Dendrite CVE-2026-63097

The attacker must hold a valid authenticated account on the target Dendrite homeserver (CVSS PR:L confirmed). They must have previously joined the target room and subsequently left - the RoomExists field evaluates true for any room the server knows about, while IsInRoom is false for departed users, creating the bypass condition. The /context endpoint must be reachable over the network (standard Matrix Client-Server API port, typically 8448 or 443). Exploitation is version-gated to Dendrite 0.13.8 and earlier; other Matrix homeserver implementations are unaffected. The data disclosed is limited to room state events (not arbitrary message history), and history visibility room settings do not compensate for this bypass since the check fails before visibility rules are applied. Exploitation is limited by the requirement for a pre-existing account; fully unauthenticated or external attackers cannot exploit this vulnerability.

The CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:L/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N) accurately reflects the attack profile: network-reachable, low complexity, no attack requirements, but constrained to authenticated users (PR:L) with limited confidentiality impact (VC:L). The requirement for a valid account on the target Dendrite instance substantially limits the realistic attacker pool compared to unauthenticated vulnerabilities. The data exposed is room state events - membership lists, room topics, and similar state - rather than full message history, capping the confidentiality damage. No EPSS score was provided in the input data; no CISA KEV listing exists, confirming no confirmed widespread active exploitation. However, a public PoC is available via the geo-chen GitHub repository and documented by VulnCheck, meaning exploitation requires minimal skill for any actor with a valid account. The real-world priority is moderate: Dendrite deployments hosting sensitive rooms (e.g., private organizational spaces) should patch promptly due to the low exploitation barrier, but the authenticated prerequisite and limited data scope prevent this from being a critical emergency.

An authenticated user who previously joined and departed a private Matrix room hosted on a Dendrite server calls GET /_matrix/client/v3/rooms/{roomId}/context/{eventId} using an event ID they observed before leaving. Because context.go checks only RoomExists rather than the full membership state, the server returns unfiltered current room state - including current membership lists, room topic, and other state events - that the /sync and /messages endpoints would correctly withhold. A public PoC demonstrating this exact sequence is available at the geo-chen GitHub repository.