+ "details": "## Summary\n\nIn impacted versions of Astro using [on-demand rendering](https://docs.astro.build/en/guides/on-demand-rendering/), request headers `x-forwarded-proto` and `x-forwarded-port` are insecurely used, without sanitization, to build the URL. This has several consequences the most important of which are:\n\n- Middleware-based protected route bypass (only via `x-forwarded-proto`)\n- DoS via cache poisoning (if a CDN is present)\n- SSRF (only via `x-forwarded-proto`)\n- URL pollution (potential SXSS, if a CDN is present) \n- WAF bypass\n\n## Details\n\nThe `x-forwarded-proto` and `x-forwarded-port` headers are used without sanitization in two parts of the Astro server code. The most important is in the `createRequest()` function. Any configuration, including the default one, is affected: \n\n[https://github.com/withastro/astro/blob/970ac0f51172e1e6bff4440516a851e725ac3097/packages/astro/src/core/app/node.ts#L97](https://github.com/withastro/astro/blob/970ac0f51172e1e6bff4440516a851e725ac3097/packages/astro/src/core/app/node.ts#L97)\n[https://github.com/withastro/astro/blob/970ac0f51172e1e6bff4440516a851e725ac3097/packages/astro/src/core/app/node.ts#L121](https://github.com/withastro/astro/blob/970ac0f51172e1e6bff4440516a851e725ac3097/packages/astro/src/core/app/node.ts#L121)\n\nThese header values are then used directly to construct URLs.\n\nBy injecting a payload at the protocol level during URL creation (via the `x-forwarded-proto` header), the entire URL can be rewritten, including the host, port and path, and then pass the rest of the URL, the real hostname and path, as a query so that it doesn't affect (re)routing.\n\nIf the following header value is injected when requesting the path `/ssr`:\n\n```\nx-forwarded-proto: https://www.malicious-url.com/?tank=\n```\n\nThe complete URL that will be created is: `https://www.malicious-url.com/?tank=://localhost/ssr`\n\nAs a reminder, URLs are created like this:\n\n```\nurl = new URL(`${protocol}://${hostnamePort}${req.url}`);\n```\n\nThe value is injected at the beginning of the string (`${protocol}`), and ends with a query `?tank=` whose value is the rest of the string, `://${hostnamePort}${req.url}`.\n\nThis way there is control over the routing without affecting the path, and the URL can be manipulated arbitrarily. This behavior can be exploited in various ways, as will be seen in the PoC section.\n\nThe same logic applies to `x-forwarded-port`, with a few differences.\n\n> [!NOTE]\n> The `createRequest` function is called every time a non-static page is requested. Therefore, all non-static pages are exploitable for reproducing the attack.\n\n## PoC\n\nThe PoC will be tested with a minimal repository:\n\n- Latest Astro version at the time (`2.16.0`)\n- The Node adapter\n- Two simple pages, one SSR (`/ssr`), the other simulating an admin page (`/admin`) protected by a middleware\n- A middleware example copied and pasted from the official Astro documentation to protect the admin page based on the path\n\n[Download the PoC repository](https://github.com/zhero-web-sec/astro-app)\n\n### Middleware-based protected route bypass - x-forwarded-proto only\n\nThe middleware has been configured to protect the `/admin` route based on [the official documentation](https://docs.astro.build/en/guides/authentication/):\n\n```ts\n// src/middleware.ts\nimport { defineMiddleware } from \"astro/middleware\";\n\nexport const onRequest = defineMiddleware(async (context, next) => {\n const isAuthed = false; // auth logic\n if (context.url.pathname === \"/admin\" && !isAuthed) {\n return context.redirect(\"/\");\n }\n return next();\n});\n```\n\n1. When tryint to access `/admin` the attacker is naturally redirected :\n ```sh\n curl -i http://localhost:4321/admin\n ```\n <img width=\"620\" height=\"102\" alt=\"image\" src=\"https://github.com/user-attachments/assets/15a7bffc-ee56-4ed9-84b2-091cf4d78351\" />\n\n2. The attackr can bypass the middleware path check using a malicious header value:\n ```sh\n curl -i -H \"x-forwarded-proto: x:admin?\" http://localhost:4321/admin\n ```\n <img width=\"1348\" height=\"159\" alt=\"image\" src=\"https://github.com/user-attachments/assets/d9d9ac1a-5efa-452b-981e-efea8a08d089\" />\n\n#### How is this possible?\n\nHere, with the payload `x:admin?`, the attacker can use the URL API parser to their advantage:\n\n- `x:` is considered the protocol\n- Since there is no `//`, the parser considers there to be no authority, and everything before the `?` character is therefore considered part of the path: `admin`\n\nDuring a path-based middleware check, the *path* value begins with a `/`: `context.url.pathname === \"/admin\"`. However, this is not the case with this payload; `context.url.pathname === \"admin\"`, the absence of a slash satisfies both the middleware check and the router and consequently allows us to bypass the protection and access the page.