Input validation error in Synapse - CVE-2021-21393
Published: April 12, 2021 / Updated: May 4, 2026
Vulnerability identifier: #VU129603
CSH Severity: Medium
CVSS v4: 6.9 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N]
CVE-ID: CVE-2021-21393
CWE-ID: CWE-20
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote attacker to cause a denial of service.
The vulnerability exists due to improper input validation in groups/communities endpoints when handling requests with crafted parameter values. A remote attacker can send specially crafted requests to cause a denial of service.
Exploitation can lead to excessive use of disk space and memory, and clients may have issues rendering large fields.
Affected software
Synapse
Fedora
matrix-synapse
Fedora
matrix-synapse
How to mitigate CVE-2021-21393
Install security update from vendor's website.
Synapse - update to 1.28.0
matrix-synapse - addressed in versions 1.38.0-2.fc34, 1.38.1-1.fc34
matrix-synapse - addressed in versions 1.38.0-2.fc34, 1.38.1-1.fc34