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Javascript SdkApplication · Matrix

CVE-2021-40823

MEDIUM · 5.9 CVSS v3.1 Published 2021-09-13
Fix available
A fix is available. Upgrade to 12.4.1 or later.
See remediation →
68/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

matrix-js-sdk before 12.4.1 contains a logic error in room key sharing where encrypted room keys can be intercepted by a malicious homeserver participating in the room. The homeserver can send crafted Matrix protocol messages to trigger key disclosure, enabling decryption of end-to-end encrypted messages that should remain confidential from the server.

MitigationUpgrade matrix-js-sdk to version 12.4.1 or later to remediate the key sharing logic flaw.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Javascript SdkApplication
Affected:< 12.4.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify the installed matrix-js-sdk version
    Locate matrix-js-sdk in your project's dependency tree: check package.json under dependencies or devDependencies, or run 'npm list matrix-js-sdk' to see the installed version
    Affected if The version listed is less than 12.4.1 (e.g., 12.4.0, 12.3.0, etc.) or shows a range like ^12.0.0 that resolves to a version below 12.4.1
  2. Verify end-to-end encryption is in use
    Check your application code or Matrix room configuration for the use of encrypted rooms (Megolm encryption). Look for usage of 'olm', 'megolm', or 'encryption' settings in your Matrix client setup
    Affected if Your application creates or participates in encrypted rooms (E2E encrypted rooms where key sharing would occur)
  3. Confirm room key sharing is active
    Review your Matrix client configuration or runtime behavior to determine if key sharing is enabled. In matrix-js-sdk, this relates to the keyRequest and keyShare logic for cross-signing or device verification flows
    Affected if Your client accepts or processes incoming key requests from other devices or users in encrypted rooms

You are affected if your installed matrix-js-sdk version is below 12.4.1 AND your application uses end-to-end encrypted rooms with active key sharing functionality.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 12.4.1 or later
Fixed in 12.4.1
Interim mitigation

Upgrade matrix-js-sdk to version 12.4.1 or later to remediate the key sharing logic flaw.

Recommended fix Moderate confidence

12.4.1

  1. Check the current version of matrix-js-sdk in your project by running `npm list matrix-js-sdk` or looking in your package.json
  2. Upgrade matrix-js-sdk to version 12.4.1 or later by running `npm install matrix-js-sdk@^12.4.1` or `yarn upgrade matrix-js-sdk@^12.4.1`
  3. After upgrading, verify the installed version matches or exceeds 12.4.1 using `npm list matrix-js-sdk`
  4. Rebuild and redeploy your application to ensure the updated SDK is used

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Javascript Sdk Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing4.0 h
  • Review / QA2.0 h
10.0 hours of engineering $1,720
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,752.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-40823 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data