Astro's middleware authentication checks based on url.pathname can be bypassed via url encoded values

A mismatch exists between how Astro normalizes request paths for routing/rendering and how the application’s middleware reads the path for validation checks. Astro internally applies `decodeURI()` to determine which route to render, while the middleware uses `context.url.pathname` without applying the same normalization (decodeURI). This discrepancy may allow attackers to reach protected routes (e.g., /admin) using encoded path variants that pass routing but bypass validation checks. https://github.com/withastro/astro/blob/ebc4b1cde82c76076d5d673b5b70f94be2c066f3/packages/astro/src/vite-plugin-astro-server/request.ts#L40-L44 ```js /** The main logic to route dev server requests to pages in Astro. */ export async function handleRequest({ pipeline, routesList, controller, incomingRequest, incomingResponse, }: HandleRequest) { const { config, loader } = pipeline; const origin = `${loader.isHttps() ? 'https' : 'http'}://${ incomingRequest.headers[':authority'] ?? incomingRequest.headers.host }`; const url = new URL(origin + incomingRequest.url); let pathname: string; if (config.trailingSlash === 'never' && !incomingRequest.url) { pathname = ''; } else { // We already have a middleware that checks if there's an incoming URL that has invalid URI, so it's safe // to not handle the error: packages/astro/src/vite-plugin-astro-server/base.ts pathname = decodeURI(url.pathname); // here this url is for routing/rendering } // Add config.base back to url before passing it to SSR url.pathname = removeTrailingForwardSlash(config.base) + url.pathname; // this is used for middleware context ``` Consider an application having the following middleware code: ```js import { defineMiddleware } from "astro/middleware"; export const onRequest = defineMiddleware(async (context, next) => { const isAuthed = false; // simulate no auth if (context.url.pathname === "/admin" && !isAuthed) { return context.redirect("/"); } return next(); }); ``` `context.url.pathname` is validated , if it's equal to `/admin` the `isAuthed` property must be true for the next() method to be called. The same example can be found in the official docs https://docs.astro.build/en/guides/authentication/ `context.url.pathname` returns the raw version which is `/%61admin` while pathname which is used for routing/rendering `/admin`, this creates a path normalization mismatch. By sending the following request, it's possible to bypass the middleware check ``` GET /%61dmin HTTP/1.1 Host: localhost:3000 ``` <img width="1920" height="1025" alt="image" src="https://github.com/user-attachments/assets/7e0eeecd-607a-4c73-b12e-5977a30c9bc4" /> **Remediation** Ensure middleware context has the same normalized pathname value that Astro uses internally, because any difference could allow it to bypass such checks. In short maybe something like this ```diff pathname = decodeURI(url.pathname); } // Add config.base back to url before passing it to SSR - url.pathname = removeTrailingForwardSlash(config.base) + url.pathname; + url.pathname = removeTrailingForwardSlash(config.base) + decodeURI(url.pathname); ``` Thank you, let @Sudistark know if any more info is needed. Happy to help :)

Source published

19 Nov 2025 20:03:21 UTC

Source modified

4 Feb 2026 03:01:27 UTC

Observed here

24 Oct 2026 13:17:17 UTC

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GitHub Advisory Database · CC-BY-4.0

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CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:L/VI:N/VA:N/SC:N/SI:N/SA:N

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