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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteA redirect guard can reject an unsafe returnTo value and still send a user off-site if another value reaches the same redirect. In a report about the Node.js package cas-authentication-user, the fallback came from the request path: a path that legacy URL parsing turned into a network-path reference. The practical review rule is to trace every input and fallback into each redirect sink—not just the parameter the guard was designed to check.
How the rejected redirect re-entered the flow
In his account, maintainer Seth Wheeler says version 0.3.0 added isSafeReturnTo to reject //host and /\host forms. But the redirect had another route to its destination: when the supplied return target was rejected, a fallback could be built from the request path.
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For the reported example, the request path /\bad.example.com was parsed by Node’s legacy url.parse as the pathname //bad.example.com. A browser interprets a URL beginning with two slashes as a network-path reference, so using that value as a redirect location can send the browser to the named host. According to Wheeler, the described authenticated flow did not require a returnTo parameter or a CAS ticket for this path to matter.
Wheeler says both redirect paths had existed since the fork’s first commit on July 30, 2019. He identifies two redirect sinks and says review focused on the query parameter while overlooking request-path input. These are details from the maintainer’s report, not independently verified repository findings. The report does not establish that anyone exploited the issue.
#1 Best Overall
What the reported 0.4.0 change does—and does not establish
Wheeler reports that version 0.4.0 parses request URLs with the WHATWG URL API against a base that cannot exist, checks whether parsing changes the origin, and substitutes / if the target moves to another origin. This approach evaluates the parsed result rather than trying to enumerate suspicious prefixes.
The author explicitly says he cannot claim the check is complete. The report describes an implementation, not proof that every redirect path is safe. The useful security question remains whether every value that reaches each redirect sink—including fallback values—is validated under the same assumptions as the browser that will follow the redirect.
Rank #2
How to review a redirect sink
- Find each sink. Locate every place the application sends a redirect, including framework helpers and response headers.
- Trace every incoming value. For each sink, list assignments, defaults, and fallback branches. Include request paths, query parameters, session values, and any other value used to construct the destination.
- Test rejection branches as hostile input. Make the preferred target invalid, then supply attacker-controlled values through every fallback source. In particular, test crafted request paths as well as query parameters.
- Validate the value that will actually be used. Do not validate one representation and then transform or reconstruct a different value for the redirect.
- Check browser-compatible parsing and policy. Where external destinations are permitted, use a maintained URL parser compatible with browser interpretation and compare parsed components against an explicit policy.
This method follows the failure chain in the report: checking one source does not secure a redirect sink if another source can reach it.
Do these 3 things before closing this tab:
1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesChoose a redirect policy that matches the need
OWASP’s guidance is to avoid user-controlled redirect destinations where possible. The following approaches trade destination flexibility against how much validation the application must own.
| Approach | Destination flexibility | Application responsibility |
|---|---|---|
| Local-only redirect | Only destinations within the application | Use a framework helper that enforces local destinations, and provide a safe in-app destination when the supplied value is not local. |
| Server-side destination ID | Only destinations represented by approved identifiers | Map a short identifier to a server-controlled destination; do not accept an arbitrary destination URL from the user. |
| Explicit external allowlist | Approved external destinations | Parse with a maintained, browser-compatible URL parser; compare scheme, canonical host, and effective port with the allowlist; reject userinfo and ambiguous inputs; constrain paths where needed; and redirect using the validated value. |
Microsoft’s ASP.NET Core documentation demonstrates the local-only pattern with LocalRedirect and IsLocalUrl; its example sends a non-local return URL to a safe in-app destination. This is guidance for ASP.NET Core, not a Node.js API recommendation. OWASP likewise advises using a maintained URL parser compatible with the redirect API and browser URL interpretation.
Quick Recap
Rank #4
Sources
- Seth Wheeler’s maintainer account (incident details and reported version 0.4.0 change).
- OWASP Unvalidated Redirects and Forwards Cheat Sheet.
- Microsoft Learn: Prevent open redirect attacks in ASP.NET Core.
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