VULNERABILITY INTELLIGENCE

CVE-2021-21333

CVSS score6.1 MEDIUM
EPSS probability1.35%
CISA KEVNot currently listed
Published2021-03-26
Last modified2026-06-17
CVSS vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N

Description

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.27.0, the notification emails sent for notifications for missed messages or for an expiring account are subject to HTML injection. In the case of the notification for missed messages, this could allow an attacker to insert forged content into the email. The account expiry feature is not enabled by default and the HTML injection is not controllable by an attacker. This is fixed in version 1.27.0.

Weakness classification

  • CWE-74: Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
  • CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

Primary references