Nomad Bridge was drained of approximately $190 million on August 1, 2022, six weeks after a routine smart contract upgrade introduced a bug that caused the bridge to accept fake messages as legitimate. The contract had been audited by Quantstamp in May to June 2022, but the audit covered the code that was running before the upgrade, not the code that was running after. The risk score immediately before the exploit would have looked clean: recent audit by a reputable firm, growing TVL, no public incidents. The audit had effectively expired the day the upgrade shipped, except no risk model had marked it expired.
Nomad was a cross-chain bridge: it let users move tokens between different blockchains. To prevent fraud, every cross-chain message carried a cryptographic proof showing it came from a legitimate source. The bridge's smart contract on the destination chain checked the proof against a list of approved sources before processing the message and releasing the tokens.
On June 21, 2022, a routine code upgrade accidentally added a blank entry to the bridge's "approved sources" list. From that moment, any message that arrived with a blank proof (the easiest thing in the world to construct) matched the blank entry and was processed as legitimate. The bridge would release tokens to anyone who asked for them in the right format.
The bug remained exploitable in production for six weeks. Nomad's monitoring infrastructure was scoped to detect compromised private keys, not smart contract logic errors, so no alert was triggered. Everything else (code on GitHub, prior audit, TVL, governance, team) looked fine. The weakest link was the audit itself, which had gone stale the instant the June 21 upgrade shipped.
On August 1, 2022, one attacker discovered the bug and executed a transaction draining 100 WBTC (approximately $2.3M at the time). Within hours, hundreds of independent actors copied the original attacker's transaction, replaced the recipient address with their own, and replayed the exploit. By the end of the incident, approximately 300 different addresses had drained around $190M from the bridge. It was the first "crowd-looted" exploit in DeFi history.
The audit Nomad publicly pointed to had been performed on a version of the code that was no longer running. Quantstamp had even flagged a related concern before the upgrade (a finding labelled QSP-19, about empty messages being incorrectly treated as legitimate). The June 21 changes were not re-audited before they shipped.
A risk score based on "recent audit by reputable firm + growing TVL + no public incidents" would have rated Nomad clean on July 31, 2022. By August 1, the same project was effectively dead. The score had not moved because the model had no mechanism for handling post-audit code drift.