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MEDIUM severity

matrix-js-sdk can be tricked into disclosing E2EE room keys to a participating homeserverGHSA-23cm-x6j7-6hq3

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

GHSA-23cm-x6j7-6hq3 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.

Also known asCVE-2021-40823
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
None indexed
Exploitation data as of Oct 12, 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
0.00%0.39%0.77%1.16%0.3%0.7%Apr 26Aug 26Oct 26

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

How urgent is this, really

GHSA-23cm-x6j7-6hq3 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 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
464Kdownloads / 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
EcosystemPackageVulnerable rangeFix
📦npmmatrix-js-sdkall versions12.4.1npm 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

  1. 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.

  2. 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 GHSA-23cm-x6j7-6hq3 is resolved across your whole dependency graph.

  3. 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
O3 Security · Impact-Aware SCA

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matrix-js-sdk can be tricked into disclosing E2EE room…