November 2025 Incident: Cardano Shows Its Resilience

Good technology can resist stress scenarios. That’s exactly what happened on 21 November 2025 when Cardano faced an incident on testnet and mainnet.

As a chain partition temporarily occurred, stake pool operators (SPOs), the Cardano Foundation, IOG, Intersect’s security council, EMURGO, and the wider Cardano ecosystem all came together to diagnose and triage the issue. The clear directions given to SPOs helped all chain maintainers to act quickly and leverage the inherent characteristics of Ouroboros, Cardano’s proof-of-stake (PoS) consensus mechanism.

The chain never went down and, after about 14 hours, the issue was resolved. The forked chain self-repaired and, as intended, the longest chain won.

Such a rapid recovery in the face of a live, unplanned, and high-stakes stress event shows just how resilient Cardano’s proof-of-stake architecture is. It also highlights the maturity of the Cardano ecosystem.

Explaining the incident

The situation started on the testnet environment: A buggy transaction was handled differently in different versions of a library. One version rejected the transaction, while the newer version accepted it. This led to a hard fork.

A fix was promptly provided to contain the situation. However, while further investigation of the bug took place, a single malformed transaction was submitted to mainnet.

Because a significant portion of the nodes had not yet implemented the fix, many nodes accepted the malformed transaction. Simultaneously, older node versions rejected the transaction.

As a consequence, a split happened on mainnet and two ledger histories started running in parallel:

• the “pig” chain version containing the malformed transaction,
• and the “chicken”, canonical chain without the malformed transaction.

As more and more nodes implemented the fix, the canonical chain started catching up. After about 14 hours, the canonical chain overtook the pig chain.

Immediately, and in accordance with the expected workings of the Ouroboros consensus mechanism, the network’s combined stake began to follow the longest, most widely-validated path. All nodes adopted the canonical chain version, merging both chains into one.

Not only did the Cardano blockchain resume its normal functioning, it operated as intended.

The incident’s impact

Block production never stopped, and the chain continued to operate. Nonetheless, the network was fragmented for several hours.

This led to four main practical consequences.

• Slower times: as some nodes produced blocks on a different chain instead of all nodes contributing to the same chain, the network slowed down.
• Inconsistent data: while the incident lasted, blockchain explorers displayed conflicting information.
• Service disruption: exchanges temporarily suspended ada deposits and withdrawals.
• User uncertainty: transaction confirmation times stretched, and decentralized finance (DeFi) applications experienced some inconsistency.

A resilient decentralized system

This was the first major consensus-level disruption in Cardano's eight-year history, and the ecosystem fully showed its maturity and ability to withstand stress events.

Within three hours of detecting the issue, engineers from IOG, Intersect, and the Cardano Foundation had already developed and deployed an emergency patch to resolve it.

SPOs then swiftly upgraded their nodes, proving not just the strength of the Cardano ecosystem but indeed the advantages of a decentralized system.

Because no central authority commands the network, the malformed transaction didn’t propagate to the entire chain. At the same time, the SPOs’ quick and decisive response allowed the healthy chain version to grow and become the dominant one.

Not only did the Cardano ecosystem resolve the situation efficiently, it did so without compromising decentralization. In fact, the events of 21 November proved that decentralization does not negatively impact the resolve of high-stress, potentially adverse scenarios.

Cardano’s Ouroboros proof-of-stake performed exactly as designed to do under duress.

• No halt: blocks continued to be produced on both branches, preventing a total network blackout and meaning the chain never really stalled.
• Canonical chain selection: as SPOs upgraded their nodes to incorporate the fix, they extended the chicken, canonical chain, activating the consensus mechanism rule of always adopting the longest chain.
• Swift resolution: within approximately 14 hours, all nodes automatically moved to the canonical chain and Cardano converged back to a single, synchronized, secure ledger.

Key takeaways

The November 2025 incident reinforced the maturity and cooperation for the greater good, common within the Cardano ecosystem. It also showed why Cardano is a leading proof-of-stake ecosystem with design principles respected across academia and businesses alike.

As we move forward, three key aspects are worth remembering:

1. Network integrity: block production continued even during the split with the Ourobors consensus mechanism inherently preventing a total block halt.
2. Governance: the key Cardano development organizations immediately coordinated efforts with SPOs, proving the ecosystem’s ability to execute critical, unified upgrades under pressure.
3. Decentralization: the SPOs’ distributed validator base rapidly and reliably executed the fix, while also preventing the automatic propagation of a malformation to all nodes.

This November incident provided a valuable, albeit costly, real-world stress scenario.

Ultimately, the entire episode reinforces the importance of decentralization and the power of Cardano’s Ouroboros consensus mechanism. Together, they work to withstand different types of attack vectors, strengthening the platform for long-term scalability and institutional adoption.