Polygon Labs said Polygon PoS nodes running outdated Bor or Heimdall clients have fallen out of consensus following the Austin and Kyoto hard forks and must upgrade to rejoin the network. Austin activated at mainnet block 91,949,700 and requires Bor v2.10.0 or later, while Kyoto activated at height 51,533,000 and requires Heimdall v0.11.0 or later. Austin addressed two Bor resource exhaustion risks tied to uncounted L1-to-L2 bridge state sync events and an unlimited TxDependency field that could crash peer nodes. Kyoto fixed vulnerabilities involving deeply nested message attacks, fee coin scanning, and checkpoint signature recovery. Both upgrades were binary-only with no state migration required, and non-forked nodes do not need to resync, though operators on older clients should upgrade and restore synchronization if needed.