The Open Redirect Hunter’s Guide: How URL Path Manipulation Leads to Critical Security Risks | by…

During a web application security assessment, I investigated how an application handled unusual URL paths and whether its redirect logic could be manipulated. Sometimes, seemingly harmless URL handling behavior can reveal weaknesses that deserve a closer look.

In this investigation, I focused on an endpoint under the /public/ path. My objective was to determine whether directory traversal sequences could influence the application's redirect behavior and potentially send users to an external website.

Understanding the Potential Vulnerability

An open redirect occurs when an application redirects users to a destination that an attacker can control without properly validating the destination URL.

Although redirects are a normal part of web applications, unsafe redirect handling can create opportunities for phishing and social engineering. A malicious link that begins with a legitimate domain may appear trustworthy to an unsuspecting user.

Investigating the /public/ Endpoint

I examined how the endpoint responded when its URL path contained directory traversal sequences such as ../. These sequences are commonly associated with path traversal vulnerabilities, but their presence alone does not prove that a vulnerability exists.

The important question was whether the application normalized the path, rejected the request, served a resource, or generated a redirect to an unexpected destination.

For testing, I used the following illustrative request path:

GET /public/../../evil.com/../.. HTTP/1.1
Host: target.example

GET /public/../../evil.com/../.. HTTP/1.1
Host: target.example

Here, target.example is a placeholder for the authorized test environment. The request was intended to help assess how the application handled an unusual path rather than to assume that an external redirect would occur.

What Makes This Behavior Interesting?

Path traversal and open redirect vulnerabilities are different security issues. Path traversal typically involves accessing resources outside an intended directory, while an open redirect involves sending a user to an untrusted destination.

However, URL normalization and redirect handling can interact in unexpected ways. If an application's redirect logic interprets manipulated input as an external destination, that behavior may indicate a security weakness.

To confirm an open redirect, I would verify that the application actually returns a redirect response, such as 302 Found, with a Location header pointing to an external domain under my control. A response status alone is not sufficient evidence.

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Potential Security Impact

If confirmed, an open redirect could help an attacker make phishing links appear more credible by using a legitimate application domain as the initial URL.

For example, a user might trust a link because it starts with a familiar website, only to be redirected elsewhere. The actual impact depends on the application's behavior, the redirect restrictions, and how the link could realistically be abused.

It is important to distinguish this potential impact from demonstrated impact. An open redirect by itself does not prove that credentials can be stolen, that malware will be delivered, or that the application has been compromised.

Severity and Vulnerability Classification

The appropriate classification for a confirmed open redirect is CWE-601: URL Redirection to Untrusted Site.

Severity depends on the circumstances. A basic open redirect without additional security impact may be rated Informational or Low by a bug bounty program. A higher rating requires evidence of meaningful additional impact, such as a demonstrated authentication-flow abuse or another security boundary being bypassed.

A CVSS score should be assigned only after evaluating the verified behavior and attack conditions. A score of 4.3 should not be treated as definitive without confirming that the selected vector accurately represents the vulnerability.

How Developers Can Fix It

The safest approach is to avoid accepting arbitrary redirect destinations. When redirects are necessary, applications should validate destinations against an explicit allowlist of permitted internal paths or trusted domains.

Developers should also use reliable URL parsing and canonicalization methods rather than relying on simple string checks. Validation must apply to the final interpreted destination, not just the original input.

Finally, unusual paths should be handled consistently, and invalid redirect requests should be rejected instead of being interpreted as unexpected external destinations.

Lessons From This Investigation

This investigation highlights an important lesson in web application security: unusual URL behavior is a starting point for testing, not proof of a vulnerability.

A convincing report needs reproducible evidence, a clear explanation of the root cause, and an impact statement that reflects what was actually demonstrated. Separating observed behavior from assumptions makes findings more useful to developers and security teams.

I continue to find that careful validation of seemingly minor application behaviors can reveal security issues that are easy to overlook during routine testing.

AI assistance:_ I used an AI-powered grammar and spelling checker to review this article. The ideas, research, technical analysis, and final content are my own._

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