CryptoSlate
ICON Foundation said the Aug. 27 replay exploit released 119,866,000 ICX and 531,600 bnUSD from foundation-held assets. Two legitimate withdrawal messages were reused 1,492 times, with 1,490 calls succeeding; no user deposits, balances or positions were accessed. The confirmed net loss was about 150.2 ETH plus 31,204 USDC, while most ICX was traced, frozen and remains in active recovery.
Why it matters
The bug changed part of a withdrawal identifier without changing the signed payload. A change to standardize withdrawal messages at 32 bytes routed part of the serial number through float64-range logic instead of exact integer arithmetic. The attacker varied unsigned high bits while the signature still verified the same low 256 bits, defeating the uniqueness check. Two calls reverted, and every successful call credited one relayer wallet.
ICON said the flaw was specific to its implementation. Other supported chains used fixed-width integers that could not create the mismatch. A November 2025 relay audit did not list the affected migration-contract source, and none of its nine findings flagged the serial-number issue.
Market impact
The 119.9M ICX release is not the confirmed loss. ICON said most ICX was traced, frozen and in active recovery, while bnUSD and SODA were recovered in full. Exchange-held ICX remains subject to revision because exact amounts held, converted or withdrawn were unavailable.
Monitoring fired at 02:08 UTC, but the contract was paused 105 minutes later, after ICX had begun moving to exchange deposit addresses. Bitvavo, Bitget and KuCoin suspended ICX transfers, and the network halted for roughly 25 hours. Recovery totals, exchange figures and the audit response are the next material updates.
Frequently asked questions
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What was the confirmed net loss from the ICON exploit?
ICON put net loss to date at about 150.2 ETH plus 31,204 USDC. The 119,866,000 ICX release was not equivalent to confirmed loss because most ICX was traced, frozen and in active recovery.
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How did the replay flaw make the same messages look unique?
A change to standardize withdrawal messages at 32 bytes routed part of the serial number through float64-range logic. The attacker varied unsigned high bits, while signature verification still covered the unchanged low 256 bits.
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Did the exploit reach user deposits, balances or trading positions?
No. ICON said the exploit released foundation-held assets and did not access user deposits, balances or positions.
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How long did ICON take to pause the affected contract?
Monitoring fired at 02:08 UTC, and the affected contract was paused at 03:53, 105 minutes after the alert. The network later halted for roughly 25 hours.
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What remains uncertain about the ICX recovery?
Exchange-held ICX figures remain subject to revision because exact amounts held, converted or withdrawn were unavailable. ICON said bnUSD and SODA were recovered in full, while most ICX was traced, frozen and in active recovery.
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