CVE-2011-5251

Step 1: Crafting the Malicious URL: The attacker constructs a URL that exploits the vulnerability. This URL typically targets the forum/login.php script with the lostpw action and includes a malicious URL in the url parameter.

Step 2: Social Engineering: The attacker distributes the crafted URL through phishing emails, social media, or other channels, enticing users to click the link, often by pretending to be a password reset request.

Step 3: User Interaction: A user clicks the malicious link, initiating the password reset process. The lostpw action is triggered.

Step 4: Redirection: The vulnerable forum/login.php script, due to the lack of proper validation, redirects the user to the URL specified in the url parameter, which is now the attacker's phishing site.

Step 5: Credential Harvesting: The user, believing they are on a legitimate vBulletin site, enters their credentials on the phishing page. The attacker captures these credentials.

The vulnerability stems from insufficient input validation and sanitization of the url parameter within the lostpw action of forum/login.php. The application fails to properly validate the destination URL, allowing an attacker to inject a malicious URL. When a user requests a password reset and is redirected, the application uses the provided url parameter without proper checks, leading to redirection to an attacker-controlled website. This allows for the creation of convincing phishing pages that mimic the legitimate vBulletin login, tricking users into entering their credentials. The root cause is a lack of input validation and output encoding for the redirect URL.