The Cardano blockchain experienced a brief but notable disruption on November 21, 2025, after a malformed transaction triggered an old software bug and caused a temporary chain split. While the issue was resolved quickly, the event highlighted how legacy code paths can still surface unexpected behavior in major proof-of-stake networks.
How the Chain Split Happened
According to Intersect, Cardano’s ecosystem governance organization, the incident began when a delegation transaction containing malformed data was submitted to the network.
Although the transaction was valid at the protocol level, it exploited a dormant bug inside an underlying software library used by older node versions.
Cardano suffered a chain partition after an abnormal transaction triggered a code bug.
Reporting the incident, one cheeky X user claimed:
"Cardano mainnet is down and nobody has noticed yet because nobody uses it".
Full story below 👇
— Cointelegraph (@Cointelegraph) November 22, 2025
Nodes running outdated software rejected the transaction, while fully updated nodes accepted it. This divergence created two temporary versions of the blockchain:
- A “healthy chain” processed by updated nodes
- A “poisoned chain” formed by unpatched nodes
As a result, block production slowed and the network briefly operated in a partitioned state.
Issue Resolved Through Emergency Node Updates
To restore chain convergence, staking pool operators (SPOs) were instructed to upgrade immediately to node versions 10.5.2 or 10.5.3.
Once the majority of block-producing nodes updated, the network re-aligned and normal block production resumed.
Intersect emphasized that no user funds were lost, and final settlement integrity was preserved throughout the incident.
Perpetrator Admits to Submitting the Triggering Transaction
In an unusual twist, an X (formerly Twitter) user named Homer J., a known Cardano staking pool operator, publicly admitted to submitting the malformed transaction.
He described the act as careless experimentation, claiming he used AI-generated instructions without understanding the consequences. He insisted that it was not an attempt to harm the network.
However, Cardano co-founder Charles Hoskinson initially referred to the event as a premeditated attack, noting that the transaction appeared intentionally crafted to exploit the vulnerable code path.
Given the nature of the disruption, the matter has been escalated to law-enforcement agencies, including the FBI, which is now investigating the incident.
A Reminder About Node Diversity and Legacy Code
The chain split underscores a recurring challenge for decentralized networks: even when protocol rules are sound, node version fragmentation can magnify the impact of subtle bugs. Cardano core developers noted that the issue would not have materialized if the majority of the network had already migrated to the newer software versions.
Still, the coordinated response from SPOs and developers ensured that the event remained contained, with no lasting damage.
