CVE-2026-52737: Zebra has sync restart poisoning from single unauthenticated peer via above-lookahead block

A malicious peer can answer Zebra’s outbound getblocks/FindBlocks request with a small two-hash inventory, then serve a syntactically valid block whose coinbase height is far above the victim’s local tip. The AboveLookaheadHeightLimit error in the sync download pipeline triggers a global sync restart rather than being scoped to the offending peer. The peer is never scored or disconnected because the error type does not carry the advertiser address.

On mainnet, each successful cycle imposes a 67-second sync restart delay. All in-flight downloads from honest peers are cancelled on each restart.