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. readheaders() deserialized the CountedHeader vector via the generic TrustedPreallocate path, which allowed up to ~1,409 entries per message. The protocol ceiling MAXFINDBLOCKHEADERSRESULTS = 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::zcashdeserialize 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::frombytes. 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 spendprefixes and V4 shieldedspends 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::zcashdeserialize() 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.
CVE-2026-40881: addr/addrv2 Deserialization Resource Exhaustion
Summary
When deserializing addr or addrv2 messages, which contain vectors of addresses, Zebra would fully deserialize them up to a maximum length (over 233,000) that was derived from the 2 MiB message size limit. This is much larger than the actual limit of 1,000 messages from the specification. Zebra would eventually check that limit but, at that point, the memory for the larger vector was already allocated. An attacker could cause out-of-memory aborts in Zebra by sending multiple such messages over different connections.
Severity
Moderate - This is a Denial of Service Vulnerability that could allow an attacker to crash a Zebra node.
Affected Versions
All Zebra versions prior to version 4.3.1.
Description
The vulnerability exists in the readaddr/addrv2 functions in codec.rs. It deserializes a vector of addresses with the zcashdeserialize() trait method, which uses as a upper bound the result of T::maxallocation(). For theses types, it was derived from dividing the max message size (2 MiB) by the minimum serialized size of one entry. For AddrV1: 2097152 / 30 = 69,904. For AddrV2: 2097152 / 9 = 233,016. Only after deserialization was the MAXADDRSINMESSAGE = 1000 limit checked.
An attacker could exploit this by: 1. Creating addr or addrv2 messages with a large number of entries. 2. Submitting them to a Zebra node, possibly through multiple connections, to attempt to get Zebra into an out-of-memory state.
Impact
Denial of Service
Attack Vector: Network. Effect: Zebra node crash. Scope: Any impacted Zebra node.
Fixed Versions
This issue is fixed in Zebra 4.3.1.
The fix changes the maxallocation() method for the relevant types to return 1,000, thus blocking larger values prior to deserialization.
Mitigation
Users should upgrade to Zebra 4.3.1 or later immediately.
There are no known workarounds for this issue. Immediate upgrade is the only way to ensure the node remains not vulnerable to the denial of service attack.
Credits
Thanks @Zk-nd3r for finding and reporting the issue, and suggesting the fix.