One of Ethereum's consensus clients has merged a fix for a validator pubkey cache poisoning attack, according to a pull request on GitHub. The patch addresses a vulnerability related to the Gloas fork, which is part of the upcoming Glamsterdam upgrade, and could result in some nodes rejecting valid signatures due to a bug in the global pubkey cache.
The vulnerability exists in a mechanism where the client stores validator public keys and their indexes in a global cache. At the fork boundary, an attacker could potentially exploit this by using three deposits just before the fork: a large top-up for an existing validator, plus two new keys with different credential types.
The attack scenario involves the attacker creating a side chain where a topped-up validator gets slashed before the fork, causing its deposit to be skipped. This would allow one validator key to be processed earlier and take another key's index. On the canonical chain, however, the normal deposit order would differ, creating a mismatch. The buggy cache would incorrectly assume both chains produced the same validators, importing keys globally and causing affected nodes to reject valid signatures from the canonical chain.
While theoretically viable, the attack is complex in practice, requiring precise slashing timing and a side chain, making accidental occurrence unlikely.
The Fix
According to the pull request, the solution involves only adding validator keys to the global pubkey cache once their activation eligibility epoch is finalized. A new HeadPubkeys structure combines keys from the head state and the global cache, ensuring nodes do not miss keys if the chain finalizes without new blocks.
Developers are still determining whether other consensus clients have similar vulnerabilities. As the Glamsterdam upgrade approaches, additional edge cases around the fork boundary continue to emerge.
