OAK — OnChain Attack Knowledge

Worked example · 2026-10

BiFi Lending — 36 days of idle interest were charged at whatever utilization the attacker set inside one block — BiFi / Ethereum — 2026-10-17

Loss
~165.6K USDT (~$165.4K), swapped to WETH (Defimon). Returned: 159,103.654838 USDC sent back to the BiFi USDT handler by c0ffeebabe.eth on 2026-10-18 17:21 UTC (traced by OAK, tx); no statement on the gap to the ~165.6K taken.
OAK Techniques observed
OAK-T9.016 (Contract Accounting-State Invariant Flaw — primary. BiFi's USDT market sat idle ~264K blocks. TokenHandler._checkFirstAction stores that gap as inactiveActionDelta and freezes actionEXR once per block; every later action in the same block re-runs interestModel.getInterestAmount, computing globalEXR = actionEXR × (1 + rate × 264K) with the rate from live deposit/borrow totals. So the attacker chose the utilization at which 36 days of interest were charged, repeatedly. See techniques/T9.016-contract-accounting-state-invariant-flaw.md). Funded by a 40.5M USDT Morpho flash loan (OAK-T9.002).
Attribution
pseudonymous. The original attacker (0x7c78a049fff71609990017b58a959e926e6ec4c6) deployed an exploit contract; one block later the MEV operator c0ffeebabe.eth replayed it from its own contract and took the funds.
Key teaching point
Accrual must use the rate that applied during the gap, not the rate at the moment someone first touches the market. Idle markets in abandoned deployments are where this bites.

Summary

The exploit ran on 2026-10-17 at 05:43 UTC (blocks 26210255–26210256, on-chain time traced by OAK); Defimon reported it about 26 hours later, on 2026-10-18. On 2026-10-18 at 17:21 UTC c0ffeebabe.eth sent 159,103.654838 USDC back to the BiFi USDT handler (traced by OAK; not in the cited sources).

Timeline (UTC)

Read it as three stages. Get in: a market left idle for 36 days. Prepare: an exploit contract. Take: a front-run replay with a flash loan. Times are from Defimon.

When What happened OAK
Before 2026-10-17 BiFi's USDT market sits idle for about 264K blocks (about 36 days). TokenHandler._checkFirstAction stores that gap as inactiveActionDelta and freezes actionEXR once per block, and each later action in the same block recomputes interest from live deposit and borrow totals. T9.016
2026-10-17, block 26210255 The original attacker 0x7c78a049fff71609990017b58a959e926e6ec4c6 deploys an exploit contract: 0x15b47e2c0daf723dec64c75b435079d3a1f8a6058b21b9a64dda4844e877a034. —
2026-10-17 05:43:11, block 26210256 c0ffeebabe.eth (0xc0ffeebabe5d496b2dde509f9fa189c25cf29671) front-runs it and replays the attack from its own contract: 0x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14. It flash-borrows 40.5M USDT from Morpho (traced by OAK; the explorer timestamps it 2026-10-17 05:43:11 UTC). T9.002
same transaction The attacker deposits the loan in growing tranches, then borrows 30M to push utilization up. Every call charges the 36-day interest at the utilization it just set. T9.016
same transaction A repay/withdraw loop cashes out the inflated deposit interest. The cycle runs twice and takes 165.6K USDT. T9.016
same transaction The USDT is swapped to WETH: 165,574.514633 USDT go to pool 0x4e68ccd3e89f51c3074ca5072bbac773960dfa36 for 68.05720311 WETH, which is unwrapped in the same transaction (traced by OAK). —
2026-10-18 17:21:59 c0ffeebabe.eth sends 159,103.654838 USDC to the BiFi USDT handler: 0x9702900bc441039c7e6937a3f3e94251072bda380bdcba38ecf6c7c3e6efeaeb (traced by OAK). response
2026-10-18 07:45:50 Defimon publishes its detection (about $165.4K lost). response
after 2026-10-18 (undisclosed) No BiFi statement, pause or recovery appears in the cited sources. response

Key addresses:

Fund flow

Ethereum only, traced by OAK with tools/explorer.py (Blockscout, keyless) on 2026-10-30, with an outgoing window to 2026-10-23. The whole extraction happens inside one transaction, sent by c0ffeebabe.eth to its own contract. The operator's identity is recorded as metadata only; the flow is described by mechanism. Edges marked traced by OAK were checked on-chain.

