Key takeaways
- A malformed transaction exploited a deserialization bug, causing a temporary chain split on the Cardano mainnet.
- Engineers have released patches and node upgrades to resolve the issue, without compromising user funds.
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The Cardano blockchain is returning to normal following a rare chain shard that temporarily split the network into two versions of its ledger.
According to a new report from Intersect, an organization supporting the long-term continuity and development of Cardano, the incident was triggered by a fraudulent and poorly formed delegation transaction on November 20.
The transaction exploited a crypto library bug dating back to 2022 on the Preview testnet, which allowed the network to split into two separate chains, including “one containing the ‘poisoned’ transaction and a ‘healthy’ chain without it.”
Cardano engineers worked through the night to release a fix after a malformed delegation transaction on the testnet revealed a bug. The next morning, the same type of transaction hit the mainnet, causing the network to split into two parallel chains.
Operators and other stakeholders quickly responded by upgrading nodes to 10.5.3, effectively bringing the network back to a single chain.
The team claims that no user funds have been affected. The wallet behind the malformed transaction has been identified, with initial forensic examinations suggesting links to a former Incentivized Testnet participant.
The FBI was contacted to investigate the event as a possible cyberattack.
“Cardano is a family, and sometimes we fight and sometimes we have bad days and good days. And I don’t forget how difficult 2025 has been for all of us,” Cardano founder Charles Hoskinson said in a statement. “The network survived. It didn’t stop.”
Following the Intersect report, user X called “Homer J” publicly admitted to triggering the malformed transaction that caused the incident.
He described it as a reckless experiment that unintentionally caused the Cardano network to split and sent an apology, saying he felt horrible about the disruption and had no malicious intent.
In response, Hoskinson said the troublemaker was now trying to backtrack after realizing the FBI was involved.
Cardano works so fast that we forked, fixed, and caught the guy in one day. He was very active in the Fake Fred discord. It was absolutely personal and now he’s trying to backtrack because he knows the FBI is already involved.
– Charles Hoskinson (@IOHK_Charles) November 21, 2025
