Executive Summary

CVE-2026-9520 is a DOM Cross-Site Scripting (XSS) vulnerability in the BlitzJS framework's sign-in functionality. It occurs because the application improperly handles the `next` URL parameter during the sign-in redirect process.

Specifically, the application uses the `next` parameter from the browser's URL without proper validation or sanitization. This allows an attacker to craft a malicious link containing a `javascript:` URI.

When a victim clicks this malicious link and logs in, the application executes the JavaScript payload in the victim's browser context via the `router.push(next)` call, leading to potential exploitation.

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Impact Analysis

This vulnerability can lead to several serious impacts including credential theft, unauthorized actions performed on behalf of the victim, and potential pivoting into internal networks.

Because the attack executes malicious JavaScript in the victim's browser, it can compromise user sessions and sensitive data.

The exploit can be initiated remotely and requires user interaction (clicking a crafted link), but no special privileges are needed.

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Detection Guidance

This vulnerability can be detected by monitoring for suspicious URLs containing the `next` parameter with potentially malicious JavaScript payloads in the sign-in redirect process of BlitzJS applications.

One approach is to inspect web server logs or proxy logs for URLs with the `next` parameter that include `javascript:` or other unsafe schemes.

For example, you can use the following command to search for suspicious `next` parameters in your web server logs:

• grep -i 'next=javascript:' /path/to/access.log
• grep -Eo 'next=[^&]+' /path/to/access.log | grep -i 'javascript:'

Additionally, monitoring for unusual redirects or unexpected JavaScript execution after login in the client-side application can help detect exploitation attempts.

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Mitigation Strategies

Immediate mitigation involves validating and sanitizing the `next` parameter used in the sign-in redirect process to ensure it only contains safe URLs within the same origin and does not allow JavaScript or other unsafe schemes.

Specifically, implement checks to confirm that the `next` parameter does not start with `javascript:` or other potentially dangerous protocols before performing any redirection.

If possible, update the BlitzJS framework or your application code to the latest version where this issue is fixed.

Until a patch is applied, consider disabling or restricting the use of the `next` parameter in URLs to trusted paths only.

Also, educate users to avoid clicking on suspicious links that include unexpected `next` parameters.

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Compliance Impact

This DOM Cross-Site Scripting (XSS) vulnerability in BlitzJS's sign-in functionality can lead to unauthorized execution of malicious scripts in a user's browser, potentially resulting in credential theft and unauthorized actions.

Such security weaknesses can impact compliance with standards and regulations like GDPR and HIPAA, which require protection of personal data and secure authentication mechanisms to prevent unauthorized access and data breaches.

Failure to address this vulnerability could lead to violations of these regulations due to potential exposure of sensitive user information and compromise of user accounts.

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