CVE-2026-44500: Allocation Amplification in Inbound Network Deserializers

Summary

Several inbound deserialization paths in Zebra allocated buffers sized against generic transport or block-size ceilings before the tighter protocol or consensus limits were enforced. An unauthenticated or post-handshake peer could therefore force the node to preallocate and parse for orders of magnitude more data than the protocol intended, across headers messages, equihash solutions in block headers, Sapling spend vectors in V5/V4 transactions, and coinbase script bytes in blocks.

Severity

Moderate - This is a Denial-of-Service Vulnerability that could allow a malicious peer to amplify per-message memory and parse cost on Zebra nodes, with effects amplified by multi-peer fan-in.

Each individual case is bounded by the 2 MiB transport ceiling or the block-size cap, so no single message causes unbounded allocation, but the cumulative gap between intended and actual limits is significant.

Affected Versions

All Zebra versions prior to 4.4.0.

Description

Zebra's network codec uses TrustedPreallocate and generic Vec deserialization to bound inbound message parsing. In several places the bound used at the deserializer was the generic transport or block-size ceiling rather than the tighter protocol or consensus rule that applies to the field, so allocation happened first and the real limit was only enforced afterwards. Four such cases were identified:

• headers message receive cap. read_headers() deserialized the CountedHeader vector via the generic TrustedPreallocate path, which allowed up to ~1,409 entries per message. The protocol ceiling MAX_FIND_BLOCK_HEADERS_RESULTS = 160 was only used on the send side, giving an ~8.8x preallocation gap on receive. Reachable before the version handshake completes since the codec is installed on raw bytes.
• Equihash solution length. Solution::zcash_deserialize decoded the solution as a generic Vec<u8> and only checked the exact consensus size (1344 bytes mainnet/testnet, 36 bytes regtest) afterwards in Solution::from_bytes. A single fixed-size header field could be inflated to nearly the full block-size ceiling before rejection.
• Sapling spend vectors in coinbase transactions. V5 spend_prefixes and V4 shielded_spends were allocated generically with block-size-derived ceilings (~5,681 / ~5,208 entries) before the consensus rule that coinbase transactions have zero Sapling spends was enforced in the verifier.
• Coinbase script bytes. Input::zcash_deserialize() read the coinbase script as a generic Vec<u8> up to the message-size cap before enforcing the consensus rule that coinbase scripts are between 2 and 100 bytes.

An attacker could exploit this by:

• Opening an inbound TCP connection (and, for the latter three cases, completing the version handshake).
• Sending one of: a headers message with a CompactSize count up to ~1,409, a block whose header carries an inflated equihash CompactSize, a tx declaring a coinbase input with a large nSpendsSapling, or a block with a coinbase input whose script length is near the message-size ceiling.
• The deserializer allocates against the loose ceiling, parses, and only then rejects.

Impact

Denial of Service

• Attack Vector: Network.
• Effect: Amplified per-message allocation and parse cost on inbound peer messages, stackable across concurrent connections. The concrete effect will be influenced by how much memory Zebra has available.
• Scope: Any affected Zebra node.

Fixed Versions

This issue is fixed in Zebra 4.4.0.

Mitigation

Users should upgrade to Zebra 4.4.0 or later immediately.

There are no known workarounds for this issue. Immediate upgrade is the only way to remove the amplified allocation surface on inbound peer messages.

Credits

Zebra thanks @Zk-nd3r for finding and reporting the issues.