Insufficient verification of data authenticity in Synapse - #VU139993
Published: July 29, 2026
Vulnerability identifier: #VU139993
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: N/A
CWE-ID: CWE-345
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Affected software:
Synapse
Synapse
Detailed vulnerability description
The vulnerability allows a remote attacker to spoof the origin of to-device messages.
The vulnerability exists due to improper origin validation in to-device message handling when processing federation messages from remote homeservers. A remote attacker can send a specially crafted to-device message with a spoofed sender field to spoof the origin of to-device messages.
Homeservers that do not federate or only participate in a closed, trusted federation are not affected.
Remediation
Install security update from vendor's website.