All transfers as a table (8)
Time (UTC)FromToAmountOAKTxBasis
2026-10-17T05:43:11ZMorpho (flash-loan source)
0xbbbbbbbbbb9cc5e90e3b3af64bdaf62c37eeffcb		Operator's replay contract
0xe08d97e151473a848c3d9ca3f323cb720472d015		40,500,000 USDTT9.0020x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14traced by OAK
2026-10-17T05:43:11ZOperator's replay contract
0xe08d97e151473a848c3d9ca3f323cb720472d015		BiFi USDT handler (UsdtHandlerProxy)
0x808c3ba97268dbf9695b1ec10729e09c7e67a9e3		141,270,327.6676 USDT (×72)T9.0160x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14traced by OAK
2026-10-17T05:43:11ZBiFi USDT handler (UsdtHandlerProxy)
0x808c3ba97268dbf9695b1ec10729e09c7e67a9e3		Operator's replay contract
0xe08d97e151473a848c3d9ca3f323cb720472d015		141,435,902.8173 USDT (×72)T9.0160x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14traced by OAK
2026-10-17T05:43:11ZOperator's replay contract
0xe08d97e151473a848c3d9ca3f323cb720472d015		Morpho (flash-loan source)
0xbbbbbbbbbb9cc5e90e3b3af64bdaf62c37eeffcb		40,500,000 USDTT9.0020x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14traced by OAK
2026-10-17T05:43:11ZOperator's replay contract
0xe08d97e151473a848c3d9ca3f323cb720472d015		USDT/WETH pool
0x4e68ccd3e89f51c3074ca5072bbac773960dfa36		165,574.5146 USDT0x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14traced by OAK
2026-10-17T05:43:11ZUSDT/WETH pool
0x4e68ccd3e89f51c3074ca5072bbac773960dfa36		Operator's replay contract
0xe08d97e151473a848c3d9ca3f323cb720472d015		68.0572 WETH0x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14traced by OAK
2026-10-17T05:43:11ZOperator's replay contract
0xe08d97e151473a848c3d9ca3f323cb720472d015		WETH unwrapped to ETH68.0572 WETH0x756c68055af7a0630aa0b56891ba4de14c56c39cbebe99e47eeb4728f4d6ad14traced by OAK
2026-10-18T17:21:59Zc0ffeebabe.eth (MEV operator, sender)
0xc0ffeebabe5d496b2dde509f9fa189c25cf29671		BiFi USDT handler (UsdtHandlerProxy)
0x808c3ba97268dbf9695b1ec10729e09c7e67a9e3		159,103.6548 USDC0x9702900bc441039c7e6937a3f3e94251072bda380bdcba38ecf6c7c3e6efeaebtraced by OAK

What the trace adds beyond the sources:

  • The extraction contract is 0xe08d97e151473a848c3d9ca3f323cb720472d015, which the sources do not name. c0ffeebabe.eth is the transaction sender and the contract is the actor. The original attacker 0x7c78a049fff71609990017b58a959e926e6ec4c6 sent a transaction with no token transfers and no created contract in the explorer data, so no funds moved through it.
  • Net amounts. Across the 72 deposit and withdraw legs the BiFi USDT handler paid out 141,435,902.81731 USDT and took in 141,270,327.66758 USDT, a net loss of 165,575.149730 USDT. The flash loan of 40,499,999.999997 USDT was repaid in full in the same transaction. The contract then sold 165,574.514633 USDT to the pool for 68.05720311 WETH. About 0.64 USDT was left over.
  • WETH was unwrapped in the same transaction (68.05720311 WETH burned to the zero address, which is a WETH withdrawal). The resulting ETH is not shown as a transfer by the explorer.
  • A return. On 2026-10-18 at 17:21:59 UTC, about 36 hours later, c0ffeebabe.eth sent 159,103.654838 USDC to the BiFi USDT handler. No source mentions this. It is 6,471 below the 165,574.51 USDT-equivalent that was sold; whether that is the operator's costs or a partial return is not stated anywhere OAK saw. This is neutral metadata, not a verdict on the operator.
  • Conflict on date. The explorer timestamps the exploit deployment at 2026-10-17 05:42:59 UTC and the replay at 2026-10-17 05:43:11 UTC. The sources and this file's header say 2026-10-18. The on-chain times are kept in the edges; Defimon's post is dated 2026-10-18.
  • Limits. No onward outgoing transfers from the replay contract in the window to 2026-10-23 (the tool returned no edges). The EOA 0xc0ffeebabe5d496b2dde509f9fa189c25cf29671 is a high-volume MEV bot, so its other transactions are not related to this incident and were not followed.

Root-cause evidence

Public references

Techniques demonstrated (2)