How severe is CVE-2021-39164?

CVE-2021-39164 has a CVSS score of 3.1/10, rated LOW. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-39164?

CVE-2021-39164 affects the following packages: matrix-synapse (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-39164?

Update matrix-synapse to 1.41.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39164 is resolved across your whole dependency graph.

How do I detect CVE-2021-39164 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-synapse, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-39164 if there is no patch (or I can't update yet)?

Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Is CVE-2021-39164 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-39164 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-39164?

CVE-2021-39164 has an EPSS (Exploit Prediction Scoring System) score of 1.5%, placing it in the 73th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-39164?

CVE-2021-39164 is classified as Information Exposure (CWE-200), CWE-863 (CWE-863). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-39164 published, and has it been updated?

CVE-2021-39164 was published on August 31, 2021 and was last updated on July 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

LOW severity

CVE-2021-39164 — matrix-synapse

LOWFix: matrix-org/synapse@cb35df9

CVE-2021-39164 is a low-severity (CVSS 3.1) Information Exposure vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Improper authorisation of members discloses room membership to non-members

Also known asGHSA-3x4c-pq33-4w3qPYSEC-2021-425

Published

Aug 31, 2021

Updated

Jul 8, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.5%probability of exploitation in next 30 days

Lower Risk-0.01%

Lower risk than most CVEs73th percentile — riskier than 73% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-39164 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐍matrix-synapse

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

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.

Patches

Server administrators should upgrade to 1.41.1 or later.

Workarounds

Administrators of servers that use a reverse proxy could, with potentially unacceptable loss of functionality, block the following endpoints:

/_matrix/client/r0/rooms/{room_id}/members with at query parameter
/_matrix/client/unstable/rooms/{room_id}/members with at query parameter

References

n/a

For more information

If you have any questions or comments about this advisory, e-mail us at [email protected].

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`matrix-synapse`	all versions	1.41.1`pip install --upgrade 'matrix-synapse==1.41.1'`

Affected Products

2 products · 3 configurations

fedorafedoraproject

2 versions

3435

Application

synapsematrix

< 1.41.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for matrix-synapse, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update matrix-synapse to 1.41.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39164 is resolved across your whole dependency graph.
Workarounds
Put an independent control in front of the weakness: restrict the affected endpoint or interface to trusted networks, require an additional authentication factor or proxy-level check, and invalidate existing sessions and credentials in case the flaw has already been used.

Frequently Asked Questions

### Impact 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. ### Patches Server administrators should upgrade to 1.41.1 or later. ### Workarounds Administrators of servers that use a reverse proxy could, with potentially unacceptable loss of functionality, block the following endpoints: * `/_matrix/client/r0/rooms/{room_id}/members` w