GeoVision GV-VMS V20 WebCam Server stack overflow vulnerability

Summary

GV-VMS V20 is a Video Monitoring Software used to gather the feeds of many surveillance cameras and manage other security devices. It is a native application accessed locally, but it is also possible to enable remote access via the "WebCam Server" feature. Once enabled, it is possible to access to the management and monitoring feature via a regular Web interface. This webersever is another native application, compiled without ASLR, which makes exploitation much easier and more likely. Most of the features require authentication before being reachable and leverage a standard login page to grant access. However the `gvapi` endpoint uses its own authentication mechanism via an `HTTP Authorization` header. It supports both `Basic` authentication and the `Digest` modes of authentication.   #### Stack-overflow via unbound copy of base64 decoded string The `b64decoder` string is sized dynamically, but it is then copied to the `Buffer` stack variable one character at the time at [0], and there's no bound-check. As such, if the decoded string is bigger than 256 characters (the size of the `Buffer` variable) then a stack overflow occurs. Because the data can be fully controlled by an attacker and lack of ASLR, this vulnerability can easily be exploited to gain full code execution as SYSTEM on the machine running the service.