CVE-2026-75899

About vulnerability

Impact

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.

Patches

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

Workarounds

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

Affected product:

AngularJS , Fastify , Node.js , React , Vue , ajv , ajv-compiler , ajv-draft-04 , ajv-errors , ajv-formats , ajv-keywords , api-extractor-model , babel-loader , cache-loader , css-loader , css-minimizer-webpack-plugin , devkit , esbuild-loader , extract-css-chunks-webpack-plugin , fast-json-stringify , fast-json-stringify-compiler , fast-uri , file-loader , fork-ts-checker-webpack-plugin , friendly-errors-webpack-plugin , hard-source-webpack-plugin , html-webpack-plugin , light-my-request , mini-css-extract-plugin , minimizer-webpack-plugin , nuxt , optimize-css-assets-webpack-plugin , postcss-loader , rushstack , schema-utils , style-resources-loader , table , thread-loader , time-fix-plugin , ts-command-line , tsdoc , url-loader , webpack , webpack-dev-middleware , webpack-dev-server , webpackbar , worker-loader

Affected packages:

fastify @ 4.29.1 (+458 more)