Better Auth's rou3 Dependency has Double-Slash Path Normalization which can Bypass disabledPaths Config and Rate Limits — better-auth
Uncontrolled Resource ConsumptionSummary
An issue in the underlying router library rou3 can cause /path and //path to be treated as identical routes. If your environment does not normalize incoming URLs (e.g., by collapsing multiple slashes), this can allow bypasses of disabledPaths and path-based rate limits.
Details
Better Auth uses better-call, which internally relies on rou3 for routing. Affected versions of rou3 normalize paths by removing empty segments. As a result:
/sign-in/email//sign-in/email///sign-in/email
…all resolve to the same route.
Some production setups automatically collapse multiple slashes. This includes:
- Vercel with Nextjs (default)
- Cloudflare - when normalize to urls origin is enabled (https://developers.cloudflare.com/rules/normalization/settings/#normalize-urls-to-origin)
In these environments and other configurations where //path reach Better Auth as /path, the issue does not apply.
Fix
Updating rou3 to the latest version resolves the issue:
- better-call previously depended on
"rou3": "^0.5.1" - The fix was introduced after that version (commit: https://github.com/h3js/rou3/commit/f60b43fa648399534507c9ac7db36d705b8874c3)
Better Auth recommends:
- Upgrading to Better Auth v1.4.5 or later, which includes the updated rou3.
- Ensuring the proxy normalizes URLs.
- If project maintainers cannot upgrade yet, they can protect their app by normalizing url before it reaches better-auth handler. See example below:
const req = new Request(...) // this would be the actual request object
const url = new URL(req.url);
const normalizedPath = url.pathname.replace(/\/+/g, "/");
if (url.pathname !== normalizedPath) {
url.pathname = normalizedPath;
// Update the raw request pathname
Object.defineProperty(req, "url", {
value: url.toString(),
writable: true,
configurable: true,
});
}
Impact
- Bypass
disabledPaths - Bypass path-based rate limits
The impact of bypassing disabled paths could vary based on a project's configuration.
| Software | From | Fixed in |
|---|---|---|
better-auth
|
- | 1.4.5 |
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