What’s Vulnerable › Vulnerabilities

CVE-2021-40823

• Severity: medium (CVSS 5.9 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.7% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-09-13 · updated 2026-06-17
• Weakness: CWE-290

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.

Affected products

• matrix-org matrix-js-sdk: before 12.4.1 — fixed in 12.4.1

How to fix

Upgrade to a fixed version: matrix-org matrix-js-sdk 12.4.1. Check the vendor advisory for mitigations.

References

• https://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1
• https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing
• https://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1
• https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing

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