The vulnerability in Zebra consensus (CVE-2026-52737) arises from the syncer promoting unvalidated FindBlocks peer responses into download schedules without plausibility checks. 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 with a 67-second delay instead of isolating and penalizing the offending peer. An unauthenticated malicious peer can exploit this by sending a small two-hash inventory and a syntactically valid block with an excessively high coinbase height repeatedly, causing indefinite sync restarts without penalty. Additional contextual validation errors also lacked misbehavior scoring, allowing repeated abuse. The issue is fixed in Zebra 4.4.2 by carrying the advertiser address in error variants, making failures peer-local with banning, and expanding misbehavior scoring to cover previously unscored errors.

An unauthenticated single peer can repeatedly stall a Zebra node's sync process by forcing global sync restarts every 67 seconds, cancelling all in-flight downloads from honest peers. This results in significant degradation of sync progress but does not crash the node or corrupt its state. The attack is repeatable indefinitely due to lack of peer penalization in vulnerable versions.

A patch is available in Zebra version 4.4.2 that addresses this vulnerability by adding peer-specific error handling and banning misbehaving peers to prevent global sync restarts. Users should upgrade to Zebra 4.4.2 or later. There are no configuration-level workarounds; mitigation depends on running the patched software. Maintaining a diverse set of honest peers may reduce attack impact but does not prevent it.