Among the total of 119 vulnerabilities with CVEs fixed by Microsoft in the March Patch Tuesday a few weeks ago, there were 29 bugs reported by us in the font-handling code of the Uniscribe library. Admittedly the subject of font-related security has already been extensively discussed on this blog both in the context of manual analysis and fuzzing. However, what makes this effort a bit different from the previous ones is the fact that Uniscribe is a little-known user-mode component, which had not been widely recognized as a viable attack vector before, as opposed to the kernel-mode font implementations included in the win32k.sys and ATMFD.DLL drivers. In this post, we outline a brief history and description of Uniscribe, explain how we approached at-scale fuzzing of the library, and highlight some of the more interesting discoveries we have made so far. All the raw reports of the bugs we’re referring to (as they were submitted to Microsoft), together with the corresponding proof-of-concept samples, can be found in the official Project Zero bug tracker. Enjoy!