CVE-2021-40824Network attack vector

Encrypted-message disclosure via unverified key-share requests

Published Sep 13, 2021 · Updated Aug 4, 2024

Key disclosure in room key sharing in Element Android and Matrix SDK for Android before 1.2.2 allows remote attackers to decrypt messages. The shared-session store keyed prior sharing by room, session, user, and device IDs without binding the record to the device identity key, so an impersonating device passed the reshare check. Exploitation requires control of a recipient account or its homeserver in an encrypted room and exposes messages originally 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

Key disclosure in room key sharing in Element Android and Matrix SDK for Android before 1.2.2 allows remote attackers to decrypt messages. The shared-session store keyed prior sharing by room, session, user, and device IDs without binding the record to the device identity key, so an impersonating device passed the reshare check. Exploitation requires control of a recipient account or its homeserver in an encrypted room and exposes messages originally sent by affected clients.

The record

CVE
CVE-2021-40824
Published
Sep 13, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-290, T1557
Attack vector
network
Privileges
unauthenticated

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 privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

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