zebrad has mempool transaction admission denial via single-peer inbound queue saturation

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 (network.listen_addr is set, which is the default).
3. Your node's mempool is active (node is synced near the chain tip).

All default configurations are affected.

Summary

A single unauthenticated P2P peer can monopolize all 25 inbound mempool download/verification slots (MAX_INBOUND_CONCURRENCY) by advertising fake transaction IDs. While the slots are occupied, all other inbound transactions from honest peers and local RPC sendrawtransaction calls are rejected with MempoolError::FullQueue. The attacker peer is never scored for misbehavior and is not disconnected, allowing sustained denial of mempool admission.

Details

The mempool download/verification pipeline at zebrad/src/components/mempool/downloads.rs uses a single bounded pool of 25 concurrent tasks. Three architectural gaps combine to produce the vulnerability:

1. No per-peer accounting: the 25 slots are shared across all peers with no cap on how many a single peer can hold.
2. No overload signaling: when FullQueue is returned, the inbound service at zebrad/src/components/inbound.rs maps it to Response::Nil, hiding the overload from the peer connection layer. The existing handle_inbound_overload disconnection logic never fires.
3. No misbehavior attribution: peer identity is not carried through the Gossip type into the download pipeline, so verification failures cannot be attributed to the originating peer.

The attacker sends inv messages advertising fake transaction IDs. Zebra queues download tasks for each ID. The attacker stays silent; each slot is held until the TRANSACTION_DOWNLOAD_TIMEOUT (20 seconds) fires. The attacker periodically sends fresh inv waves to re-fill slots as they expire.

Two additional slot-holding techniques have been independently demonstrated: invalid-prevout transactions that park in AwaitOutput for 60 seconds, and expensive shielded proof verification with auth-variant cache bypass. All three techniques are addressed by the same per-peer accounting fix.

Patches

zebrad 4.5.0

The fix adds per-peer queue accounting to the mempool download pipeline. A single peer is limited to a fraction of MAX_INBOUND_CONCURRENCY (e.g., 5 slots out of 25). FullQueue is surfaced as an overload signal to the peer connection layer. Peer identity is plumbed through the Gossip type for misbehavior attribution.

Workarounds

There is no complete configuration-level workaround. Reducing network.peerset_initial_target_size limits the total inbound peer count but does not prevent a single peer from holding all mempool slots.

Impact

Mempool transaction admission is denied for all honest peers and local RPC clients while the attack is sustained. Block validation and chain synchronization continue normally. The attacker needs only one TCP connection and minimal bandwidth (~1 KB/s of fake inv messages). The node recovers immediately when the attacker stops. This does not affect consensus, funds, or on-disk state.

Credit

Reported by @dingledropper via a private GitHub Security Advisory submission.