CVE-2021-40823Network attack vector

Room-key disclosure via unverified device resharing

Published Sep 13, 2021 · Updated Aug 4, 2024

Information disclosure in matrix-js-sdk before 12.4.1 allows remote attackers to obtain E2EE room keys through crafted requests. The MegolmEncryption resharing path records user and device IDs but fails to bind that record to the device's cryptographic identity key. A malicious homeserver in an encrypted room, or an attacker controlling a recipient account, can impersonate that device and decrypt messages sent by affected clients.

CVSS severity5.9
Medium
EPSS probability0.66%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Information disclosure in matrix-js-sdk before 12.4.1 allows remote attackers to obtain E2EE room keys through crafted requests. The MegolmEncryption resharing path records user and device IDs but fails to bind that record to the device's cryptographic identity key. A malicious homeserver in an encrypted room, or an attacker controlling a recipient account, can impersonate that device and decrypt messages sent by affected clients.

The record

CVE
CVE-2021-40823
Published
Sep 13, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-290, T1557
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 13, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

4.3NIST NVDAV:N/AC:M/Au:N/C:P/I:N/A:N
5.9NIST NVDCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-290Authentication Bypass by Spoofing
T1557Adversary-in-the-Middle

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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