GHSA-4fc2-h7jh-287c: 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 seco