GHSA-v56r-hwv5-mxg6 is a high-severity (CVSS 7.1) Improper Input Validation vulnerability in matrix-synapse. A fix is available for matrix-synapse — see the affected versions and patch details below.
Synapse vulnerable to federation denial of service via malformed events
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-v56r-hwv5-mxg6.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-v56r-hwv5-mxg6 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
matrix-synapseReal-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
A malicious server can craft events with a depth outside the integer range allowed by Canonical JSON. When such an event is received by Synapse version up to 1.127.0, it prevents it from federating with other servers. The vulnerability has been exploited in the wild.
Patches
Fixed in Synapse v1.127.1.
Workarounds
Closed federation environments of trusted servers or non-federating installations are not affected.
For more information
If you have any questions or comments about this advisory, please email us at security at element.io.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | matrix-synapse | all versions | 1.127.1pip install --upgrade 'matrix-synapse==1.127.1' |
Detection & mitigation playbook
Open-source dependencyDetect
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 1.127.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-v56r-hwv5-mxg6 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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-v56r-hwv5-mxg6 can be triaged on real exposure rather than presence alone.
Tailored to GHSA-v56r-hwv5-mxg6. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-v56r-hwv5-mxg6 in your dependencies?
O3 Security finds GHSA-v56r-hwv5-mxg6 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.