CVE-2021-21333: HTML injection in email and account expiry notifications
Impact 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.
Patches This issue is fixed in #9200.
Workarounds For the missed messages notifications:
The notif.html, notifmail.html, and room.html templates can be overridden with custom templates that manually escapes the variables using JInja2's escape filter. See the email.templatedir setting.
For the account expiry notifications:
1. Account expiry can be disabled via the accountvalidity.enabled setting.
2. The noticeexpiry.html template can be overridden with a custom template that manually escapes the variables using JInja2's escape filter. See the email.templatedir setting.
Other sources
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 Software
Remediation
Patch Available
Event History
Frequently Asked Questions
What is CVE-2021-21333?
CVE-2021-21333 is a vulnerability in Synapse, a Matrix reference homeserver, that allows an attacker to intercept email notifications for missed messages or expiring accounts.
How severe is CVE-2021-21333?
CVE-2021-21333 has a severity level of 6.1 (medium).
What is the affected software for CVE-2021-21333?
The affected software for CVE-2021-21333 includes Matrix Synapse versions up to and excluding 1.27.0, as well as Fedoraproject Fedora version 34.
How can I fix CVE-2021-21333?
To fix CVE-2021-21333, upgrade Matrix Synapse to version 1.27.0 or higher.
What is the CWE classification of CVE-2021-21333?
The CWE classifications for CVE-2021-21333 are CWE-79 (Improper Neutralization of Input During Web Page Generation) and CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component).