AUGUST 4, 2026
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Case File

CVE-2021-21333

MEDIUM · CVSS 6.1 EPSS 1.39% Public Exploit

Source: NVD + CISA KEV + EPSS (historical backfill) · Published 2021-03-26 · Last synced 2026-08-04

CyberRota Analysis

AI analysis pending.

Public Exploit Signal

A public exploit, PoC, GitHub repository or Metasploit reference was detected for this CVE.

Note: these links are listed for security research and verification purposes only.

CVE
CVE-2021-21333
Severity
MEDIUM
CVSS
6.1
EPSS
1.39%

Original NVD 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.