What’s Vulnerable › Vulnerabilities

CVE-2021-21333: HTML injection in email and account expiry notifications

• Severity: medium (CVSS 6.1 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.4% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-03-26 · updated 2026-06-17
• Weakness: CWE-74

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.

Affected products

• Fedoraproject Fedora: 34
• matrix-org synapse: before 1.27.0 — fixed in 1.27.0

How to fix

Upgrade to a fixed version: matrix-org synapse 1.27.0. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df
• https://github.com/matrix-org/synapse/pull/9200
• https://github.com/matrix-org/synapse/releases/tag/v1.27.0
• https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fm
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
• https://github.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73df
• https://github.com/matrix-org/synapse/pull/9200
• https://github.com/matrix-org/synapse/releases/tag/v1.27.0
• https://github.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fm
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

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