Input validation error in Synapse - #VU139988
Published: July 29, 2026
Vulnerability identifier: #VU139988
CSH Severity: Low
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/U:Clear
CVE-ID: N/A
CWE-ID: CWE-20
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Affected software:
Synapse
Synapse
Detailed vulnerability description
The vulnerability allows a remote user to cause a denial of service.
The vulnerability exists due to improper input validation in the MSC4186 Sliding Sync response handling when processing room metadata and user profile data. A remote user can join a room with invalid name, avatar, or profile data to cause a denial of service.
Instances that only have trusted local users and either do not federate or only participate in a closed, trusted federation are not affected.
Remediation
Install security update from vendor's website.