CVE-2026-75899

fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding

Workarounds

Reject untrusted URIs whose host component contains an encoded percent sign (%25) before passing them to normalize() or resolve().

fast-uri decodes a hostname's percent escapes twice in a single normalize() or resolve() call: once during parsing and again during authority recomposition. A nested percent-encoded host therefore survives the first decode and is turned into a live destination by the second, so normalize('http://%256c%256f%2563%2561%256c%2568%256f%2573%2574/') returns http://localhost/. Applications that normalize or resolve an untrusted URI before an SSRF check, redirect validation, or host allowlist can be steered to a different destination, including internal addresses such as loopback or a cloud metadata endpoint, than the encoded input appeared to contain. This is an incomplete-fix variant of CVE-2026-6322, whose encoded-authority-delimiter fix introduced the second decode.

Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.

CVE-2026-75899 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.

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Fixed in fast-uri 2.4.5, 3.1.6, and 4.1.3.

1. What is CVE-2026-75899? CVE-2026-75899 is a high-severity server-side request forgery (SSRF) vulnerability in fast-uri (npm), affecting versions >= 2.4.1, < 2.4.5. It is fixed in 2.4.5, 3.1.6, 4.1.3. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
2. How severe is CVE-2026-75899? CVE-2026-75899 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-2026-75899? fast-uri (npm) versions >= 2.4.1, < 2.4.5 is affected.
4. Is there a fix for CVE-2026-75899? Yes. CVE-2026-75899 is fixed in 2.4.5, 3.1.6, 4.1.3. Upgrade to this version or later.
5. Is CVE-2026-75899 exploitable, and should I be worried? Whether CVE-2026-75899 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-2026-75899 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-2026-75899?
   • 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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