CVE-2021-21333

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.

• CWE-74
• CWE-79

• https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73dfPatch, Third Party Advisory
• https://github.com/matrix-org/synapse/pull/9200Patch, Third Party Advisory
• https://github.com/matrix-org/synapse/releases/tag/v1.27.0Third Party Advisory
• https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fmPatch, Third Party Advisory
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
• https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73dfPatch, Third Party Advisory
• https://github.com/matrix-org/synapse/pull/9200Patch, Third Party Advisory
• https://github.com/matrix-org/synapse/releases/tag/v1.27.0Third Party Advisory
• https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fmPatch, Third Party Advisory
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

Published

Mar 26, 2021

Last Modified

Jun 17, 2026

Status

Modified

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
• Autonomous PentestingAI agents that find and validate exploitable vulnerabilities like this one across your applications.
• PR ReviewsPentest every pull request so vulnerable code is caught before it ships to production.
• AI Penetration TestingHow AI-driven penetration testing continuously covers your attack surface.

• CVE-2021-21327GLPI is an open-source asset and IT management software pack…7.5
• CVE-2021-21328Vapor is a web framework for Swift. In Vapor before version …5.3
• CVE-2021-21329RATCF is an open-source framework for hosting Cyber-Security…9.8
• CVE-2021-21330aiohttp is an asynchronous HTTP client/server framework for …6.1
• CVE-2021-21331The Java client for the Datadog API before version 1.0.0-bet…3.3
• CVE-2021-21332Synapse is a Matrix reference homeserver written in python (…8.2
• CVE-2021-21334In containerd (an industry-standard container runtime) befor…6.3
• CVE-2021-21335In the SPNEGO HTTP Authentication Module for nginx (spnego-h…9.8
• CVE-2021-21336Products.PluggableAuthService is a pluggable Zope authentica…6.5
• CVE-2021-21337Products.PluggableAuthService is a pluggable Zope authentica…6.1
• CVE-2021-21338TYPO3 is an open source PHP based web content management sys…6.1
• CVE-2021-21339TYPO3 is an open source PHP based web content management sys…7.5