# CVE-2021-39164: Improper authorisation of /members discloses room membership to non-members

> Unauthorized users can view room member lists in shared-history rooms if they know the room ID and use a vulnerable homeserver account.

- **Severity:** Low 3.1 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 31 Aug 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-3x4c-pq33-4w3q, PYSEC-2021-425, UBUNTU-CVE-2021-39164

## What is CVE-2021-39164?

CVE-2021-39164 is a low-severity missing authorization vulnerability in Fedoraproject Fedora, published on 31 Aug 2021. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Matrix is an ecosystem for open federated Instant Messaging and Voice over IP. In versions 1.41.0 and prior, unauthorised users can access the membership (list of members, with their display names) of a room if they know the ID of the room. The vulnerability is limited to rooms with `shared` history visibility. Furthermore, the unauthorised user must be using an account on a vulnerable homeserver that is in the room. Server administrators should upgrade to 1.41.1 or later in order to receive the patch. One workaround is available. Administrators of servers that use a reverse proxy could, with potentially unacceptable loss of functionality, block the endpoints: `/_matrix/client/r0/rooms/{room_id}/members` with `at` query parameter, and `/_matrix/client/unstable/rooms/{room_id}/members` with `at` query parameter.

Weakness: CWE-863 (Incorrect Authorization), CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor)

## Which versions are affected by CVE-2021-39164?

3 open source packages ship an affected version, including matrix-synapse. It affects 2 products.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `matrix-synapse` | PyPI | All versions | 1.41.1 |
| `matrix-synapse` | PyPI | All versions | 1.41.1 |
| `matrix-synapse` | PyPI | All versions | 1.41.1 |

Products: Fedoraproject Fedora, Matrix Synapse.

## How do I fix CVE-2021-39164?

Upgrade matrix-synapse to 1.41.1 or later.

## Is CVE-2021-39164 being exploited?

There is no known exploitation of CVE-2021-39164 in the wild and no public exploit code.

## Other vulnerabilities in Fedoraproject Fedora

- [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild)
- [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild)
- [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild)
- [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild)
- [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-39164 and fix it if it is affected.

What it is: CVE-2021-39164 is a low-severity missing authorization vulnerability in Fedoraproject Fedora, published on 31 Aug 2021. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2021-39164 in the wild and no public exploit code.

Affected packages and fixed versions:
- matrix-synapse (PyPI): affected all versions; fixed in 1.41.1
- matrix-synapse (PyPI): affected all versions; fixed in 1.41.1
- matrix-synapse (PyPI): affected all versions; fixed in 1.41.1

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-2021-39164/ (Markdown: https://insights.sbomify.com/CVE-2021-39164/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-39164
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-39164
- OSV: https://osv.dev/vulnerability/CVE-2021-39164

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-39164/
Summaries are generated and may contain mistakes; the official description above is the record.