GHSA-4c8g-83qw-93j6 is a high-severity (CVSS 7.5) CWE-436 vulnerability in fast-uri. O3 Security confirms whether GHSA-4c8g-83qw-93j6 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
fast-uri vulnerable to host confusion via failed IDN canonicalization
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-4c8g-83qw-93j6.
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-4c8g-83qw-93j6 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 369,023 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
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
fast-urinpmDescription
Impact
fast-uri versions >= 2.3.1, <= 4.0.0 fail to canonicalize Unicode/IDN hostnames for HTTP-family URLs. The IDN conversion path calls URL.domainToASCII(...) on the global WHATWG URL constructor, where that helper does not exist. The resulting TypeError is silently routed into parsed.error, but parse(), normalize(), and equal() all return with the host left in its original Unicode form.
For example, http://127。0。0。1/ is treated by fast-uri as host 127。0。0。1, while Node's WHATWG URL parser and fetch() canonicalize the same input to 127.0.0.1.
Applications that use fast-uri to enforce host-based policy (denylists, loopback filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or fetch() consumers see a policy/use desync and can be steered to an unintended destination.
Patches
Upgrade to fast-uri v4.0.1, v3.1.3, or v2.4.2
Workarounds
None. Upgrade to the patched version.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fast-uri | ≥ 4.0.0&&< 4.0.1 | 4.0.1 |
| 📦npm | fast-uri | ≥ 3.0.0&&< 3.1.3 | 3.1.3 |
| 📦npm | fast-uri | ≥ 2.3.1&&< 2.4.2 | 2.4.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fast-uri. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update fast-uri to 4.0.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-4c8g-83qw-93j6 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 pinpoints whether GHSA-4c8g-83qw-93j6 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-4c8g-83qw-93j6. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
This Important flaw in `fast-uri` allows a remote attacker to bypass host-based security policies. Applications that rely on `fast-uri` for URL parsing and policy enforcement, such as denylists or redirect validations, can be circumvented due to inconsistent handling of Unicode hostnames. This discrepancy between…
Frequently Asked Questions
Is GHSA-4c8g-83qw-93j6 in your dependencies?
O3 detects GHSA-4c8g-83qw-93j6 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.