better-auth (npm) before 1.4.2 allows an external request to configure baseURL when it is not otherwise defined (e.g., BETTER_AUTH_URL is unset). An attacker able to make the very first request to the server after startup can poison the router's base path, causing all routes to return 404 for all users (denial of service). The issue is not reachable when baseURL is explicitly configured or on typical managed hosting platforms.

Affected: npm: better-auth

# Summary

Affected versions of Better Auth allow an external request to configure `baseURL` when it isn’t defined through any other means. This can be abused to poison the router’s base path, causing all routes to return 404 for all users.

This issue is only exploitable when `baseURL` is not explicitly configured (e.g., `BETTER_AUTH_URL` is missing) *and* the attacker is able to make the very first request to the server after startup. In properly configured environments or typical managed hosting platforms, this fallback behavior cannot be reached.

# Details

A combination of `X-Forwarded-Host` and `X-Forwarded-Proto` is implicitly trusted. This allows the first request to configure baseURL whenever it is not explicitly configured.

Here's the code that reads the headers:

<img width="631" height="219" alt="headers" src="https://github.com/user-attachments/assets/b3fb0078-a62f-4058-9d0b-4afbd30c4953" />

Here's the call to `getBaseURL()`, the result is assigned to `ctx.baseURL`.

<img width="838" height="414" alt="write" src="https://github.com/user-attachments/assets/a7b4dd17-75c3-49ef-9d08-6a2079d6a0ea" />

Here's the router receiving the poisoned `basePath`:

<img width="594" height="372" alt="router" src="https://github.com/user-attachments/assets/5fdf2862-9cd1-4b96-b146-18e67d904157" />

`X-Forwarded-Host` and `X-Forwarded-Proto` can be used to modify the pathname of a parsed URL object which forms `baseURL`. `basePath` is then derived from the pathname of `baseURL`.…