil's case, the attacker executed 29 fraudulent withdrawals from a privacy pool.

Both protocols made the same fundamental mistake: deploying verifier contracts with verification keys from a broken or skipped trusted setup ceremony.

The Trusted Setup Problem

Groth16 requires a trusted setup ceremony — a multi-party computation (MPC) that generates the proving and verification keys. The ceremony produces "toxic waste" (secret random values) that must be destroyed. If anyone retains these values, they can forge proofs.

Common failure modes:

Failure Impact Real Example
Skipped MPC ceremony entirely gamma == delta (test values deployed) FOOMCASH, Veil
Missing Phase 2 contribution Verification key constants not randomized FOOMCASH CLI step skipped
Single-party "ceremony" One person holds toxic waste Multiple small protocols
Reused ceremony across circuits Keys don't match the actual circuit Theoretical but likely

5-Point ZK Verification Audit Checklist

1. Verify gamma ≠ delta On-Chain 

// Add this check to your verifier deployment script
function validateVerificationKey(
    uint256[2] memory gamma,
    uint256[2] memory delta
) internal pure {
    require(
        gamma[0] != delta[0] || gamma[1] != delta[1],
        "CRITICAL: gamma == delta breaks soundness"
    );
}
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Or check post-deployment:

# Python script to verify deployed verifier keys
from web3 import Web3

w3 = Web3(Web3.HTTPProvider("https://eth.llamarpc.com"))

# Read gamma and delta from contract storage
# Slot layout depends on verifier implementation
verifier = "0xc043865fb4D542E2bc5ed5Ed9A2F0939965671A6"

# For snarkjs-generated verifiers, check the vk() function
# or read IC, gamma, delta constants directly
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2. Validate Trusted Setup Artifacts 

#!/bin/bash
# Verify ceremony artifacts before deployment
# Requires snarkjs

# Check Phase 1 (Powers of Tau)
snarkjs powersoftau verify pot_final.ptau

# Check Phase 2 (Circuit-specific)
snarkjs groth16 verify verification_key.json public.json proof.json

# Extract and compare gamma/delta from verification key
python3 -c "
import json
vk = json.load(open('verification_key.json'))
gamma = vk['vk_gamma_2']
delta = vk['vk_delta_2']
assert gamma != delta, 'FATAL: gamma == delta in verification key!'
print('✅ gamma ≠ delta — soundness preserved')
print(f'gamma: {gamma}')
print(f'delta: {delta}')
"
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3. CI/CD Pipeline Guard 

# .github/workflows/zk-deploy-guard.yml
name: ZK Verification Key Safety Check
on:
  push:
    paths:
      - 'contracts/verifiers/**'
      - 'circuits/**'

jobs:
  verify-keys:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - name: Check gamma ≠ delta
        run: |
          for vk_file in $(find . -name "verification_key.json"); do
            python3 scripts/check_vk_soundness.py "$vk_file"
          done
      - name: Verify ceremony transcript
        run: snarkjs groth16 verify verification_key.json public.json proof.json
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4. Multi-Party Ceremony Verification

Before trusting any ZK protocol, verify:

  • Number of participants — More is better; 1 honest participant is sufficient
  • Ceremony transcript — Published and independently verifiable
  • Participant attestations — Each participant confirms toxic waste destruction
  • Circuit hash — Ceremony was for the correct circuit, not a different one

5. Runtime Proof Validation Beyond Pairing 

// Additional runtime checks in your ZK contract
function collect(
    uint256[2] memory a,
    uint256[2][2] memory b,
    uint256[2] memory c,
    bytes32 nullifierHash,
    address recipient
) external {
    // Check 1: Nullifier hasn't been used
    require(!nullifiers[nullifierHash], "Already withdrawn");

    // Check 2: Proof elements are on the curve (not point at infinity)
    require(a[0] != 0 || a[1] != 0, "Invalid proof element A");
    require(c[0] != 0 || c[1] != 0, "Invalid proof element C");

    // Check 3: Verify the proof
    require(verifier.verifyProof(a, b, c, publicInputs), "Invalid proof");

    // Check 4: Rate limiting (defense in depth)
    require(
        block.timestamp >= lastWithdrawal + MIN_WITHDRAWAL_INTERVAL,
        "Rate limited"
    );

    nullifiers[nullifierHash] = true;
    lastWithdrawal = block.timestamp;

    // Transfer funds
    token.transfer(recipient, amount);
}
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Beyond FOOMCASH: The Growing ZK Attack Surface

As ZK-based protocols proliferate — ZK bridges, ZK rollups, ZK identity, ZK governance — the attack surface grows. Key areas to watch:

  1. PLONK/Halo2 setup issues: While these systems use universal (not circuit-specific) setups, they still have configuration pitfalls
  2. Recursive proof composition: Verifying proofs of proofs adds layers where misconfiguration can hide
  3. Cross-chain ZK bridges: Verification keys deployed on different chains must match; a mismatch is exploitable
  4. ZK coprocessors: Off-chain computation verified on-chain — the verification contract is a single point of failure

Lessons for Security Researchers

If you're auditing ZK protocols:

  1. Always check gamma vs delta — It's the lowest-hanging fruit and the most devastating
  2. Don't assume the cryptography is correct — Implementation ≠ specification
  3. Trace the trusted setup chain — From ceremony to deployed bytecode
  4. Test with forged proofs — If your forged proof passes, the verifier is broken
  5. Check all deployment chains — FOOMCASH was vulnerable on both Ethereum and Base

The FOOMCASH exploit wasn't a failure of zero-knowledge cryptography. Groth16 is sound when implemented correctly. It was a failure of deployment hygiene — a missing CLI step in the trusted setup that left the verifier contract mathematically broken.

In a space where "code is law," the code has to actually be correct. A single misconfigured elliptic curve point turned a $2.26M protocol into an open ATM.

DreamWork Security researches smart contract vulnerabilities across EVM and Solana ecosystems. Follow for weekly deep dives into DeFi exploits, audit techniques, and security best practices.

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