MGB OpenSource Guestbook version 0.7.0.2 contains a SQL injection vulnerability in the email.php file. The vulnerability allows unauthenticated attackers to execute arbitrary SQL queries through the 'id' parameter. Attackers can send crafted GET requests with malicious SQL payloads to extract sensitive database information. The vulnerability enables extraction of table and column names from the database. No authentication is required to exploit this vulnerability, making it particularly dangerous.