`CVE-2021-40824`

Affected software	2 products, listed below
Severity	5.9 Medium CVSS 3.1 2 scores
Fixed in	`1.2.2`
Weakness	CWE-290
Published	13 Sep 2021
Updated	13 Oct 2026

Description

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 clients.

CWE-327

Exploitation status

No public exploit

No exploit has been reported in the sources we check. Published 5 years ago.

CISA KEV not listed
VulnCheck KEV not listed
CISA Vulnrichment no assessment published
Indexed PoC none indexed
EPSS 0.66% chance in 30 days

Waratek protection

Outside Waratek scope

A Java vulnerability, but not one Waratek protects. It affects only Java on a user's desktop (applets, Web Start, JavaFX, the installer), sits in a product the Waratek agent can't be installed into, or is in a package we don't expect in business applications. The decision below says which.

Protection none
Action not pursued none
Review human Researcher
Record active

Decided by	A researcher's recorded decision for this specific CVE
Finding	A researcher set this CVE's status to Outside Waratek scope.

Decided by

A researcher's recorded decision for this specific CVE

Finding

A researcher set this CVE's status to Outside Waratek scope.

Waratek research note

written 2026-11-13

Written by a Waratek engineer who reviewed this CVE. Their decision sets the status above, ahead of every automated rule except a withdrawal.

Comments

matrix-android-sdk2 (Element Android) is an Android library. It runs inside a phone app, not in a server JVM, so there is no start-up line to add -javaagent to. In scope only because it is published to Maven Central.

CVSS

5.9 MEDIUM v3.1 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N 2 metrics

We show the highest score from the newest CVSS version. Scores from different versions can't be compared, and the first score listed is sometimes a 0.0 placeholder.

Version Score Band Vector Assigner Type

CVSS 3.1 5.9 MEDIUM CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N NVD Primary published

CVSS 2.0 4.3 none AV:N/AC:M/Au:N/C:P/I:N/A:N NVD Primary

Version	Score	Band	Vector	Assigner	Type
CVSS 3.1	5.9	MEDIUM	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N`	NVD	Primary	published
CVSS 2.0	4.3	none	`AV:N/AC:M/Au:N/C:P/I:N/A:N`	NVD	Primary

Affected software and releases

Maven packages

Affected and fixed versions, from OSV.dev

Package Introduced Fixed Last affected

org.matrix.android:matrix-android-sdk2 0 1.2.2 unbounded

Package	Introduced	Fixed	Last affected
`org.matrix.android:matrix-android-sdk2`	0	`1.2.2`	unbounded

Products and versions

3 products, 3 version entries

Source	Vendor	Product	Scheme	Affected versions
nvd	matrix	element	generic	＜ 1.2.2
nvd	matrix	matrix-android-sdk2	generic	＜ 1.2.2
osv	org.matrix.android	matrix-android-sdk2	generic	＜ 1.2.2

Source

Vendor

Product

Scheme

Affected versions

nvd

matrix

element

generic

＜ 1.2.2

nvd

matrix

matrix-android-sdk2

generic

＜ 1.2.2

osv

org.matrix.android

matrix-android-sdk2

generic

＜ 1.2.2

References

URL	Tags
https://github.com/matrix-org/matrix-android-sdk2/releases/tag/v1.2.2	WEB
https://matrix.org/blog/2021/09/13/vulnerability-disclosure-key-sharing	WEB

URL

Timeline

Published	2021-09-13	By the CVE Program.
NVD record modified	2026-06-17	NVD's own last-modified date for this record.
Last VRT activity	2026-10-13	Most recent commit touching this CVE's classification, patch or rule file.
Classification created	2026-11-13	First commit adding this CVE's research note.

Published

2021-09-13

By the CVE Program.

NVD record modified

2026-06-17

NVD's own last-modified date for this record.

Last VRT activity

2026-10-13

Most recent commit touching this CVE's classification, patch or rule file.

Classification created

2026-11-13

First commit adding this CVE's research note.

How VRT decided (4 of 24 rules ran)

4 of 24 rules ran. The rest didn't run because an earlier rule had already settled the CVE.

# Rule Outcome Status Detail

identity: Does this identifier describe a vulnerability at all? the first rule that matches decides

1 rejected no match CVE Program withdrawal of this identifier

scope: Is it a vulnerability Waratek protects? the first rule that matches decides

2 armr-patch-file no match An ARMR patch file on disk for this CVE

3 armr-secure-rule-file no match An ARMR security-rule template on disk for this CVE

4 manual-classification decided it matched Outside Waratek scope A researcher set this CVE's status to Outside Waratek scope.

5 manual classification not reached Manual classification of this CVE in the legacy dataset

6 non-java-upstream not reached A root cause in non-Java code: an OS kernel, a C library, or a bundled runtime

7 oracle-component not reached The defect being in a component of a tracked product the agent does not reach: JavaFX, the Java SE installer, the JInitiator browser plug-in, the native WebLogic proxy plug-in, or a native part of Oracle Application Server or Collaboration Suite (HTTP Server, Web Cache, Internet Directory, mail and calendar servers). Oracle gives these no CPE of their own and names them only in the description.

8 java-client-side not reached Oracle's own risk-matrix note saying the CVE is reached only through untrusted code in the Java sandbox (applets, Web Start) or only through a client installation of Java, so a server running its own trusted code is not exposed

9 osv-maven not reached An OSV.dev record of this CVE affecting a package published to Maven Central. When every such package is one business applications do not use (Minecraft mods and launchers, Fastjson) and no tracked product is named by CPE, it settles the CVE as Outside Waratek scope instead

10 cna-maven-package not reached An affected Maven artifact named by the CNA itself

11 known-cpe-product not reached An NVD CPE naming a product ARMR tracks

12 cna-cpe-product not reached A CPE the CNA published naming a tracked product. Oracle publishes these on advisory day, months before NVD.

13 oracle-advisory-family not reached A tracked product family named in Oracle's own risk matrix. CPE cannot carry a suite name, only a module.

14 cna-named-product not reached A tracked product named in words by the CVE's own CNA, on a record with no CPE yet. Apache and Eclipse publish these on advisory day, before NVD adds CPE data.

15 non-deployable-product not reached The only affected product being a sealed appliance image, where Java runs but no -javaagent can be attached

16 unmatched-product not reached No tracked product matched above, and none present at all

disposition: What do we do about one that is? every rule runs

17 patch-hint not reached A reference pointing at the upstream fix: a commit, a PR, a Red Hat bug

18 open-source-maven not reached The affected package being open source on Maven, so the fix can be read

19 poc-derivable not reached An indexed exploit clearing the evidence bar to reverse-engineer, where there is no source to read

20 no-exploit not reached Nothing to build a rule from yet: no published exploit, no CISA record of exploitation and no source-level change to read

21 secure-rule-match not reached A vulnerability class already blocked by an ARMR security rule

22 exploited-no-fix-or-poc not reached Somebody exploiting this CVE (a CISA or VulnCheck KEV listing, or a CISA verdict of active exploitation), with no fix, source code or PoC to build a rule from

23 below-action-threshold not reached Medium or Low severity, no CISA or VulnCheck KEV listing, no PoC clearing the evidence bar, no CISA verdict, no Oracle advisory

24 no-exploit-published not reached Exploit code for this candidate, published in a reviewed catalogue or a GitHub repository with 5 or more stars, or in use per CISA or VulnCheck; not only a CISA verdict, a forecast, or nothing at all

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