How severe is CVE-2021-40823?

CVE-2021-40823 has a CVSS score of 5.9/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-40823?

CVE-2021-40823 affects the following packages: matrix-js-sdk (npm). Ecosystems affected: npm.

How do I fix CVE-2021-40823?

Update matrix-js-sdk to 12.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-40823 is resolved across your whole dependency graph.

How do I detect CVE-2021-40823 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-js-sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-40823 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-40823 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-40823 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-40823?

CVE-2021-40823 has an EPSS (Exploit Prediction Scoring System) score of 0.7%, placing it in the 50th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-40823?

CVE-2021-40823 is classified as CWE-290 (CWE-290). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-40823 published, and has it been updated?

CVE-2021-40823 was published on September 13, 2021 and was last updated on September 14, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

MEDIUM severity

CVE-2021-40823 — matrix-js-sdk

MEDIUMFix: matrix-org/matrix-js-sdk@894c248

CVE-2021-40823 is a medium-severity (CVSS 5.9) CWE-290 vulnerability in matrix-js-sdk. A fix is available for matrix-js-sdk — see the affected versions and patch details below.

matrix-js-sdk can be tricked into disclosing E2EE room keys to a participating homeserver

Also known asGHSA-23cm-x6j7-6hq3

Published

Sep 13, 2021

Updated

Sep 14, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.7%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs50th percentile — riskier than 50% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-40823 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

166other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

matrix-js-sdknpm

564Kdownloads / week

Description

Impact

A logic error in the room key sharing functionality of matrix-js-sdk before 12.4.1 allows a malicious Matrix homeserver† participating in an encrypted room to steal room encryption keys from affected Matrix clients participating in that room. This allows the homeserver to decrypt end-to-end encrypted messages sent by affected clients.

† Or anyone with access to the account of the original recipient of an encrypted message.

Known clients affected (via their use of vulnerable versions of matrix-js-sdk):

Element Web (1.8.2 and earlier)
Element Desktop (1.8.2 and earlier)
SchildiChat Web (1.7.32-sc1 and earlier)
SchildiChat Desktop (1.7.32-sc1 and earlier)
Cinny (1.2.0 and earlier)

Patch

This was fixed in https://github.com/matrix-org/matrix-js-sdk/commit/894c24880da0e1cc81818f51c0db80e3c9fb2be9.

Workarounds

To prevent a homeserver from being able to steal the room keys, vulnerable clients can be taken offline or signed out. If signing out, care should be taken to either set up Secure Backup or export E2E room keys in order to preserve access to past messages.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`matrix-js-sdk`	all versions	12.4.1`npm install matrix-js-sdk@12.4.1`

Affected Products

1 product · 1 configurations

Application

javascript sdkmatrix

< 12.4.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-js-sdk, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update matrix-js-sdk to 12.4.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-40823 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact A logic error in the room key sharing functionality of matrix-js-sdk before 12.4.1 allows a malicious Matrix homeserver† participating in an encrypted room to steal room encryption keys from affected Matrix clients participating in that room. This allows the homeserver to decrypt end-to-end encrypted messages sent by affected clients. † Or anyone with access to the account of the original recipient of an encrypted message. Known clients affected (via their use of vulnerable versions of matrix-js-sdk): - Element Web (1.8.2 and earlier) - Element Desktop (1.8.2 and earlier) - Schi