CVE-2021-21394

Name: CVE-2021-21394
Creator: NVD / NIST
Published: 2021-04-12T21:15:14.013+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 6.5/10EPSS 1.54%

Last modified Jun 17, 2026

CVE-2021-21394 is a medium-severity vulnerability rated 6.5/10 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 1.54% 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 1.28.0 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.

Metrics

CVSS 3.1

6.5/10

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

EPSS Probability

1.54%

71.7th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

Affected Software

Vendor	Product	Versions
Matrix	Synapse	> 0.17.0, < 1.28.0
Fedoraproject	Fedora	34

References

https://github.com/matrix-org/synapse/pull/9321Patch, Third Party Advisory
https://github.com/matrix-org/synapse/pull/9393Patch, Third Party Advisory
https://github.com/matrix-org/synapse/security/advisories/GHSA-w9fg-xffh-p362Patch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
https://pypi.org/project/matrix-synapse/Product, Third Party Advisory
https://github.com/matrix-org/synapse/pull/9321Patch, Third Party Advisory
https://github.com/matrix-org/synapse/pull/9393Patch, Third Party Advisory
https://github.com/matrix-org/synapse/security/advisories/GHSA-w9fg-xffh-p362Patch, Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY/
https://pypi.org/project/matrix-synapse/Product, Third Party Advisory

Timeline

Published: Apr 12, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-21394?

How severe is CVE-2021-21394?

CVE-2021-21394 has a CVSS score of 6.5/10 (MEDIUM severity). The EPSS model estimates a 1.54% probability of exploitation in the next 30 days.

How do I fix CVE-2021-21394?

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-2021-21394?

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

Scan your code now

Source: NVD / NIST