How severe is GHSA-fh66-fcv5-jjfr?

No CVSS score has been assigned to GHSA-fh66-fcv5-jjfr yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by GHSA-fh66-fcv5-jjfr?

GHSA-fh66-fcv5-jjfr affects the following packages: matrix-synapse (PyPI), matrix-synapse (PyPI). Ecosystems affected: PyPI.

How do I fix GHSA-fh66-fcv5-jjfr?

Update matrix-synapse to 1.138.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fh66-fcv5-jjfr is resolved across your whole dependency graph.

How do I detect GHSA-fh66-fcv5-jjfr 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 GHSA-fh66-fcv5-jjfr if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is GHSA-fh66-fcv5-jjfr actively exploited in the wild?

No public exploit code has been indexed for GHSA-fh66-fcv5-jjfr 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 GHSA-fh66-fcv5-jjfr?

GHSA-fh66-fcv5-jjfr has an EPSS (Exploit Prediction Scoring System) score of 0.5%, placing it in the 41th 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 GHSA-fh66-fcv5-jjfr?

GHSA-fh66-fcv5-jjfr is classified as CWE-1287 (CWE-1287). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was GHSA-fh66-fcv5-jjfr published, and has it been updated?

GHSA-fh66-fcv5-jjfr was published on October 8, 2025 and was last updated on July 7, 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

Synapse's invalid device keys degrade federation functionalityGHSA-fh66-fcv5-jjfr

Fix: element-hq/synapse#17097

GHSA-fh66-fcv5-jjfr is a CWE-1287 vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.

Also known asCVE-2025-61672PYSEC-2026-1612

Published

Oct 8, 2025

Updated

Jul 7, 2026

Affected

2 pkgs

Patched

2 / 2

Exploits

None indexed

Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-fh66-fcv5-jjfr.

EPSS Exploitation Probability

via FIRST.org ↗

0.5%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs41th percentile — riskier than 41% of all scored CVEsHighest risk

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

Real-World Exposure

2 pkgs affected

🐍matrix-synapse🐍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

Lack of validation for device keys in Synapse before 1.138.3 and in Synapse 1.139.0 allow an attacker registered on the victim homeserver to degrade federation functionality, unpredictably breaking outbound federation to other homeservers.

Patches

Patched in Synapse 1.138.3, 1.138.4, 1.139.1, and 1.139.2.

Note that even though 1.138.3 and 1.139.1 fix the vulnerability, they inadvertently introduced an unrelated regression. For this reason, it is recommend to skip these releases and upgrading straight to 1.138.4 and 1.139.2.

Workarounds

The vulnerability can only be exploited by users registered on the victim homeserver.

Affected Packages

2 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`matrix-synapse`	all versions	1.138.3`pip install --upgrade 'matrix-synapse==1.138.3'`
🐍PyPI	`matrix-synapse`	≥ 1.139.0rc2&&< 1.139.1	1.139.1`pip install --upgrade 'matrix-synapse==1.139.1'`

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 1.138.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fh66-fcv5-jjfr is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact Lack of validation for device keys in Synapse before 1.138.3 and in Synapse 1.139.0 allow an attacker registered on the victim homeserver to degrade federation functionality, unpredictably breaking outbound federation to other homeservers. ### Patches Patched in Synapse 1.138.3, 1.138.4, 1.139.1, and 1.139.2. Note that even though 1.138.3 and 1.139.1 fix the vulnerability, they inadvertently introduced an unrelated regression. For this reason, it is recommend to skip these releases and upgrading straight to 1.138.4 and 1.139.2. ### Workarounds The vulnerability can only be ex

O3 Security · Impact-Aware SCA

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