ICON Foundation Details Replay Exploit That Released Millions in Tokens
Quick Look
- The ICON Foundation disclosed a replay exploit on August 27 that released 119,866,000 ICX and 531,600 bnUSD from foundation-held assets after two legitimate withdrawal messages were reused 1,492 times.
- The foundation reported a net loss of about 150.2 ETH and 31,204 USDC, with most ICX traced and frozen.
- The exploit stemmed from a flaw in withdrawal message handling that allowed attackers to alter identifiers without changing signed payloads, and was detected seven minutes after it began, though the network was not halted until 25 hours later.
AI-generated summary
Why It Matters
The ICON Foundation manages the ICON blockchain network and its associated tokens, including ICX and bnUSD. The organization regularly issues postmortems on security incidents to improve transparency and system resilience.
According to the ICON Foundation, the Aug. 27 ICON replay exploit released 119,866,000 ICX and 531,600 bnUSD from foundation-held assets after two legitimate withdrawal messages were reused 1,492 times. Its Aug. 30 postmortem said 1,490 calls succeeded, while no user deposits, balances or positions were accessed.
The headline-sized ICX release is not the same as the confirmed loss. ICON put net loss to date at about 150.2 ETH plus 31,204 USDC, with the vast majority of the ICX traced, frozen and in active recovery. The foundation said bnUSD and SODA were recovered in full, but exchange-held amounts remain subject to revision. That distinction matters because ICON had not received exact exchange figures for how much ICX was held, converted or withdrawn.
The flaw let the attacker change part of a withdrawal identifier without changing the signed payload being verified. ICON traced the mismatch to a change intended to standardize withdrawal messages at 32 bytes, which routed part of the serial number through float64-range logic rather than exact integer arithmetic.
As a result, the contract's uniqueness check looked at high bits the attacker could vary, while cryptographic verification covered the unchanged low 256 bits. The signed payload and signature remained identical within each replay set, but the altered unsigned portion made the calls appear unique. Two calls reverted; every successful call credited the same relayer wallet. ICON said the flaw was specific to its implementation because other supported chains used fixed-width integers that could not produce the same mismatch.
Detection of the ICON replay exploit came before containment
ICON's monitoring system fired at 02:08 UTC, seven minutes after the exploit began. Technical staff started investigating at about 03:40, a 92-minute gap. The affected contract was paused at 03:53, 105 minutes after the alert.
The attacker had begun splitting ICX across exchange deposit addresses at 02:44, according to ICON, and the distribution continued until about 05:20. The foundation said an ICON-side pause could not stop movement of funds already swept into exchange custody.
The network was halted at 06:18:54 and resumed at about 07:51 the next day, roughly 25 hours later. Public notices from Bitvavo, Bitget and KuCoin confirm that ICX deposits and withdrawals were suspended around the incident, though none identifies itself as holding attacker funds or verifies the amount frozen.
What to Watch
AI outlook — possibilities, not facts
ICON will implement stricter validation for withdrawal message uniqueness to prevent reuse of signed payloads.
Very likely · Within weeks
ICON will improve monitoring and response times to reduce the gap between exploit detection and contract pausing.
Likely · Within days
Open Questions
- What specific changes will ICON make to prevent similar replay exploits in the future?
- Are any exchange-held ICX funds still at risk or subject to further loss?
- Will affected users be compensated for any losses incurred during the exploit?
