Auth bypass in Matrix-Org Dendrite
CVE-2026-63097
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.
Vulnerability class: Broken Access Control
Published · last modified .
CVSS v3 metric
CVSS v3 base score 4.3 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N.
EPSS exploit prediction
EPSS: 0.003 (17.8th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 18th percentile — 17.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: 0.000
Affected products
- Matrix-Org Dendrite — versions 0
Weakness classification (CWE)
References
- github.com/geo-chen/oss/blob/main/dendrite.md (technical-description, exploit)
- www.vulncheck.com/advisories/dendrite-syncapi-context-endpoint-post-leave-state… (third-party-advisory)
Frequently asked questions
- What is CVE-2026-63097?
- CVE-2026-63097 is a medium-severity vulnerability in Matrix-Org Dendrite, classified under Incorrect Authorization. CVSS score: 4.3/10. Published 2026-07-17.
- How severe is CVE-2026-63097?
- Medium severity. CVSS v3 base score is 4.3 out of 10.