CVE-2021-40823

Name: CVE-2021-40823
Creator: NVD / NIST
Published: 2021-09-13T19:15:19.137+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 5.9/10EPSS 0.64%

Last modified Jun 17, 2026

CVE-2021-40823 is a medium-severity vulnerability rated 5.9/10 on the CVSS scale. A logic error in the room key sharing functionality of matrix-js-sdk (aka Matrix Javascript SDK) before 12.4.1 allows a malicious Matrix homeserver present in an encrypted room to steal room encryption keys (via crafted Matrix protocol messages) that were originally sent by affected Matrix clients participating in that room. This allows the homeserver to decrypt end-to-end encrypted messages sent by affected clients.. EPSS estimates a 0.64% chance of exploitation in the next 30 days.

Description

A logic error in the room key sharing functionality of matrix-js-sdk (aka Matrix Javascript SDK) before 12.4.1 allows a malicious Matrix homeserver present in an encrypted room to steal room encryption keys (via crafted Matrix protocol messages) that were originally sent by affected Matrix clients participating in that room. This allows the homeserver to decrypt end-to-end encrypted messages sent by affected clients.

Metrics

CVSS 3.1

5.9/10

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

EPSS Probability

0.64%

46.0th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-290

Affected Software

Vendor	Product	Versions
Matrix	Javascript Sdk	< 12.4.1

References

https://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1Third Party Advisory
https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharingVendor Advisory
https://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1Third Party Advisory
https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharingVendor Advisory

Timeline

Published: Sep 13, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-40823?

How severe is CVE-2021-40823?

CVE-2021-40823 has a CVSS score of 5.9/10 (MEDIUM severity). The EPSS model estimates a 0.64% probability of exploitation in the next 30 days.

How do I fix CVE-2021-40823?

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-2021-40823?

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

Scan your code now

Source: NVD / NIST