In my last blog post I introduced the new Windows feature, Administrator Protection and how it aimed to create a secure boundary for UAC where one didn’t exist. I described one of the ways I was able to bypass the feature before it was released. In total I found 9 bypasses during my research that have now all been fixed. In this blog post I wanted to describe the root cause of 5 of those 9 issues, specifically the implementation of UI Access, how this has been a long standing problem with UAC that’s been under-appreciated, and how it’s being fixed now. A Question of Accessibility Prior to Windows Vista any process running on a user’s desktop could control any window created by another, such as by sending window messages. This behavior could be abused if a privileged user, such as SYSTEM, displayed a user interface on the desktop. A limited user could control the UI and potentially elevate privileges. This was referred to as a Shatter Attack, and was usually fixed by removing user interface components from privileged code. As UAC encouraged running processes at different privilege levels on the same desktop, Microsoft introduced an additional feature, User Interface Privacy Isolation (UIPI). This used the Mandatory Integrity Control feature in UAC to limit what windows a process could interact with. If the integrity level of a process was lower than the process which created a window then it would be blocked from operations such as sending messages to that window. As an additional protection, Vista no longer ran user processes on the “service” desktop so that even if UIPI was inadequate a user interface exposed by a service process was not accessible to limited processes. To take an example, a limited user process has an assigned integrity level of “Medium” while a UAC administrator process is “High”. In this case UIPI would block the limited user process sending messages to any window created by the administrator process, excluding a…
