Beter Auth has an Open Redirect via Scheme-Less Callback Parameter

Summary

The application is vulnerable to an open redirect due to improper validation of the callbackURL parameter in the email verification endpoint and any other endpoint that accepts callback url. While the server blocks fully qualified URLs (e.g., evil.com), it incorrectly allows scheme-less URLs (e.g., //malicious-site.com). This results in the browser interpreting the URL as malicious-site.com, leading to unintended redirection.

bypass for : github.com/better-auth/better-auth/security/adv…

Affected Versions

All versions prior to 1.1.19

Details

The application’s email verification endpoint (/auth/verify-email) accepts a callbackURL parameter intended to redirect users after successful email verification. While the server correctly blocks fully qualified external URLs (e.g., evil.com), it improperly allows scheme-less URLs (e.g., //malicious-site.com). This issue occurs because browsers interpret //malicious-site.com as malicious-site.com, leading to an open redirect vulnerability.

An attacker can exploit this flaw by crafting a malicious verification link and tricking users into clicking it. Upon successful email verification, the user will be automatically redirected to the attacker's website, which can be used for phishing, malware distribution, or stealing sensitive authentication tokens.

Impact

Phishing & Credential Theft – Attackers can redirect users to a fake login page, tricking them into entering sensitive credentials, which can then be stolen.

Session Hijacking & Token Theft – If used in OAuth flows, an attacker could redirect authentication tokens to their own domain, leading to account takeover.