Misskey CVE-2025-66402

Exploitation requires an authenticated account on the target Misskey instance (PR:L) and the ability to add specific private posts - followers-only or direct notes the attacker cannot normally view - to the attacker's own favorites or clips, which presupposes obtaining those posts' note IDs/URLs (the advisory notes pinned private post IDs can be harvested from the victim's user page). The attacker then triggers the standard favorites/clips export job, which in affected versions skipped visibility enforcement. Limiting factors: a valid local account is mandatory, the attacker must already know or discover the private note identifiers, and only confidentiality of exposed notes is affected - there is no integrity or availability impact and no unauthenticated path.

This is a missing-authorization flaw (CWE-862) where the favorites/clips export feature in Misskey fails to re-apply note visibility checks, letting an authenticated user exfiltrate the contents of followers-only and direct posts they were never permitted to read. The provided CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:N) scores 7.1 and is internally consistent with the description: network reachable, low attack complexity, low privileges (a valid account), no user interaction, and a pure confidentiality impact with no integrity or availability effect. A working PoC is published in the GHSA advisory and the patch commit is public, so publicly available exploit code exists; however, there is no CISA KEV listing and no EPSS value was supplied, so there is no evidence of active or widespread exploitation - the signals point to a disclosed, easily reproducible but not-yet-weaponized data-exposure bug rather than an urgent mass-exploitation threat. The practical ceiling on impact is that the attacker must first obtain the target's private note IDs (e.g., via pinned private posts shown on the user page) and must have an account on the same instance, which makes this a targeted-disclosure risk rather than a trivial bulk scrape.

An authenticated user on a Misskey instance obtains the IDs/URLs of another user's followers-only or direct posts (for example from pinned private posts visible on the victim's profile), adds those posts to their own favorites or clips, and then uses the built-in export feature; the exported file contains the full private post contents despite the attacker never having view permission. A step-by-step PoC confirming this (verified on v2025.11.1) is published in the GHSA advisory, and the fix adds visibility-query enforcement to the export processors.