Heimdall fault caused a temporary block finality disruption on Polygon PoS

A technical issue in Polygon’s Heimdall component recently led to a roughly one-hour disruption in block finality on the Proof-of-Stake (PoS) chain. The incident began when a validator exited the network, triggering an unforeseen condition in Heimdall the layer responsible for validation and state synchronization. The Polygon Foundation quickly responded, issuing a technical report that outlined both immediate fixes and longer-term improvements.

Incident overview

The disruption lasted about an hour and occurred due to an unhandled state condition following a validator’s exit. During this time, Heimdall built on Tendermint lost sync with Bor (Polygon’s block producer layer), leading to a temporary halt in finality. Block production and verification were impaired until the issue was resolved. The Foundation has since released a detailed timeline and remediation plan.

Impact on network, infrastructure and users

Several RPC providers experienced synchronization errors, which in turn affected dApps, wallets, and end-users. Incorrect state data was temporarily served, resulting in failed transactions, higher latency, and service interruptions. The event highlighted the risks of over-reliance on centralized infrastructure and single points of failure within the network stack.

Root cause and fixes

The fault originated in a state process handling mechanism within Heimdall that had not been adequately tested for validator exit scenarios. This prevented nodes from achieving finality and caused RPCs to return inaccurate data. The incident also revealed gaps in integration testing following earlier Heimdall upgrades.

Immediate technical fixes

The Foundation released a patch for Heimdall, performed controlled node restarts, and improved state-process handling to prevent a recurrence. These short-term measures were coupled with the publication of a full incident report and a commitment to enhanced testing protocols.

Operational and longer-term measures

Longer-term solutions include optimizations to dynamic gas limits and the introduction of bulk state sync transactions. More significantly, Polygon is planning a architectural merge of Heimdall and Bor (often referred to as v3) to reduce complexity and eliminate single points of failure. These steps are aimed at strengthening synchronization and improving network resilience.