CVE-2026-52732

ZEBRA is a Zcash node written entirely in Rust. Prior to 4.5.0, one unauthenticated P2P peer can monopolize all 25 MAX_INBOUND_CONCURRENCY slots in Zebra's inbound mempool download and verification pipeline. In zebrad/src/components/mempool/downloads.rs, the bounded queue was shared globally without per-peer accounting, while peer identity was not carried through Gossip and FullQueue responses were mapped to Response::Nil instead of reaching overload disconnection handling. An attacker can advertise fake transaction identifiers and remain silent so each task holds a slot until TRANSACTION_DOWNLOAD_TIMEOUT, then periodically refill the queue as slots expire. While saturated, honest peer transactions and local sendrawtransaction requests are rejected with MempoolError::FullQueue, although block validation and synchronization continue. This issue is fixed in version 4.5.0.

ZEBRA is a Zcash node written entirely in Rust. Prior to 4.5.0, one unauthenticated P2P peer can monopolize all 25 MAX_INBOUND_CONCURRENCY slots in Zebra's inbound mempool download and verification pipeline. In zebrad/src/components/mempool/downloads.rs, the bounded queue was shared globally without per-peer accounting, while peer identity was not carried through Gossip and FullQueue responses were mapped to Response::Nil instead of reaching overload disconnection handling. An attacker can advertise fake transaction identifiers and remain silent so each task holds a slot until TRANSACTION_DOWNLOAD_TIMEOUT, then periodically refill the queue as slots expire. While saturated, honest peer transactions and local sendrawtransaction requests are rejected with MempoolError::FullQueue, although block validation and synchronization continue. This issue is fixed in version 4.5.0.

### 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.

### 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.

ZEBRA es un nodo Zcash escrito completamente en Rust. Antes de la versión 4.5.0, un par P2P no autenticado puede monopolizar las 25 ranuras MAX_INBOUND_CONCURRENCY en la tubería de descarga y verificación de la mempool entrante de Zebra. En zebrad/src/components/mempool/downloads.rs, la cola limitada se compartía globalmente sin contabilidad por par, mientras que la identidad del par no se transmitía a través de Gossip y las respuestas FullQueue se mapeaban a Response::Nil en lugar de alcanzar el manejo de desconexión por sobrecarga. Un atacante puede anunciar identificadores de transacción falsos y permanecer en silencio para que cada tarea retenga una ranura hasta TRANSACTION_DOWNLOAD_TIMEOUT, luego rellenar periódicamente la cola a medida que las ranuras expiran. Mientras está saturado, las transacciones de pares honestos y las solicitudes locales de sendrawtransaction son rechazadas con MempoolError::FullQueue, aunque la validación y sincronización de bloques continúan. Este problema está solucionado en la versión 4.5.0.