One of the early web application vulnerabilities I remember was CVE-2000-0411 [CVE]. This was a simple vulnerability in a CGI-bin script used to send e-mail. The vulnerability was pretty straightforward: Adding additional parameters to the URL leaked environment variables. Other vulnerabilities allowed setting the "To" address and were heavily abused by spammers. And if parameters were passed directly to the sendmail binary, similar vulnerabilities could allow arbitrary code execution.