How severe is CVE-2021-41281?

CVE-2021-41281 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-41281?

CVE-2021-41281 affects the following packages: matrix-synapse (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-41281?

Update matrix-synapse to 1.47.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41281 is resolved across your whole dependency graph.

How do I detect CVE-2021-41281 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-2021-41281 if there is no patch (or I can't update yet)?

Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Is CVE-2021-41281 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-41281 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-2021-41281?

CVE-2021-41281 has an EPSS (Exploit Prediction Scoring System) score of 1.6%, placing it in the 75th 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-2021-41281?

CVE-2021-41281 is classified as Path Traversal (CWE-22). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2021-41281 published, and has it been updated?

CVE-2021-41281 was published on November 23, 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

HIGH severity

CVE-2021-41281 — matrix-synapse

HIGHFix: matrix-org/synapse@91f2bd0

CVE-2021-41281 is a high-severity (CVSS 7.5) Path Traversal vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Path traversal in Matrix Synapse

Also known asGHSA-3hfw-x7gx-437cPYSEC-2021-436

Published

Nov 23, 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 ↗

1.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs75th percentile — riskier than 75% of all scored CVEsHighest risk

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

How urgent is this, really

CVE-2021-41281 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

Synapse instances with the media repository enabled can be tricked into downloading a file from a remote server into an arbitrary directory, potentially outside the media store directory.

The last two directories and file name of the path are chosen randomly by Synapse and cannot be controlled by an attacker, which limits the impact.

Homeservers with the media repository disabled are unaffected. Homeservers configured with a federation whitelist are also unaffected.

Patches

Server administrators should upgrade to 1.47.1 or later.

Workarounds

Server administrators using a reverse proxy could, at the expense of losing media functionality, block the following endpoints:

/_matrix/media/r0/download/{serverName}/{mediaId}
/_matrix/media/r0/download/{serverName}/{mediaId}/{fileName}
/_matrix/media/r0/thumbnail/{serverName}/{mediaId}

Alternatively, non-containerized deployments can be adapted to use the hardened systemd config, located at contrib/systemd/override-hardened.conf.

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	1.47.1`pip install --upgrade 'matrix-synapse==1.47.1'`

Affected Products

2 products · 3 configurations

fedorafedoraproject

2 versions

3435

Application

synapsematrix

< 1.47.1

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 1.47.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-41281 is resolved across your whole dependency graph.
Workarounds
Resolve every user-supplied path to its canonical form and reject anything that escapes the intended directory, and run the component under an account that has no read or write access outside the directory it legitimately serves.

Frequently Asked Questions

### Impact Synapse instances with the media repository enabled can be tricked into downloading a file from a remote server into an arbitrary directory, potentially outside the media store directory. The last two directories and file name of the path are chosen randomly by Synapse and cannot be controlled by an attacker, which limits the impact. Homeservers with the media repository disabled are unaffected. Homeservers configured with a federation whitelist are also unaffected. ### Patches Server administrators should upgrade to 1.47.1 or later. ### Workarounds Server administrators using