Zebra has sync restart poisoning from single unauthenticated peer via above-lookahead block

Am I affected

You are affected if:

1. You run zebrad up to and including v4.4.1.
2. Your node accepts inbound P2P connections and is syncing or catching up to the chain tip.

Summary

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.

Details

The bug is the interaction of three layers:

1. The syncer promotes unvalidated FindBlocks peer responses into concrete download schedules without checking that the advertised hashes are plausible chain extensions.

2. When a downloaded block's coinbase height exceeds tip + VERIFICATION_PIPELINE_DROP_LIMIT, the sync downloader returns BlockDownloadVerifyError::AboveLookaheadHeightLimit. This error variant carries only the block height and hash, not the advertiser peer address.

3. The sync error handler in handle_block_response only sends misbehaviour scores for BlockDownloadVerifyError::Invalid errors that carry an advertiser_addr and have a nonzero misbehavior_score(). AboveLookaheadHeightLimit falls through to the default restart-worthy path, cancelling all in-flight downloads and waiting 67 seconds before restarting sync.

The attacker needs only an unauthenticated P2P connection (post-handshake), a tiny payload (one two-hash inv message plus one small block per cycle), and no mining capability, funds, or valid chain data. The peer is never penalised, so the attack is repeatable indefinitely.

Additionally, several other pre-consensus sync-layer errors had zero misbehaviour scores even when peer-attributed. Contextual validation failures (InvalidDifficultyThreshold, TimeTooEarly, TimeTooLate, NonSequentialBlock) and locktime failures from block-serving peers all scored zero, allowing repeated abuse without penalty.

Patches

Patched in Zebra 4.4.2. The fix:

• Carries advertiser_addr through AboveLookaheadHeightLimit and InvalidHeight error variants.
• Makes above-lookahead and invalid-height failures peer-local (the block is dropped and the peer is banned with score 100) rather than triggering a global sync restart.
• Expands misbehavior_score() across BlockError, VerifyBlockError, and CommitBlockError to cover contextual validation failures that previously scored zero.

Workarounds

No configuration-level workaround is available. The attack is mitigated by having a diverse honest peer set, but cannot be prevented while the vulnerable code is running.

Impact

A single unauthenticated peer can repeatedly stall a syncing Zebra node by forcing 67-second global sync restart cycles. All unrelated in-flight downloads are cancelled on each restart. The node does not crash and no state is corrupted, but sync progress is significantly degraded for the duration of the attack.

Credit

Reported by @ipwning.