Misskey CVE-2026-46713

Exploitation requires the target Misskey instance (version 12.37.0 up to 2026.5.4) to process federated ActivityPub activities that use JSON-LD/Linked Data signatures - i.e., the server must be participating in federation and accept and validate signed activities from remote actors, which is the default and intended operating mode for a fediverse server. Per the CVSS vector (AV:N/AC:L/PR:N/UI:N), no authentication or user interaction is needed and attack complexity is low. The main limiting factor is federation reachability: the attacker must be able to deliver activities to the victim instance, so instances that block, allow-list, or disable federation with the attacker's server are not exposed via that path. No specific non-default configuration toggle is called out in the available data beyond running an affected version with federation enabled.

The signals are largely consistent toward high priority: the CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N) indicates remote, unauthenticated, low-complexity exploitation with no user interaction, and the impact metrics stress integrity (VI:H, SI:H) with limited confidentiality (VC:L) and no availability impact, yielding a 9.2 Critical rating. This aligns with a signature-spoofing bug whose primary danger is forged/authoritative-looking activities rather than data destruction. Mitigating the raw severity: exploitation requires participation in the federated network (the target instance must accept federation from the attacker), and no EPSS score, POC, or KEV entry is provided, so there is no evidence of active or automated mass exploitation at this time. The 'Information Disclosure' tag partly conflicts with the vector's integrity-dominant profile - the realistic worst case is impersonation/spoofing of activities rather than bulk data leakage. Net: treat as a real, high-priority patch for any internet-facing federated Misskey instance, but current evidence points to potential targeted abuse rather than confirmed widespread exploitation.

A malicious or attacker-controlled fediverse server crafts a JSON-LD ActivityPub activity that exploits the compaction/signature mismatch so that forged content appears to be validly signed by a trusted actor. The victim Misskey instance accepts and processes the spoofed activity (e.g., attributing a post or action to another user or instance), enabling impersonation and integrity manipulation across the federated network. No public exploit has been identified at time of analysis, and the network, low-complexity, unauthenticated vector means exploitation requires only that the target federates with the attacker.