CVE-2021-40824: A logic error in the room key sharing functionality of Element Android before 1.2.2 and matrix-android-sdk2 (a

Q: How do I fix CVE-2021-40824?

element: upgrade to 1.2.2; matrix-android-sdk2: upgrade to 1.2.2. Exact commands for each OS are on this page.

Q: Is CVE-2021-40824 being exploited?

CVE-2021-40824 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-40824 affect?

CVE-2021-40824 affects element, matrix-android-sdk2. Severity is medium (CVSS 5.9). A logic error in the room key sharing functionality of Element Android before 1.2.2 and matrix-android-sdk2 (aka Matrix SDK for Android) before 1.2.2 allows a malicious Matrix homeserver present in an encrypted room to steal room encryption

Q: How urgent is CVE-2021-40824?

Priority 39/100. Remotely exploitable, no login, no user action

Routine

MEDIUM 5.9REMOTEUPGRADEUPGRADE

CVE-2021-40824 is a medium-severity flaw (CVSS 5.9) affecting element, matrix-android-sdk2. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

A logic error in the room key sharing functionality of Element Android before 1.2.2 and matrix-android-sdk2 (aka Matrix SDK for Android) before 1.2.2 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 attacker to decrypt end-to-end encrypted messages sent by affected c

Why this priority

Remotely exploitable, no login, no user action

A fixed release or distro patch exists.

Risk facts

CVSS score	5.9 (Medium)
Attack vector	Network (remote)
Attack complexity	High
Privileges needed	None
User interaction	None
EPSS exploit chance	0.7% (top 48%)
CISA exploited list	No
Weakness	CWE-290
Published	2021-09-13
Last modified	2026-06-17

Publisher (CNA): MITRE · Vendor: matrix

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-09-13 19:15 UTCPublished CVE record published
2026-06-17 04:07 UTCUpdated Record last modified

matrix element UPGRADE

Affected: < 1.2.2

Fixed in: 1.2.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy element || rpm -q element || element --version

2. UPGRADE to 1.2.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade element

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade element

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

matrix matrix-android-sdk2 UPGRADE

Affected: < 1.2.2

Fixed in: 1.2.2

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy matrix-android-sdk2 || rpm -q matrix-android-sdk2 || matrix-android-sdk2 --version

2. UPGRADE to 1.2.2 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade matrix-android-sdk2

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade matrix-android-sdk2

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-40824?

element: upgrade to 1.2.2; matrix-android-sdk2: upgrade to 1.2.2. Exact commands for each OS are on this page.

Is CVE-2021-40824 being exploited?

CVE-2021-40824 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-40824 affect?

CVE-2021-40824 affects element, matrix-android-sdk2. Severity is medium (CVSS 5.9). A logic error in the room key sharing functionality of Element Android before 1.2.2 and matrix-android-sdk2 (aka Matrix SDK for Android) before 1.2.2 allows a malicious Matrix homeserver present in an encrypted room to steal room encryption

How urgent is CVE-2021-40824?

Priority 39/100. Remotely exploitable, no login, no user action

Software pages

Android vulnerabilities

Related CVEs

CVE-2025-27606 MEDIUM 5.1 UPGRADE Element Android is an Android Matrix Client provided by Element
CVE-2024-26132 MEDIUM 4.0 UPGRADE Element Android is an Android Matrix Client
CVE-2024-26131 HIGH 8.4 UPGRADE Element Android is an Android Matrix Client
CVE-2022-41904 MEDIUM 6.4 UPGRADE Element iOS is an iOS Matrix client provided by Element
CVE-2021-44538 CRITICAL 9.8 UPGRADE UPGRADE The olm_session_describe function in Matrix libolm before 3.2.7 is vulnerable to a buffer overflow

References

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