\n\n### SSRF\n\nAs seen, the request URL is built from untrusted input via the `x-forwarded-protocol` header, if it turns out that this URL is subsequently used to perform external network calls, for an API for example, this allows an attacker to supply a malicious URL that the server will fetch, resulting in server-side request forgery (SSRF).\n\nExample of code reusing the \"origin\" URL, concatenating it to the API endpoint :\n\n<img width=\"601\" height=\"418\" alt=\"image\" src=\"https://github.com/user-attachments/assets/9c374b2c-841c-48d6-98f1-3b3f5b060802\" />\n\n### DoS via cache poisoning\n\nIf a CDN is present, it is possible to force the caching of bad pages/resources, or 404 pages on the application routes, rendering the application unusable.\n\nA `404` cab be forced, causing an error on the `/ssr` page like this : `curl -i -H \"x-forwarded-proto: https://localhost/vulnerable?\" http://localhost:4321/ssr`\n<img width=\"998\" height=\"108\" alt=\"image\" src=\"https://github.com/user-attachments/assets/4bab58e5-3045-4e25-9aa2-2f72a0832d86\" />\n\nSame logic applies to `x-forwarded-port` : `curl -i -H \"x-forwarded-port: /vulnerable?\" http://localhost:4321/ssr`\n\n#### How is this possible?\n\nThe router sees the request for the path `/vulnerable`, which does not exist, and therefore returns a `404`, while the potential CDN sees `/ssr` and can then cache the `404` response, consequently serving it to all users requesting the path `/ssr`.\n\n### URL pollution\n\nThe exploitability of the following is also contingent on the presence of a CDN, and is therefore cache poisoning.\n\nIf the value of `request.url` is used to create links within the page, this can lead to Stored XSS with `x-forwarded-proto` and the following value:\n\n```\nx-forwarded-proto: javascript:alert(document.cookie)//\n```\n\nresults in the following URL object:\n\n<img width=\"444\" height=\"202\" alt=\"image\" src=\"https://github.com/user-attachments/assets/c2990626-da5b-4868-9093-dbb9b34780ba\" />\n\nIt is also possible to inject any link, always, if the value of `request.url` is used on the server side to create links.\n\n```\nx-forwarded-proto: https://www.malicious-site.com/bad?\n```\n\n**The attacker is more limited with `x-forwarded-port`**\n\nIf the value of `request.url` is used to create links within the page, this can lead to broken links, with the header and the following value:\n\n```\nX-Forwarded-Port: /nope?\n```\n\nExample of an Astro website: \n<img width=\"1627\" height=\"298\" alt=\"Capture d’écran 2025-11-03 à 22 07 14\" src=\"https://github.com/user-attachments/assets/02de5e67-f48d-4bf4-810d-6b0714ad2c12\" />\n\n### WAF bypass\n\nFor this section, Astro invites users to read previous research on the React-Router/Remix framework, in the section \"Exploitation - WAF bypass and escalations\". This research deals with a similar case, the difference being that the vulnerable header was `x-forwarded-host` in their case:\n\n[https://zhero-web-sec.github.io/research-and-things/react-router-and-the-remixed-path](https://zhero-web-sec.github.io/research-and-things/react-router-and-the-remixed-path)\n\nNote: A section addressing DoS attacks via cache poisoning using the same vector was also included there.\n\n### CVE-2025-61925 complete bypass\n\nIt is possible to completely bypass the vulnerability patch related to the `X-Forwarded-Host` header.\n\nBy sending `x-forwarded-host` with an empty value, the `forwardedHostname` variable is assigned an empty string. Then, during [the subsequent check](https://github.com/withastro/astro/blob/7a5f28006e9b1f6ad77c7884991ba551ca9ff35b/packages/astro/src/core/app/node.ts#L107), the condition fails because `forwardedHostname ` returns `false`, its value being an empty string:\n\n```\nif (forwardedHostname && !App.validateForwardedHost(...))\n\n```\n\nConsequently, the implemented check is bypassed. From this point on, since the request has no `host` (*its value being an empty string*), the path value is retrieved by the URL parser to set it as the `host`. This is because the `http/https` schemes are considered special schemes by the [WHATWG URL Standard Specification](https://url.spec.whatwg.org/#scheme-state), requiring an `authority state`.\n\nFrom there, the following request on the example SSR application (astro repo) yields an SSRF:\n<img width=\"1878\" height=\"456\" alt=\"Capture d’écran 2025-11-06 à 21 18 26\" src=\"https://github.com/user-attachments/assets/c5cca89c-9c65-46f6-bf70-cd7a90a9e0d9\" />\n*empty `x-forwarded-host` + the target `host` in the path*\n\n## Credits\n\n- Allam Rachid ([zhero;](https://zhero-web-sec.github.io/research-and-things/))\n- Allam Yasser (inzo)",
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