CVE-2021-21333

Name: CVE-2021-21333
Creator: NVD / NIST
Published: 2021-03-26T20:15:11.953+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.1/10EPSS 1.39%

Last modified Jun 17, 2026

CVE-2021-21333 is a medium-severity vulnerability rated 6.1/10 on the CVSS scale. Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. EPSS estimates a 1.39% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

6.1/10

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:N/I:H/A:N

EPSS Probability

1.39%

68.9th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Matrix	Synapse	< 1.27.0
Fedoraproject	Fedora	34

References

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/

Timeline

Published: Mar 26, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-21333?

How severe is CVE-2021-21333?

CVE-2021-21333 has a CVSS score of 6.1/10 (MEDIUM severity). The EPSS model estimates a 1.39% probability of exploitation in the next 30 days.

How do I fix CVE-2021-21333?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-21333?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST