Add PoC exploit for CVE-2018-6789

[postmodern]

Add a PoC exploit for CVE-2018-6789, a buffer overflow vulnerability in the base64d function part of the Exim SMTP listener that may allow for remote code execution.

Reference PoCs

• https://github.com/c0llision/exim-vuln-poc
• https://github.com/synacktiv/Exim-CVE-2018-6789/blob/master/exploit.c

Vuln App

• https://github.com/skysider/VulnPOC/tree/master/CVE-2018-6789/Environment

How to Submit a PoC

See the CONTRIBUTING file for instructions on how to submit a PoC exploit.