The vulnerability arises from Zebra's syncer promoting unvalidated FindBlocks peer responses into download schedules without verifying the plausibility of advertised hashes. When a block's coinbase height exceeds the allowed lookahead limit, the sync downloader returns an AboveLookaheadHeightLimit error that lacks the advertiser peer address. Consequently, the sync error handler triggers a global sync restart rather than isolating the offending peer. An unauthenticated malicious peer can exploit this by repeatedly sending a small inventory and a syntactically valid block with an excessively high coinbase height, causing repeated 67-second sync restarts and cancelling all honest peer downloads. The peer is never penalized, allowing indefinite exploitation. The patch in Zebra 4.4.2 addresses this by carrying the advertiser address in errors, banning offending peers with a high misbehavior score, and expanding misbehavior scoring to other contextual validation failures.

An unauthenticated single peer can repeatedly stall the sync process of a Zebra node by forcing global sync restarts with a 67-second delay each cycle. This cancels all ongoing downloads from honest peers, significantly degrading sync progress. The node remains operational without crashing or corrupting state. The attack requires minimal resources from the attacker and can be repeated indefinitely due to lack of peer penalty in vulnerable versions.

A patch is available in Zebra version 4.4.2 that fixes this vulnerability by improving error handling and peer banning. Users should upgrade to Zebra 4.4.2 or later to remediate this issue. No configuration-level workaround exists. Maintaining a diverse set of honest peers can mitigate the attack impact but does not prevent exploitation while running vulnerable versions.