# CVE-2021-40823: matrix-js-sdk can be tricked into disclosing E2EE room keys to a participating homeserver

> A malicious homeserver can steal encryption keys from clients, allowing decryption of end-to-end encrypted messages in the Matrix JavaScript SDK.

- **Severity:** Medium 5.9 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 13 Sep 2021
- **Updated:** 4 Aug 2024
- **Also known as:** CLEANSTART-2024-FT51773, DEBIAN-CVE-2021-40823, GHSA-23cm-x6j7-6hq3, UBUNTU-CVE-2021-40823

## What is CVE-2021-40823?

CVE-2021-40823 is a medium-severity vulnerability in n/a, published on 13 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.

### 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.

Weakness: CWE-290 (Authentication Bypass by Spoofing)

## Which versions are affected by CVE-2021-40823?

2 open source packages ship an affected version, including matrix-js-sdk. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `matrix-js-sdk` | npm | All versions | 12.4.1 |
| `matrix-js-sdk` | npm | All versions | 12.4.1 |

Products: Matrix Javascript Sdk.

## How do I fix CVE-2021-40823?

Upgrade matrix-js-sdk to 12.4.1 or later.

## Is CVE-2021-40823 being exploited?

There is no known exploitation of CVE-2021-40823 in the wild and no public exploit code.

## Other vulnerabilities in n/a

- [CVE-2022-39250](https://insights.sbomify.com/CVE-2022-39250/): Matrix JavaScript SDK vulnerable to key/device identifier confusion in SAS verification (high 8.6)
- [CVE-2022-39251](https://insights.sbomify.com/CVE-2022-39251/): Matrix Javascript SDK vulnerable to Olm/Megolm protocol confusion (high 8.6)
- [CVE-2022-36059](https://insights.sbomify.com/CVE-2022-36059/): Prototype pollution in matrix-js-sdk (high 8.2)
- [CVE-2023-28427](https://insights.sbomify.com/CVE-2023-28427/): Prototype pollution in matrix-js-sdk (high 8.2)
- [CVE-2022-39249](https://insights.sbomify.com/CVE-2022-39249/): Matrix Javascript SDK vulnerable to impersonation via forwarded Megolm sessions (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-40823 and fix it if it is affected.

What it is: CVE-2021-40823 is a medium-severity vulnerability in n/a, published on 13 Sep 2021. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-40823 in the wild and no public exploit code.

Affected packages and fixed versions:
- matrix-js-sdk (npm): affected all versions; fixed in 12.4.1
- matrix-js-sdk (npm): affected all versions; fixed in 12.4.1

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2021-40823/ (Markdown: https://insights.sbomify.com/CVE-2021-40823/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-40823
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-40823
- OSV: https://osv.dev/vulnerability/CVE-2021-40823

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-40823/
Summaries are generated and may contain mistakes; the official description above is the record.