CVE-2021-40823
MEDIUMCVE-2021-40823 is a medium-severity (CVSS 5.9) CWE-290 vulnerability in matrix-js-sdk. O3 Security confirms whether CVE-2021-40823 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
matrix-js-sdk can be tricked into disclosing E2EE room keys to a participating homeserver
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.
matrix-js-sdknpmDescription
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 Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | matrix-js-sdk | all versions | 12.4.1 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-js-sdk. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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.
How O3 protects you
O3 pinpoints whether CVE-2021-40823 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2021-40823. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-40823 in your dependencies?
O3 detects CVE-2021-40823 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.