On November 21, 2025, the Cardano blockchain experienced a temporary chain split due to a malformed delegation transaction that exploited a software flaw. This incident led to a divergence in the blockchain, effectively splitting the network into two distinct chains. One chain contained the "poisoned" transaction, while the other remained "healthy".
According to Intersect, Cardano's governance organization, the split occurred because the malformed transaction passed validation on newer node versions but was rejected by nodes running older software. This exploited a bug in an underlying software library that was not trapped by validation code. The vulnerability was related to a cryptographic library bug dating back to 2022, which had been previously observed on the Preview testnet.
Cardano engineers responded swiftly by releasing a hotfix and urging node operators to upgrade to version 10.5.3. This upgrade effectively converged the network back to a single chain. While block production continued on both chains during the split, the capacity was reduced, leading to network congestion. Some exchanges, such as Upbit, temporarily halted ADA deposits and withdrawals.
Cardano co-founder Charles Hoskinson characterized the event as a "premeditated attack from a disgruntled stake pool operator" seeking to harm the reputation of Input Output Global, Cardano's developer. Hoskinson stated that all Cardano users were impacted and that it would take weeks to fully resolve the issues. He also mentioned that the FBI had been contacted to investigate the incident as a potential cyberattack.
Following the incident, an X user named "Homer J" publicly admitted to triggering the malformed transaction. He apologized for the disruption, stating that it was a careless experiment without malicious intent. The wallet responsible for the transaction has been identified, with early forensics suggesting ties to a former Incentivized Testnet participant.
The price of ADA, Cardano's native token, experienced a drop of over 6% following the chain split. The incident has raised questions about the network's resilience and the vulnerabilities in its underlying software. However, the Cardano network demonstrated resilience amidst the technical difficulties, and developers insist that block production continued as normal. Hoskinson praised the network's design after its recovery, emphasizing that "the network survived".
