Improper access control in Synapse - #VU139990
Published: July 29, 2026
Vulnerability identifier: #VU139990
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: N/A
CWE-ID: CWE-284
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Affected software:
Synapse
Synapse
Detailed vulnerability description
The vulnerability allows a remote attacker to disclose event IDs from an unjoined room.
The vulnerability exists due to improper access control in the federation /timestamp_to_event endpoint when handling requests for the nearest event to a timestamp. A remote attacker can send a request for a room ID it knows to disclose event IDs from an unjoined room.
Only homeservers participating in federation are affected.
Remediation
Install security update from vendor's website.