CVE-2021-39163

What it is

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 name, avatar, topic and number of members of a room if they know the ID of the room. This vulnerability is limited to homeservers where the vulnerable homeserver is in the room and untrusted users are permitted to create groups (communities). By default, only homeserver administrators can create groups. However, homeserver administrators can already access this information in the database or using the admin API. As a result, only homeservers where the configuration setting `enable_group_creation` has been set to `true` are impacted. Server administrators should upgrade to 1.41.1 or higher to patch the vulnerability. There are two potential workarounds. Server administrators can set `enable_group_creation` to `false` in their homeserver configuration (this is the default value) to prevent creation of groups by non-administrators. Administrators that are using a reverse proxy could, with partial loss of group functionality, block the endpoints `/_matrix/client/r0/groups/{group_id}/rooms` and `/_matrix/client/unstable/groups/{group_id}/rooms`.

At a glance

Exploitability

Can be exploited over the network with a low-privilege account. Limited direct impact.

Exploit odds

0.9% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-08-31 16:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 04:03 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 0.9% in the next 30 days (EPSS).
• Severity: 3.1 LOW (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-08-31 16:15 UTC

Last modified

2026-06-17 04:03 UTC

Exploit probability

0.9% · EPSS, next 30 days

Weakness

CWE-200 · Information exposure

Record status

Modified

Affected

• Matrix Synapse < 1.41.1
• Fedoraproject Fedora 34
• Fedoraproject Fedora 35

Attack profile CVSS 3.1 · 3.1

Attack vector

Network

Complexity

High

Privileges needed

Low

User interaction

None

Scope

Unchanged

Confidentiality

Low

Integrity

None

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-39163.md and follow it. Check whether this project is affected by CVE-2021-39163. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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