BrokenType

BrokenType is a set of tools designed to test the robustness and security of font rasterization software, especially codebases prone to memory corruption issues (written in C/C++ and similar languages). It consists of the following components:

• TrueType program generator - a Python script for generating random, but valid TrueType programs.
• TTF/OTF mutator - a semi-“smart” binary font file mutator written in C++.
• TTF/OTF loader for Windows GDI - a utility for loading and testing custom fonts with Windows GDI / Uniscribe.
• TTF/OTF loader for DirectWrite - a utility for loading and testing custom fonts with the Microsoft DirectWrite API.
• TTF loader for Windows FontSub.dll - a utility for loading and testing custom fonts with the Font Subsetting library.
• Linux PE loader for FontSub.dll - a minimal PE loader for Linux, tailored for fuzzing the Windows subsetter.
• PDF font embedders - Python scripts for generating PDF documents which embed the specified fonts and display all of their glyphs.

The description and usage instructions of the utilities can be found in their corresponding READMEs.

The programs and scripts were successfully used in 2015-2019 to discover and report 20 vulnerabilities in the font rasterization code present in the Windows kernel (win32k.sys and atmfd.dll drivers), 19 security flaws in the user-mode Microsoft Uniscribe library, as well as 9 bugs in the FontSub.dll library and several issues in DirectWrite. The fuzzing efforts were discussed in the following Google Project Zero blog posts:

• A year of Windows kernel font fuzzing #1: the results (June 2016)
• A year of Windows kernel font fuzzing #2: the techniques (July 2016)
• Notes on Windows Uniscribe Fuzzing (April 2017)

and the “Reverse engineering and exploiting font rasterizers” talk given in September 2015 at the 44CON conference in London. The two most notable issues found by the tool were CVE-2015-2426 and CVE-2015-2455 - an OTF bug collision with an exploit found in the Hacking Team leak, and a TTF bug collision with KeenTeam’s exploit for pwn2own 2015.

Disclaimer

This is not an official Google product.