CVE-2026-76172
HIGHCVE-2026-76172 is a high-severity (CVSS 7.5) CWE-177 vulnerability in fast-uri. O3 Security confirms whether CVE-2026-76172 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
fast-uri vulnerable to host confusion via percent-encoded scheme normalization
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 CVE-2026-76172.
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
CVE-2026-76172 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 367,996 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
fast-uri is a URI parser for Node.js. During parsing it runs a legacy decoding pass over the scheme component and never re-escapes the result, and serialization writes the scheme back out verbatim, unlike the host component which is re-escaped. As a result an input whose scheme carries percent-encoded slashes parses as a scheme with no authority, so the parsed host and error are both undefined, yet resolving or normalizing that same input emits a network-path reference whose authority is attacker-chosen and re-parses to that host. An application that allowlists on the parsed host, or treats a reference with no authority as safe to resolve against its base, gets the opposite of what it checked, giving an off-site redirect, server-side request forgery, or address-policy bypass. The legacy decoder also expands non-standard escape forms, widening the issue past upstream filters, and control characters in the scheme can reach the output as raw carriage return and line feed. The affected versions are 2.3.1 up to but not including 2.4.5, 3.0.0 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which reject a scheme that is not valid after decoding. Users should upgrade to a patched version.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | fast-uri | ≥ 2.3.1&&< 2.4.5 | 2.4.5 |
| 📦npm | fast-uri | ≥ 3.0.0&&< 3.1.6 | 3.1.6 |
| 📦npm | fast-uri | ≥ 4.0.0&&< 4.1.3 | 4.1.3 |
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 2.4.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-76172 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 CVE-2026-76172 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 CVE-2026-76172. 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.
Frequently Asked Questions
Is CVE-2026-76172 in your dependencies?
O3 detects CVE-2026-76172 across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.