Cross-site scripting in Synapse - CVE-2021-21333
Published: March 25, 2021 / Updated: May 4, 2026
Vulnerability identifier: #VU129604
CSH Severity: Low
CVSS v4: 5.1 [CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N]
CVE-ID: CVE-2021-21333
CWE-ID: CWE-79
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vulnerability details
The vulnerability allows a remote user to inject forged content into notification emails.
The vulnerability exists due to improper neutralization of input during web page generation in email notification templates when rendering missed message notifications. A remote user can send crafted content to inject forged content into notification emails.
The account expiry notification path is also affected, but that injection is not controllable by an attacker.
Affected software
Synapse
Fedora
matrix-synapse
Fedora
matrix-synapse
How to mitigate CVE-2021-21333
Install security update from vendor's website.
Synapse - update to 1.27.0
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