Misskey EUVD-2026-52488

Exploitation requires that the target Misskey instance has federation (ActivityPub) enabled and is actively processing incoming signed JSON-LD messages from external federated servers - this is the default operational mode for Misskey deployments. The attacker must be able to deliver a crafted ActivityPub message to the target, either by operating a malicious federated server reachable by the target or by relaying a manipulated message through the federation network. AC:H indicates the exploit requires carefully structured JSON-LD payloads that produce divergent context resolution between the verification and processing phases, which is technically demanding. No authentication to the target instance is required (PR:N). Instances running versions prior to 12.37.0 are explicitly not affected. Instances with federation disabled are not exposed to this attack vector.

The CVSS 4.0 score of 8.9 is driven by the network attack vector (AV:N), no required privileges (PR:N), no user interaction (UI:N), and high integrity impact on both the vulnerable system (VI:H) and subsequent systems (SI:H), reflecting that fraudulent federated activities could propagate across the ActivityPub network. The AC:H rating is the primary mitigating factor - crafting a valid exploit requires precise JSON-LD payload construction to exploit the context divergence between the two processing phases, which is technically non-trivial. No KEV listing is present, and no public POC has been identified, indicating no observed widespread exploitation. EPSS data is not provided in the input. Despite the high CVSS score, the combination of AC:H complexity, the niche deployment footprint of self-hosted Misskey instances, and the absence of public exploit code places this at moderate near-term mass-exploitation risk. For operators of federated Misskey instances, however, integrity of federated activities is foundational to platform trust, making this a genuine priority despite the lack of observed exploitation.

An attacker operates a malicious ActivityPub-compatible federated server and crafts a JSON-LD signed message designed to exploit the context mismatch in Misskey's verification pipeline - for example, embedding a forged follow relationship or spoofed activity from a trusted federated peer. The message's cryptographic signature passes verification under one JSON-LD context interpretation, but when the message is processed under a different context during compaction, the fraudulent payload content is accepted as legitimate, allowing the attacker's counterfeit activity to persist in the target instance. No public exploit code is known at time of analysis.