Better Auth Open Redirect Vulnerability in originCheck Middleware Affects Multiple Routes

Summary

An open redirect has been found in the originCheck middleware function, which affects the following routes: /verify-email, /reset-password/:token, /delete-user/callback, /magic-link/verify, /oauth-proxy-callback.

Details

In the matchesPattern function, url.startsWith( can be deceived with a url that starts with one of the trustedOrigins.

		const matchesPattern = (url: string, pattern: string): boolean => {
			if (url.startsWith("/")) {
				return false;
			}
			if (pattern.includes("*")) {
				return wildcardMatch(pattern)(getHost(url));
			}
			return url.startsWith(pattern);
		};

Open Redirect PoCs

export const auth = betterAuth({
	baseURL: 'http://localhost:3000',
	trustedOrigins: [
		"http://trusted.com"
	],
	emailAndPassword: {
		...
	},
})

/reset-password/:token

<img width="481" alt="image" src="https://github.com/user-attachments/assets/46e7871a-1dad-4375-af94-0446e29aaab6" /> <br/> <img width="518" alt="image 1" src="https://github.com/user-attachments/assets/83abfb53-6fc9-4d1f-918d-9b4ce093c808" />

/verify-email

<img width="549" alt="image" src="https://github.com/user-attachments/assets/7dd424b7-42a4-4616-aa73-fcc2e3eeb309" /> <br/> <img width="436" alt="image" src="https://github.com/user-attachments/assets/54f11636-0a3e-4e83-9a09-57c5e8ba98cd" />

/delete-user/callback

<img width="545" alt="image" src="https://github.com/user-attachments/assets/2ff1b217-d069-48fb-81c1-f8c8792d34a4" /> <br/> <img width="492" alt="image" src="https://github.com/user-attachments/assets/71df11db-9d38-4f34-abe1-add9d60b3486" />

/magic-link/verify

<img width="379" alt="image" src="https://github.com/user-attachments/assets/6b6b6a8a-59b6-4a65-9df3-57d5b2f6eb0f" /> <br/> <img width="413" alt="image" src="https://github.com/user-attachments/assets/82a5c9c6-2ea0-44eb-af48-40732657b59e" />

/oauth-proxy-callback

<img width="548" alt="image" src="https://github.com/user-attachments/assets/d8d2ee51-e9fd-4337-bec3-a70afd1ceacb" /> <br/> <img width="544" alt="image" src="https://github.com/user-attachments/assets/f097d406-b965-4f85-b124-9b0ef1cc2689" />

Impact

Untrusted open redirects in various routes.