Exploit-DB #47094
Description: Microsoft DirectWrite / AFDKO - Heap-Based Buffer Overflow in OpenType Font Handling in readFDSelect
Type: dos
Platform: windows
Date: 2019-07-10
Author: Google Security Research

Exploit:
-----=====[ Background ]=====-----

AFDKO (Adobe Font Development Kit for OpenType) is a set of tools for examining, modifying and building fonts. The core part of this toolset is a font handling library written in C, which provides interfaces for reading and writing Type 1, OpenType, TrueType (to some extent) and several other font formats. While the library existed as early as 2000, it was open-sourced by Adobe in 2014 on GitHub [1, 2], and is still actively developed. The font parsing code can be generally found under afdko/c/public/lib/source/*read/*.c in the project directory tree.

At the time of this writing, based on the available source code, we conclude that AFDKO was originally developed to only process valid, well-formatted font files. It contains very few to no sanity checks of the input data, which makes it susceptible to memory corruption issues (e.g. buffer overflows) and other memory safety problems, if the input file doesn't conform to the format specification.

We have recently discovered that starting with Windows 10 1709 (Fall Creators Update, released in October 2017), Microsoft's DirectWrite library [3] includes parts of AFDKO, and specifically the modules for reading and writing OpenType/CFF fonts (internally called cfr/cfw). The code is reachable through dwrite!AdobeCFF2Snapshot, called by methods of the FontInstancer class, called by dwrite!DWriteFontFace::CreateInstancedStream and dwrite!DWriteFactory::CreateInstancedStream. This strongly indicates that the code is used for instancing the relatively new variable fonts [4], i.e. building a single instance of a variable font with a specific set of attributes. The CreateInstancedStream method is not a member of a public COM interface, but we have found that it is called by d2d1!dxc::TextConvertor::InstanceFontResources, which led us to find out that it can be reached through the Direct2D printing interface. It is unclear if there are other ways to trigger the font instancing functionality.

One example of a client application which uses Direct2D printing is Microsoft Edge. If a user opens a specially crafted website with an embedded OpenType variable font and decides to print it (to PDF, XPS, or another physical or virtual printer), the AFDKO code will execute with the attacker's font file as input. Below is a description of one such security vulnerability in Adobe's library exploitable through the Edge web browser.

-----=====[ Description ]=====-----

The readFDSelect() function in afdko/c/public/lib/source/cffread/cffread.c is designed to read and parse the FDSelect table of an input OpenType font. It is called by cfrBegFont(), the standard entry point function for the "cfr" (CFF Reader) module of AFDKO. The relevant part of the function is shown below:

--- cut ---
  2352          case 3: {
  2353              int nRanges = read2(h);
  2354
  2355              gid = read2(h);
  2356              while (nRanges--) {
  2357                  int fd = read1(h);
  2358                  long next = read2(h);
  2359
  2360                  while (gid < next)
  2361                      h->glyphs.array[gid++].iFD = (unsigned char)fd;
  2362              }
  2363          } break;
--- cut ---

In the handling of FDSelect Type 3 (see [5]), the code doesn't consider the size of the h->glyphs array and writes to it solely based on the FDSelect information. If the values read from the input stream in lines 2353, 2355 and 2358 exceed the number of glyphs in the font, the array may be overflown in line 2361, corrupting adjacent objects on the heap. The h->glyphs array is initialized in readCharStringsINDEX() based on the number of CharStrings found in the font:

--- cut ---
  1791      dnaSET_CNT(h->glyphs, index.count);
--- cut ---

-----=====[ Proof of Concept ]=====-----

The proof of concept font contains an FDSelect table with the following initial values:

- nRanges = 0x0001
- gid = 0x0000
- fd = 0x00
- next = 0xffff (modified from the original 0x0586)

By increasing the value of "next" from 1414 to 65535, we cause the loop in lines 2360-2361 to go largely out of bounds and overflow the h->glyphs array.

-----=====[ Crash logs ]=====-----

A 64-bit build of "tx" compiled with AddressSanitizer, started with ./tx -cff poc.otf prints out the following report:

--- cut ---
=================================================================
==235715==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x7fb8259a282a at pc 0x[PII_REDACTED]30 bp 0x7ffe5b379f50 sp 0x7ffe5b379f48
WRITE of size 1 at 0x7fb8259a282a thread T0
    #0 0x54012f in readFDSelect afdko/c/public/lib/source/cffread/cffread.c:2361:48
    #1 0x529a3d in cfrBegFont afdko/c/public/lib/source/cffread/cffread.c:2791:13
    #2 0x50928d in cfrReadFont afdko/c/tx/source/tx.c:137:9
    #3 0x508cc3 in doFile afdko/c/tx/source/tx.c:429:17
    #4 0x506b2e in doSingleFileSet afdko/c/tx/source/tx.c:488:5
    #5 0x4fc91e in parseArgs afdko/c/tx/source/tx.c:558:17
    #6 0x4f9470 in main afdko/c/tx/source/tx.c:1631:9
    #7 0x7fb8246e02b0 in __libc_start_main
    #8 0x41e5b9 in _start

0x7fb8259a282a is located 42 bytes to the right of 143360-byte region [0x7fb82597f800,0x7fb8259a2800)
allocated by thread T0 here:
    #0 0x4c63f3 in __interceptor_malloc
    #1 0x6c9ac2 in mem_manage afdko/c/public/lib/source/tx_shared/tx_shared.c:73:20
    #2 0x5474a4 in dna_manage afdko/c/public/lib/source/cffread/cffread.c:271:17
    #3 0x7de64e in dnaGrow afdko/c/public/lib/source/dynarr/dynarr.c:86:23
    #4 0x7dec75 in dnaSetCnt afdko/c/public/lib/source/dynarr/dynarr.c:119:13
    #5 0x53e6fa in readCharStringsINDEX afdko/c/public/lib/source/cffread/cffread.c:1791:5
    #6 0x5295be in cfrBegFont afdko/c/public/lib/source/cffread/cffread.c:2769:9
    #7 0x50928d in cfrReadFont afdko/c/tx/source/tx.c:137:9
    #8 0x508cc3 in doFile afdko/c/tx/source/tx.c:429:17
    #9 0x506b2e in doSingleFileSet afdko/c/tx/source