How severe is CVE-2021-40823?

CVE-2021-40823 has a CVSS 3.x base score of 5.9, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2021-40823 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (47th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-40823?

CVE-2021-40823 affects Matrix Javascript Sdk. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-40823?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2021-40823 published?

CVE-2021-40823 was published on 2021-09-13 and last updated on 2026-06-17.

CVE-2021-40823

CVE-2021-40823 is a medium-severity vulnerability in Matrix Javascript Sdk with a CVSS 3.x base score of 5.9. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-290.

Key facts

Severity: Medium (CVSS 3.x base score 5.9)
CVSS v2: 4.3
EPSS exploit prediction: 1% (47th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-290
Affected product: Matrix Javascript Sdk
Published: 2021-09-13
Last modified: 2026-06-17

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.

Frequently asked questions

What is CVE-2021-40823?: 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.
How severe is CVE-2021-40823?: CVE-2021-40823 has a CVSS 3.x base score of 5.9, rated medium severity. It is exploitable over network with high attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2021-40823 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (47th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-40823?: CVE-2021-40823 affects Matrix Javascript Sdk. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-40823?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2021-40823 published?: CVE-2021-40823 was published on 2021-09-13 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:matrix:javascript_sdk:*:*:*:*:*:*:*:*

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