HOMEVULNERABILITIESCVE-2021-40823
MEDIUM

CVE-2021-40823

CWE-290Published: September 13, 2021· Updated: Jun 17, 2026

5.9
CVSS v3.1

Official Description

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.

NVD Source

Technical Analysis

CVE-2021-40823 can be exploited remotely over the network without requiring physical or adjacent access, significantly expanding the attack surface for threat actors.

The vulnerability requires no privileges and no user interaction, making it a prime target for automated exploitation campaigns and worm-like propagation.

A successful exploit results in complete confidentiality breach (data exposure), with a CVSS base score of 5.9.

CVSS v3.1 Vector Breakdown

Exploitability
Attack VectorNetwork
Attack ComplexityHigh
Privileges Req.None
User InteractionNone
ScopeUnchanged
Impact
ConfidentialityHigh
IntegrityNone
AvailabilityNone
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

Affected Vendors & Products

matrix1 product(s)
javascript sdk
Source: NVD CPE · 1 total CPE entries

Exploit & PoC Resources

NO KNOWN EXPLOITNo public exploit confirmed at this time
Exploit-DB
Offensive Security exploit database
GitHub PoC Search
Community PoC repositories
Rapid7 / Metasploit
Metasploit module search
PacketStorm
Security advisories & exploits
NVD Detail
Official NIST entry
MITRE CVE
CVE Program entry
External links open in a new tab. Always verify in a controlled environment before use.

Official Patches & Advisories

ADVISORYhttps://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1
ADVISORYhttps://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing
ADVISORYhttps://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1
ADVISORYhttps://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing

All References (4)

→
https://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1Third Party Advisory
→
https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharingVendor Advisory
→
https://github.com/matrix-org/matrix-js-sdk/releases/tag/v12.4.1Third Party Advisory
→
https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharingVendor Advisory

Quick Facts

CVE IDCVE-2021-40823
CVSS Score5.9 / 10
SeverityMEDIUM
WeaknessCWE-290
CISA KEVNo
Affected1 vendor(s)
PublishedSep 13, 2021

Related CVEs (CWE-290)

CVE-2026-39858
10.0
CVE-2020-5415
10.0
CVE-2025-34063
10.0
CVE-2026-6213
10.0
CVE-2026-54782
10.0
CVE-2019-16871
9.8

Recommended Actions

  • →Apply vendor patches immediately
  • →Monitor CVE-2021-40823 in threat intel feeds
  • →Review IDS/IPS signatures for exploitation attempts
Data sourced from NVD (NIST), CISA KEV, and EPSS (FIRST). Analysis generated by CTIWatch. CVE data is provided under the NVD usage policy.
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