Which L1 suffered a chain split?

TLDR

Cardano (ADA) is the L1 that suffered a chain split.

1. A malformed delegation transaction triggered incompatible views across node versions, briefly creating two parallel chains per a detailed post-mortem and reporting on the split in two by a single transaction (analysis).
2. The network stayed live but degraded and reconverged within hours after patches and coordination, with coverage of the 14-hour incident and recovery steps (incident recap).

Deep Dive

1. What Happened

Cardano experienced a temporary chain split when a malformed delegation certificate was accepted by newer nodes but rejected by older ones, causing two competing forks until the network reconverged. A detailed account explains how this single transaction exploited a legacy deserialization bug and split validator perspectives (analysis).

Reports note the event lasted hours, with degraded throughput and delayed inclusion, before stakeholders rolled out patches and stake migrated back to a single canonical chain (incident recap). Law enforcement attention was also raised due to the nature of the exploit attempt, adding scrutiny to the incident (analysis).

2. Why It Matters

The incident spotlights upgrade coordination and client diversity as resilience levers for L1s. A single client with version skew can tolerate faults differently than multi-client ecosystems, and this event became a live test of that trade-off (analysis).

There were short-term operational impacts. Coverage noted major exchanges briefly paused ADA deposits and withdrawals during reconvergence before resuming after patches were applied (exchange impact report). The broader takeaway is that even without a full halt, chain splits can disrupt user experience and infrastructure until the network re-aligns.

Conclusion

The L1 that suffered a chain split was Cardano. The network remained live, recovered within hours, and the episode underlines how client versioning and upgrade discipline affect fault tolerance. The practical implication is short-lived disruption for users and a renewed focus on resilience practices for validators and service providers.