Various users have been reporting Ethereum nodes running on Parity Technologies’ ETH Client have been “randomly falling out of sync.” After investigation the issue, Parity issued an emergency fix warning an attack may be underway.

According to blockchain services firm Bitfly, multiple reports pointed out nodes using Parity seemingly simply stopped synchronizing with the Ethereum mainnet. Responding to Bitfly’s tweet one user confirmed his nodes stopped syncing, but restarting Parity seemed to be a temporary fix.

The issue seems to have first been spotted on mainnet block 11355 after GitHub user Peter Prascher revealed on the platform his client would stop syncing and only resumed after a restart, showing errors when it wasn’t keeping up with the Ethereum blockchain.

Parity Technologies reacted to the reports revealing that after investigating them, it found “there may be an attack underway.” In response it issued a new release.

The organization has urged all of its users to update to the newest version as soon as possible, whether or not they’re experiencing issues with their Ethereum nodes. Shortly after ETC Cooperative asked Ethereum Classic Parity node operators to upgrade as well, adding the vulnerability also affected the cryptocurrency.

ETC Cooperative later explained how the attack worked:

The attack used a cache poisoning vulnerability, where a carefully crafted invalid block could stall the Parity-Ethereum client. MultiGeth and Hyperledger Besu are not vulnerable.

The attack came shortly before the Ethereum network is set to undergo a scheduled hard fork, dubbed “Muir Glacier.” The update has been scheduled for block number 9,200,000, which is expected to occur on January 1, 2020.

The hard fork will delay the planned increase of Ethereum’s mining difficulty, dubbed the “Ice Age” by an estimated 611 days. The goal is to prepare for the cryptocurrency’s transition from a Proof-of-Work (PoW) consensus algorithm to a Proof-of-Stake (PoS) consensus algorithm.

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