I ran into this link:
Does this mean that Cisco has an open source license and anyone can modify the code?
No it doesn't. As far as i am aware all Cisco code is proprietary. What that link is referring to are the bits of software that Cisco use on the ASA that are open source eg.
Apache software provides an HTTP server. So Cisco have used the Apache open source software for their HTTP server on the ASA. Because it is open source they have to notify the licensing and use of the software.