Affected: litestar-org litestar

Litestar is an Asynchronous Server Gateway Interface (ASGI) framework. In versions prior to 2.22.0, an attacker can bypass the allowed hosts validation by omitting the Host header and supplying an X-Forwarded-Host header set to a whitelisted domain. The AllowedHostsMiddleware trusts the X-Forwarded-Host header as a fallback when the Host header is absent. Since X-Forwarded-Host is a client-controllable header, this enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation. Any application using AllowedHostsConfig is affected when deployed without a reverse proxy that strips X-Forwarded-Host, or when accepting HTTP/1.0 connections. This issue has been fixed in version 2.22.0.

Affected: PyPI: litestar

### Summary

`AllowedHostsMiddleware` trusts the `X-Forwarded-Host` header as a fallback when the `Host` header is absent. Since `X-Forwarded-Host` is a client-controllable header, an attacker can bypass the allowed hosts validation by omitting the `Host` header and supplying an `X-Forwarded-Host` header set to a whitelisted domain. This enables host header injection attacks such as password reset poisoning, cache poisoning, and server-side request routing manipulation.

### Details

In `AllowedHostsMiddleware.__call__`, the host value used for validation is resolved as follows:

https://github.com/litestar-org/litestar/blob/main/litestar/middleware/allowed_hosts.py#L68

```python headers = MutableScopeHeaders(scope=scope) if host := headers.get("host", headers.get("x-forwarded-host", "")).split(":")[0]: if self.allowed_hosts_regex.fullmatch(host): await self.app(scope, receive, send) return ```

When `Host` is absent (e.g., HTTP/1.0 clients, misconfigured proxies, or raw TCP connections), the middleware falls back to `X-Forwarded-Host` without any verification that the request actually passed through a trusted reverse proxy.

An attacker can send a request with no `Host` header and set `X-Forwarded-Host` to any whitelisted domain, bypassing the entire allowed hosts check. The application then processes the request as if it originated from a trusted host.

This is particularly dangerous when applications use the resolved host value for: - Generating passw…