Better Auth allows bypassing the trustedOrigins Protection which leads to ATO — better-auth
URL Redirection to Untrusted Site ('Open Redirect')Summary
A bypass was found for wildcard or absolute URLs trustedOrigins configurations and opens the victims website to a Open Redirect vulnerability, where it can be used to steal the reset password token of a victims account by changing the "callbackURL" parameter value to a website owned by the attacker.
Details
Absolute URLs
The issue here appears in the middleware, specifically. This protection is not sufficiente and it allows attackers to exploit a open redirect vulnerability, by using the payload /\/example.com. We can check this is a valid URL ( or it will be a valid URL because the URL parser fix it for us ), by checking the image bellow:
// trustedOrigins = [ "https://example.com" ]
validateURL("https://attacker.com", "callbackURL") // ❌ APIError, No Redirect
validateURL("/\/attacker.com", "callbackURL") // ✅ Redirect to http://attacker.com
Regex
The issue here is because the regex is not strong enough [^/\\]*?\.example\.com[/\\]*? ( this is the regex it will be created if we have a wildcard as the trustedOrigins config ), but we can bypass by using a payload like:
// trustedOrigins = [ "*.example.com" ]
┌──────────────────┐ ┌────────────────┐ ┌─────────────────┐
│ None of [ "/\" ] │ ────▶ │ ".example.com" │ ────▶ │ One of [ "/\" ] │
└──────────────────┘ └────────────────┘ └─────────────────┘
demo .example.com / ✅ Redirect to https://example.com
demo .attacker.com / ❌ APIError, no redirect
http:attacker.com? .example.com / ✅ Redirect to http://attacker.com
This works because : and ? are special chars in a URL, so when the URL parser sees, http: it will fix our happily fix our URL to http://attacker.com? and make .example.com as parameter, thus, bypassing this check.
PoC
We can PoC the open redirect by using the demo.better-auth.com.
If we access the URL bellow, we are redirected to example.com:
- https://demo.better-auth.com/api/auth/reset-password/x?callbackURL=//example.com
Impact
Every single website using the better-auth library, is vulnerable to un-auth open redirect and more importantilly, vulnerable to potential one click account take over vulnerability, as the attacker can send the victim a email to reset their account while changing the "redirectTo" parameter here, and when the victim clicks on the link, the reset token is sent to the attackers website, which then a attacker could use that token to reset the password of the victims account.
| Software | From | Fixed in |
|---|---|---|
better-auth
|
- | 1.1.21 |
- https://github.com/better-auth/better-auth/blob/ddebd0358d74376ea64541512d0167dd4377f182/packages/better-auth/src/api/middlewares/origin-check.ts#L53
- https://github.com/better-auth/better-auth/commit/b381cac7aafd6aa53ef78b6ab771ebfa24643c80
- https://github.com/better-auth/better-auth/security/advisories/GHSA-vp58-j275-797x
Deep Security Visibility Without the Complexity
SynScan provides clear, real-time security insights so you can monitor your attack surface, spot risks early, and act fast—without extra complexity.
- No setup fees
- 5-min deployment
- Cancel anytime
Frequently Asked Questions
A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
A vulnerability is the underlying weakness. An exploit is the method or code used to take advantage of it. A zero-day is a vulnerability that is unknown to the vendor or has no publicly available fix when attackers begin using it. In practice, risk increases sharply when exploitation becomes reliable or widespread.
Recurring findings usually come from incomplete Asset Discovery, inconsistent patch management, inherited images, and configuration drift. In modern environments, you also need to watch the software supply chain: dependencies, containers, build pipelines, and third-party services can reintroduce the same weakness even after you patch a single host. Unknown or unmanaged assets (often called Shadow IT) are a common reason the same issues resurface.
Use a simple, repeatable triage model: focus first on externally exposed assets, high-value systems (identity, VPN, email, production), vulnerabilities with known exploits, and issues that enable remote code execution or privilege escalation. Then enforce patch SLAs and track progress using consistent metrics so remediation is steady, not reactive.
SynScan combines attack surface monitoring and continuous security auditing to keep your inventory current, flag high-impact vulnerabilities early, and help you turn raw findings into a practical remediation plan.