CVE-2021-21394
5.3 · MEDIUMOverview
CVE-2021-21394 is a medium-severity vulnerability affecting matrix synapse. It was published on April 12, 2021 and has a CVSS 3.1 base score of 5.3 (MEDIUM).
This vulnerability has a CVSS 3.1 base score of 5.3, rated MEDIUM. It can be exploited remotely over the network. Some level of privileges is 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 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.
Remediation
Check the references section for vendor advisories and patches from matrix. Update synapse to the latest patched version. 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.
Affected Products
| Vendor | Product | Versions | Status |
|---|---|---|---|
| matrix | synapse | >= 0, < 1.28.0 | Affected |
References
Frequently Asked Questions
What is CVE-2021-21394?
CVE-2021-21394 is a medium-severity vulnerability affecting matrix synapse. It was published on April 12, 2021 and has a CVSS 3.1 base score of 5.3 (MEDIUM).
How severe is CVE-2021-21394?
This vulnerability has a CVSS 3.1 base score of 5.3, rated MEDIUM. It can be exploited remotely over the network. Some level of privileges is required for exploitation.
How do I fix or remediate CVE-2021-21394?
Check the references section for vendor advisories and patches from matrix. Update synapse to the latest patched version. 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-2021-21394?
CyberStrike's AI-powered security agents can automatically detect CVE-2021-21394 across your infrastructure using autonomous pentesting, DAST scanning, and HackBrowser. The platform continuously monitors for known vulnerabilities and provides actionable remediation guidance prioritized by real-world exploitability.
How CyberStrike Helps
AI agents map your attack surface to find vulnerabilities like this one.
Automated penetration testing that runs continuously, not just quarterly.
AI-driven PR review catches vulnerable dependencies before they ship.
Browser-based exploitation validates findings with real proof-of-concept.
Related MEDIUM CVEs from 2021
View all →Improper Neutralization of Special Elements used in a Command ('Command Injection') vulnerability in
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is
Integer Overflow in memory allocating functions. Zephyr versions >= 1.14.2, >= 2.4.0 contain Integer
Magento versions 2.4.2 (and earlier), 2.4.1-p1 (and earlier) and 2.3.6-p1 (and earlier) are affected
Certain NETGEAR devices are affected by out-of-bounds reads and writes. This affects R6400 before 1.
NETGEAR R6400 devices before 1.0.1.70 are affected by a stack-based buffer overflow by an authentica
Next.js is an open source website development framework to be used with the React library. In affect
Authenticated Stored Cross-Site Scripting (XSS) vulnerability in YITH Maintenance Mode (WordPress pl
Multiple Authenticated Stored Cross-Site Scripting (XSS) vulnerabilities in YITH Maintenance Mode (W
An attacker with physical access to Boston Scientific Zoom Latitude Model 3120 can remove the hard d
Grafana is an open-source platform for monitoring and observability. In affected versions if an atta
A cross-site scripting (XSS) vulnerability has been reported to affect QNAP device running QmailAgen