GHSA-pmc3-p9hx-jq96 is a medium-severity (CVSS 6.5) CWE-693 vulnerability in github.com/refraction-networking/utls. A fix is available for github.com/refraction-networking/utls — see the affected versions and patch details below.
uTLS ServerHellos are accepted without checking TLS 1.3 downgrade canaries
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- 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-pmc3-p9hx-jq96.
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-pmc3-p9hx-jq96 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 377,636 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
github.com/refraction-networking/utlsReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Description
Before version 1.7.0, utls did not implement the TLS 1.3 downgrade protection mechanism specified in RFC 8446 Section 4.1.3 when using a utls ClientHello spec. This allowed an active network adversary to downgrade TLS 1.3 connections initiated by a utls client to a lower TLS version (e.g., TLS 1.2) by modifying the ClientHello message to exclude the SupportedVersions extension, causing the server to respond with a TLS 1.2 ServerHello (along with a downgrade canary in the ServerHello random field). Because utls did not check the downgrade canary in the ServerHello random field, clients would accept the downgraded connection without detecting the attack. This attack could also be used by an active network attacker to fingerprint utls connections.
Fix Commit or Pull Request
refraction-networking/utls#337, specifically refraction-networking/utls@f8892761e2a4d29054264651d3a86fda83bc83f9
References
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/refraction-networking/utls | all versions | 1.7.0go get github.com/refraction-networking/utls@v1.7.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/refraction-networking/utls, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/refraction-networking/utls to 1.7.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-pmc3-p9hx-jq96 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-pmc3-p9hx-jq96 can be triaged on real exposure rather than presence alone.
Tailored to GHSA-pmc3-p9hx-jq96. 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-pmc3-p9hx-jq96 in your dependencies?
O3 Security finds GHSA-pmc3-p9hx-jq96 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.