CVE-2026-75975

fast-uri vulnerable to server-side request forgery via malformed IPv6 normalization

Workarounds

Reject untrusted URLs whose host is a bracketed IPv6 literal before passing them to fast-uri, or route outbound requests against an explicit allowlist of addresses rather than trusting the normalized host.

fast-uri does not validate the complete RFC 3986 grammar for bracketed IPv6 literals, so a malformed literal with invalid trailing text is silently truncated to a different valid IPv6 address with no error reported. For example, normalize('http://[::not-valid]/private') returns http://[::]/private, and [fc00::not-hex] and [fe80::not-hex] collapse to [fc00::] and [fe80::]. An application that normalizes an untrusted URL before an outbound request, redirect, or host-policy check can be routed to a local or private address such as loopback (::1), unique-local, or link-local. Because parse().error is unset for these inputs, checking it does not protect the consumer.

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-75975 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.4.5, 3.1.6, 4.1.3); upgrading removes the vulnerable code path.

Kodem intelligence

Already deployed Kodem?

Upgrade to fast-uri 2.4.5, 3.1.6, or 4.1.3. Malformed IPv6 literals are now rejected with a host error instead of being normalized to a valid address.

1. What is CVE-2026-75975? CVE-2026-75975 is a high-severity improper input validation vulnerability in fast-uri (npm), affecting versions >= 2.3.1, < 2.4.5. It is fixed in 2.4.5, 3.1.6, 4.1.3. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-75975? CVE-2026-75975 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of fast-uri are affected by CVE-2026-75975? fast-uri (npm) versions >= 2.3.1, < 2.4.5 is affected.
4. Is there a fix for CVE-2026-75975? Yes. CVE-2026-75975 is fixed in 2.4.5, 3.1.6, 4.1.3. Upgrade to this version or later.
5. Is CVE-2026-75975 exploitable, and should I be worried? Whether CVE-2026-75975 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-75975 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-75975?
   • Upgrade fast-uri to 2.4.5 or later
   • Upgrade fast-uri to 3.1.6 or later
   • Upgrade fast-uri to 4.1.3 or later

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