ZEBRAD · CVE-2026-52732 · Package
FIX → 4.5.0
A single unauthenticated peer can fill all 25 inbound mempool download slots by advertising fake transaction IDs, so honest transactions and local sendrawtransaction calls are rejected while the attack lasts. The attacker is never scored or disconnected, so the denial can be sustained with about 1 KB/s of traffic.
first public May 29, 2026 · 2 sources
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Any node running zebrad at or below 4.4.1 that accepts inbound P2P connections and has its mempool active, which is the default configuration. Nodes that do not accept inbound connections, or that are not synced to the chain tip, are not exposed. Block validation and chain sync are not affected.
Not affected if listen-addr-unset, not-synced-to-tip
Who cares p2p-node, self-hosted, zcash, zebrad
zebrad --version and compare against the affected range <= 4.4.1.network.listen_addr is set in the zebrad config (it is set by default).Upgrade to 4.5.0
4.5.0.You are affected if:
zebrad up to and including v4.4.1.network.listen_addr is set, which is the default).All default configurations are affected.
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.
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:
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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.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.
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.
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.
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.
Reported by @dingledropper via a private GitHub Security Advisory submission.
This page as .md · .json · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
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