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CVE-2021-21333

6.1 · MEDIUM

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse).

Published Mar 26, 2021CWE-74EPSS 1.35%

Overview

CVE-2021-21333 is a medium-severity vulnerability. It was published on March 26, 2021 and has a CVSS 3.1 base score of 6.1 (MEDIUM).

This vulnerability has a CVSS 3.1 base score of 6.1, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

GitHubgithub.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73dfGitHubgithub.com/matrix-org/synapse/pull/9200GitHubgithub.com/matrix-org/synapse/releases/tag/v1.27.0GitHubgithub.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fmReflists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/GitHubgithub.com/matrix-org/synapse/commit/e54746bdf7d5c831eabe4dcea76a7626f1de73dfGitHubgithub.com/matrix-org/synapse/pull/9200GitHubgithub.com/matrix-org/synapse/releases/tag/v1.27.0GitHubgithub.com/matrix-org/synapse/security/advisories/GHSA-c5f8-35qr-q4fmReflists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/

How CyberStrike helps

Frequently asked questions

What is CVE-2021-21333?

CVE-2021-21333 is a medium-severity vulnerability. It was published on March 26, 2021 and has a CVSS 3.1 base score of 6.1 (MEDIUM).

How severe is CVE-2021-21333?

This vulnerability has a CVSS 3.1 base score of 6.1, rated MEDIUM. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2021-21333?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2021-21333?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2021-21333 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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