Fix open redirect vulnerability in LoginController

[Copilot]

Addresses critical security vulnerability where unvalidated redirect URLs in login flow allowed attackers to redirect users to external malicious sites after authentication.

Changes

• Added isValidRedirectUrl() validation method that:

  • Rejects URLs with schemes (http://, https://, javascript:, data:, etc.)
  • Rejects protocol-relative URLs (//example.com)
  • Only allows internal paths starting with /

• Protected both redirect entry points:

  • showLoginForm(): validates $_GET['redirect'] parameter
  • login(): validates $_POST['redirect_url'] parameter

• Introduced DEFAULT_REDIRECT_URL constant to eliminate hardcoded fallback duplication

Example

// Before: vulnerable to external redirects
$ViewData = [
    'RedirectUrl' => $_GET['redirect'] ?? '/admin/dashboard'
];

// After: validated against malicious URLs
$requestedRedirect = $_GET['redirect'] ?? self::DEFAULT_REDIRECT_URL;
$redirectUrl = $this->isValidRedirectUrl($requestedRedirect) 
    ? $requestedRedirect 
    : self::DEFAULT_REDIRECT_URL;
$ViewData = ['RedirectUrl' => $redirectUrl];

Invalid redirect attempts now safely fall back to /admin/dashboard.

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