Zebra has pre-handshake buffer capacity reservation based on attacker-claimed body length

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).

Summary

The P2P codec's Codec::decode() method calls src.reserve(body_len + HEADER_LEN) after parsing a 24-byte protocol header, using the attacker-claimed body_len field. This reserves up to MAX_PROTOCOL_MESSAGE_LEN (~2 MiB) of virtual buffer capacity per connection before any body bytes arrive and before the handshake completes.

However, BytesMut::reserve() sets virtual capacity without committing physical memory pages. The operating system does not allocate physical RAM until bytes are actually written into the buffer. Since the attacker never sends body bytes, the reserved capacity remains uncommitted. Reproduction of the reporter's PoC (256 threads, 30 seconds of sustained connections) showed negligible RSS impact on the Zebra process.

Zebra's existing mitigations further constrain the practical attack surface: per-IP connection limits (max_connections_per_ip = 1), a per-connection accept rate of approximately one per second, and a 3-second handshake timeout that cleans up idle connections.

Details

At zebra-network/src/protocol/external/codec.rs:406, after parsing the 24-byte header and validating the network magic and body length against MAX_PROTOCOL_MESSAGE_LEN, the codec calls src.reserve(body_len + HEADER_LEN). The codec is constructed on the bare TCP stream before negotiate_version() runs, so the reservation is reachable from any TCP peer that can send 24 bytes.

No legitimate Zcash handshake message (version, verack) is anywhere close to 2 MiB. The codec makes no distinction between pre-handshake and post-handshake message types when sizing the reservation.

Patches

The fix defers large buffer reservations until after the handshake completes, or caps the per-message reservation for pre-handshake messages to what version/verack actually require.

Workarounds

No workaround is needed. The existing per-IP rate limiting, handshake timeout, and connection limits effectively mitigate the practical impact.

Impact

Minimal. The reservation affects virtual address space only, not physical memory. Zebra's existing connection-management mitigations (per-IP limits, accept rate, handshake timeout) further constrain the attack. The code path is worth cleaning up for defense-in-depth but does not produce a measurable denial-of-service effect.

Credit

Reported by @ouicate via a private GitHub Security Advisory submission.