Improper access control in Synapse - #VU139996
Published: July 29, 2026
Vulnerability identifier: #VU139996
CSH Severity: Low
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:A/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U/U:Clear
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 user to change the destination of room aliases.
The vulnerability exists due to improper access control in room alias handling when participating in federated rooms. A remote user can cause a room alias to be redirected to a different room to change the destination of room aliases.
Homeservers that do not federate or only participate in a closed, trusted federation are not affected. Exploitation requires a malicious federated homeserver and either conspiring local authenticated users or a local user being tricked into joining a malicious federated room.
Remediation
Install security update from vendor's website.