GHSA-569Q-MPPH-WGWW

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:

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

Here's the router receiving the poisoned basePath:

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. Once the router basePath is poisoned it fails to match & route incoming requests.

Repro

Start a better-auth server with no baseURL configuration.

Send the following request as the first request to the server:

curl -i --location 'https://example.com/api/auth/ok' \
--header 'X-Forwarded-Proto: some:' \
--header 'X-Forwarded-Host: junk'

The better-auth API check endpoint returns 404.

Now send a regular request without the X-Forwarded-Proto and X-Forwarded-Host headers.

curl -i --location 'https://example.com/api/auth/ok'

The better-auth API check endpoint still returns 404.

Example result

We have modified the basePath for the router until the server is restarted. An attacker can repeatedly send these attack requests aiming to persistently exploit the vulnerability.

Kodem intelligence

1. What is GHSA-569Q-MPPH-WGWW? GHSA-569Q-MPPH-WGWW is a low-severity security vulnerability in better-auth (npm), affecting versions < 1.4.2. It is fixed in 1.4.2.
2. Which versions of better-auth are affected by GHSA-569Q-MPPH-WGWW? better-auth (npm) versions < 1.4.2 is affected.
3. Is there a fix for GHSA-569Q-MPPH-WGWW? Yes. GHSA-569Q-MPPH-WGWW is fixed in 1.4.2. Upgrade to this version or later.
4. Is GHSA-569Q-MPPH-WGWW exploitable, and should I be worried? Whether GHSA-569Q-MPPH-WGWW is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether GHSA-569Q-MPPH-WGWW is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix GHSA-569Q-MPPH-WGWW? Upgrade better-auth to 1.4.2 or later.