Polygon issued Heimdall repair while its bridge remained stopped

Polygon’s proof-of-stake network operators received Heimdall v0.2.7 upgrade instructions at 08:07 UTC on March 12, 2022, moving the chain’s consensus and state-sync recovery beyond the temporary Bor workaround deployed after a March 10 halt. The repair was material, but incomplete: Polygon’s operator notice explicitly said to keep the Heimdall bridge service stopped until further notice.

That distinction defines the March 12 development. Validators were not told that every service had returned to normal. They were told to install version 0.2.7, restart Heimdall and its REST server, verify that block height was increasing, and recover from a March 12 snapshot if their node produced an AppHash mismatch. The update was a coordinated recovery step for network operators while bridge functions remained restricted.

How a state-sync fix became a chain halt

Polygon PoS used two closely connected components. Heimdall, built on Tendermint, handled validator consensus and communications for state synchronization between Ethereum bridge contracts and Polygon. Bor produced user-facing blocks but relied on Heimdall to select producer committees and define blocks of work known as spans.

Polygon’s March 28 postmortem said the incident began when a large state-sync transaction exposed a data-size check error in Heimdall v0.2.5. Developers released v0.2.6 on March 10, reducing the relevant limit from roughly 100 kilobytes to 50 kilobytes. Polygon later acknowledged that the change required a mandatory upgrade or hard fork but was mistakenly released as optional.

When nodes running different rules encountered the transaction, their application states diverged. Polygon identified an AppHash mismatch at Heimdall height 8,588,756. Heimdall stopped progressing, and Bor eventually exhausted the spans already supplied by Heimdall and halted too.

A Bor v0.2.14 hotfix hard-coded roughly 50 spans and restarted block production. Contemporaneous reporting on March 11 described that measure as temporary and placed the stoppage at roughly eight hours. Block production, however, was not the same as full infrastructure recovery: Polygon’s own timeline said bridges remained offline after Bor resumed.

What v0.2.7 changed on March 12

Polygon’s later technical account said Heimdall v0.2.7 overwrote the 50 hard-coded spans and added a limited rollback that returned nodes to the last working application state at height 8,588,755. Operators were instructed to restart gradually and isolate the recovering network from nodes still running v0.2.6. The goal was to rebuild the two-thirds-plus-one validator consensus threshold and replay state-sync transactions under one consistent rule set.

The March 12 forum instructions reveal the limits of the recovery as it happened. Operators were given new seed peers, commands for checking whether height advanced, and a snapshot path for nodes that could not replay cleanly. Replies posted on March 13 reported peer-connectivity and synchronization problems. Those reports do not establish a network-wide failure, but they caution against treating the patch notice itself as proof of uninterrupted service.

Polygon’s later timeline records that internal and external RPC nodes returned after the v0.2.7 rollout. Polygon also said no user funds or data were lost. The latter was the project’s incident finding; this reconstruction does not substitute it for an independent forensic audit.

Later context

On March 13, 2022, Polygon announced that the bridge, deposits and withdrawals had resumed and that pending state-sync transactions had been processed. On March 17, validators received Heimdall v0.2.8, a hard fork intended to reconcile the temporary spans at block 8,664,000 and restore normal span creation.

The bounded conclusion for March 12 is therefore a partial but consequential recovery: Polygon distributed the consensus-layer repair and brought more node infrastructure back, while deliberately keeping the bridge stopped pending validation. The episode exposed how an error in state synchronization could propagate through Polygon’s coupled Heimdall-and-Bor architecture into user-visible downtime.