Vulnerability · Published Jul 17, 2026 · Assigned by VulnCheck

CVE-2026-63097Dendrite 0.13.8 syncapi /context Endpoint Post-Leave State Exposure

Medium 4.3

In plain English

A medium-severity incorrect authorization in dendrite from matrix-org. Proof-of-concept exploit code is public. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 19% of all known vulnerabilities).

Severity (CVSS 3.1)4.3Medium

Exploit odds (30 days)0.3%top 81.1% of all CVEs

Exploited in the wildNo reportsCISA SSVC: poc

Attack toolingNoneno public tooling known

Official description

Dendrite through 0.13.8 contains an improper access control vulnerability in the syncapi /context endpoint (syncapi/routing/context.go) that allows authenticated local users to access post-leave room state events by exploiting a flawed membership check that evaluates only the RoomExists field while ignoring IsInRoom, HasBeenInRoom, and Membership fields. Attackers who have left a room can call the rooms context API endpoint for a previously permitted event and receive unfiltered current room state that the /messages and /sync endpoints correctly withhold.

Affected products (as reported)

Vendor	Product	Versions
matrix-org	dendrite	0 – through 0.13.8

Weakness: CWE-863 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N

References

Researcher Disclosure technical-description, exploit
https://www.vulncheck.com/advisories/dendrite-syncapi-context-endpoint-post-leave-state-exposure third-party-advisory