Security readout for executives and security teams
Plain-English summary
This flaw could let a malicious Matrix homeserver inside an encrypted room obtain encryption keys from vulnerable Matrix JavaScript SDK-based clients. If successful, the homeserver could read end-to-end encrypted messages sent by affected clients, weakening a core confidentiality promise.
Executive priority
Prioritize remediation for organizations relying on Matrix end-to-end encryption for sensitive communications. The issue directly affects message confidentiality, but available sources do not show active exploitation or provide a CVSS score.
Technical view
matrix-js-sdk before 12.4.1 had a logic error in room key sharing. A malicious homeserver present in an encrypted room could send crafted Matrix protocol messages and steal room encryption keys originally sent by affected clients, enabling decryption of their end-to-end encrypted messages.
Likely exposure
Exposure is likely limited to Matrix clients or applications using matrix-js-sdk versions before 12.4.1 with encrypted rooms and room key sharing behavior.
Exploitation context
The source bundle does not cite active exploitation, and KEV is false. The attack requires a malicious Matrix homeserver already present in an encrypted room, then abuse of crafted Matrix protocol messages.
Researcher notes
Evidence names matrix-js-sdk before 12.4.1 only. The bundle does not include CVSS, CWE, exploit status, or a broader affected product list. Avoid assuming all Matrix clients are affected unless they use the vulnerable SDK version.
Mitigation direction
- Upgrade matrix-js-sdk to version 12.4.1 or later.
- Check Matrix project guidance for any client-specific remediation steps.
- Prioritize clients used for sensitive encrypted communications.
- Assess encrypted rooms that include external or untrusted homeservers.
Validation and detection
- Inventory applications and clients using matrix-js-sdk.
- Check lockfiles and deployed bundles for versions before 12.4.1.
- Confirm whether affected clients participate in encrypted Matrix rooms.
- Review Matrix disclosure and release notes for remediation confirmation.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CVE-2021-40823 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Unknown
- CVSS
- Not scored
- Known Exploited
- No
- Published
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS and timeline data
No CVSS vectors or timeline events were available in the normalized CVE source material.
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1CVE reference · x_refsource_MISC
- https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharingCVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.