fix(p2p): frame gossipsub msgId and restrict allowedTopics (A-1256)#24214
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The gossipsub msgId was SHA256(topic || data) with no framing between the topic string and the message bytes, and no allowedTopics was set. A peer could publish on an unsubscribed topic T+data[0] with data[1:], whose (topic, data) concatenation is byte-identical to a real message on topic T, hashing to the same msgId. gossipsub transformed it and inserted the id into seenCache before the subscription check, so the genuine proposal/attestation was later dropped as a duplicate, suppressing a time-sensitive consensus message. - Frame the topic length into the msgId input (uint32be(topicLen) || topic || data) so the (topic, data) boundary is unambiguous and a boundary-shifted pair no longer collides. - Set exact allowedTopics on the gossipsub config so an unsubscribed-topic message is rejected before transform / msgId / seenCache insertion. Defense in depth: either fix alone breaks the attack. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Fixes A-1256.
Problem
The gossipsub full message id was
SHA256(topic || data)[0..20]with no framing between the topic string and the message bytes, andLibP2PServiceset noallowedTopics. Raw concatenation isn't injective: for a real message(T, D), a peer can craftT' = T + D[0],D' = D[1:]soT' || D'is byte-identical toT || D→ same msgId. ChainSafe gossipsub transforms the arbitrary-topic message and inserts that id intoseenCachebefore the subscription check (it isn't delivered to us, since we're not subscribed toT'). When the genuine proposal/attestation arrives onT, gossipsub drops it as a duplicate before application validation, peer scoring, or handling — suppressing a time-sensitive consensus message within the slot.Fix (defense in depth — either alone breaks the attack)
uint32be(topicLen) || topic || data(encoding.ts). The topic length pins the(topic, data)boundary, so a boundary-shifted pair no longer collides. (getMsgIdFn's parameter is narrowed toPick<Message, 'topic' | 'data'>— the only fields it reads — which stays assignable to gossipsub'smsgIdFnslot.)allowedTopics(libp2p_service.ts) to the subscribed Aztec topic strings. Verified against the installed gossipsub: the allowlist is enforced inhandleReceivedRpcbeforehandleReceivedMessage/seenCache.put, so an unsubscribed-topic message is dropped before transform / msgId / seenCache.Test
encoding.test.ts: builds a realP2PMessage.toMessageData()buffer (confirmingdata[0] === 0x00), constructs the shifted(T', D'), and asserts the two msg ids now differ (they collided before the framing change); plus a determinism check.Compatibility
msgIdis computed locally for dedup; changing the function doesn't change any on-wire format. During a rolling upgrade, mixed nodes briefly compute different ids for the same message (minor IHAVE/IWANT inefficiency), with no correctness impact.