CVE-2026-76172

fast-uri vulnerable to host confusion via percent-encoded scheme normalization

Workarounds

None. Upgrade to the patched version.

fast-uri decodes percent-encoded characters in the scheme component with the legacy global unescape() and serializes the result back as raw characters, without re-escaping it or validating it as a scheme. A scheme that decodes to characters outside the RFC 3986 scheme grammar can therefore introduce structure the original input did not contain.

For example, %2f%2fevil.example:/pwn parses with no authority (parse().host is undefined), but resolve() and normalize() return //evil.example:/pwn, which reparses with host evil.example. The %uXXXX form (%u002f%u002fevil.example:/pwn) produces the same result, and a scheme containing %0d%0a reaches the output as a raw CR LF.

Applications that normalize or resolve untrusted URLs before a redirect check, host allowlist, or outbound request decision, especially ones that treat a missing authority as same-origin, can be steered to an attacker-chosen authority, and a normalized URI placed in a response header can carry an injected CR LF.

CVE-2026-76172 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 >= 4.1.3, or >= 3.1.6 in the v3.x release line, or >= 2.4.5 in the v2.x release line.

1. What is CVE-2026-76172? CVE-2026-76172 is a high-severity security 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.
2. How severe is CVE-2026-76172? CVE-2026-76172 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-2026-76172? fast-uri (npm) versions >= 2.3.1, < 2.4.5 is affected.
4. Is there a fix for CVE-2026-76172? Yes. CVE-2026-76172 is fixed in 2.4.5, 3.1.6, 4.1.3. Upgrade to this version or later.
5. Is CVE-2026-76172 exploitable, and should I be worried? Whether CVE-2026-76172 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-2026-76172 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-2026-76172?
   • 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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