CVE-2025-27143

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., https://evil.com), it incorrectly allows scheme-less URLs (e.g., //malicious-site.com). This results in the browser interpreting the URL as https://malicious-site.com, leading to unintended redirection.

bypass for : https://github.com/better-auth/better-auth/security/advisories/GHSA-8jhw-6pjj-8723

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., https://evil.com), it improperly allows scheme-less URLs (e.g., //malicious-site.com). This issue occurs because browsers interpret //malicious-site.com as https://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.

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.

Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.

Kodem intelligence

1. What is CVE-2025-27143? CVE-2025-27143 is a medium-severity open redirect vulnerability in better-auth (npm), affecting versions < 1.1.20. It is fixed in 1.1.20. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
2. Which versions of better-auth are affected by CVE-2025-27143? better-auth (npm) versions < 1.1.20 is affected.
3. Is there a fix for CVE-2025-27143? Yes. CVE-2025-27143 is fixed in 1.1.20. Upgrade to this version or later.
4. Is CVE-2025-27143 exploitable, and should I be worried? Whether CVE-2025-27143 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 CVE-2025-27143 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 CVE-2025-27143? Upgrade better-auth to 1.1.20 or later.