Zebra CVE-2025-52736: NU5 Same-Header Body Poisoning Denial-of-Service

Vulnerability Overview Vulnerability Name: Block suppression via NU5 same-header body poisoning of sent-hash cache CVE ID: CVE-2025-52736 Severity: High Description: When sending blocks, Zebra records block hashes in to cache them before context validation is complete. If validation fails, the hash is not removed. A remote unauthenticated node can send a block with the same header hash but a different transaction body, causing Zebra to treat it as a duplicate and reject it. This prevents the node from progressing to that height unless it restarts or reorganizes. Scope Affected Versions: (Rust): <= 6.0.0 (Rust): <= 4.4.1 Fixed Versions: (Rust): 7.0.0 (Rust): 4.5.0 Affected Users: Nodes running Zebra 4.4.1 and earlier Nodes accepting inbound P2P connections (default configuration) Nodes processing blocks above the checkpoint height (pre-activation of non-finalized state) Remediation Patch: In Zebra 4.4.2, the issue was fixed by cleaning up entries in the failed non-finalized write path within . Workarounds: Reducing the number of inbound peer connections ( ) can narrow the attack surface, but does not eliminate it entirely. Restarting the node clears the cache in memory, allowing valid blocks to be fetched again. Technical Details Attack Flow: 1. The attacker observes a new block header (from any peer). 2. The attacker constructs a poisoned block body by flipping a byte in the coinbase extra data field. The block hash remains unchanged. 3. The attacker advertises the block's hash to the target node via . 4. The target node requests the block via ; the attacker provides the poisoned block body. 5. The target node adds the hash to before validation. 6. The task rejects the block body because fails (auth_data_root mismatch). 7. The hash is not removed from . 8. When a valid canonical block arrives from an honest peer or RPC, sees the hash in the cache and returns . 9. The node gets stuck at height N-1. Secondary Variant: When chain pruning ( ) removes a chain from but leaves its hash in , it creates the same locking situation, leading to pruning of sub-chains. Impact Remote unauthenticated P2P peers can permanently block a target Zebra node at a specific block height. The node forks from the network, causing downstream consumers (such as lightwallets, wallets, and mining infrastructure) that rely on this node to see a stalled chain. The attack requires winning a propagation race: delivering the poisoned block body before honest peers deliver the canonical block. A well-placed attacker (with low latency to the target and monitoring for new blocks from other peers) can reliably win this race. In a persistent form, the attacker repeats this process for each new block, leaving the target permanently behind. Recovery requires restarting the node (clearing the cache in memory) or waiting for a reorganization on the affected height (on the canonical chain). References ZIP-244: Transaction Identifier and Signature Validation. Defines txid_v5 and auth_digest separation. zcash GHSA-qpm-qfm-gq35. Analogous advisory in zcashd. CWE-460: Improper Cleanup on Thrown Exception CWE-471: Modification of Assumed-Immutable Data Zebra at : Affected . Zebra at : Hash added to sent-hashes before write validation. Credits Independently reported by @jpuering (primary, with ZIP-244 modifiability analysis and zcash cross-reference) and @x15-eth (first reporter, with E2E reproduction and control experiments).

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Zebra CVE-2025-52736: NU5 Same-Header Body Poisoning Denial-of-Service

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