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Microsoft Font Subsetting - DLL Heap Corruption in MakeFormat12MergedGlyphList



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-----=====[ Background ]=====-----

The Microsoft Font Subsetting DLL (fontsub.dll) is a default Windows helper library for subsetting TTF fonts; i.e. converting fonts to their more compact versions based on the specific glyphs used in the document where the fonts are embedded. It is used by Windows GDI and Direct2D, and parts of the same code are also found in the t2embed.dll library designed to load and process embedded fonts.

The DLL exposes two API functions: CreateFontPackage and MergeFontPackage. We have developed a testing harness which invokes a pseudo-random sequence of such calls with a chosen font file passed as input. This report describes a crash triggered by a malformed font file in the fontsub.dll code through our harness.

-----=====[ Description ]=====-----

We have encountered the following crash in fontsub!MakeFormat12MergedGlyphList:

--- cut ---
(48e4.50e0): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
00007fff`c086908a 458904ca        mov     dword ptr [r10+rcx*8],r8d ds:000001a4`4ebf1000=????????

0:000> ? r10
Evaluate expression: 1805184796672 = 000001a4`4d660800

0:000> ? rcx
Evaluate expression: 2826496 = 00000000`002b2100

0:000> ? r8d
Evaluate expression: 5 = 00000000`00000005

0:000> dd r10
000001a4`4d660800  00000000 00000000 00000020 00000003
000001a4`4d660810  00000021 00000004 00000022 00000005
000001a4`4d660820  00000023 00000006 00000024 00000007
000001a4`4d660830  00000025 00000008 00000026 00000009
000001a4`4d660840  00000027 0000000a 00000028 0000000b
000001a4`4d660850  00000029 0000000c 0000002a 0000000d
000001a4`4d660860  0000002b 0000000e 0000002c 0000000f
000001a4`4d660870  0000002d 00000010 0000002e 00000011

0:000> !heap -p -a r10
    address 000001a44d660800 found in
    _DPH_HEAP_ROOT @ 1a44c521000
    in busy allocation (  DPH_HEAP_BLOCK:         UserAddr         UserSize -         VirtAddr         VirtSize)
                             1a44c5255b0:      1a44d660800          1590800 -      1a44d660000          1592000
    00007fffcf6530df ntdll!RtlDebugAllocateHeap+0x000000000000003f
    00007fffcf60b52c ntdll!RtlpAllocateHeap+0x0000000000077d7c
    00007fffcf59143b ntdll!RtlpAllocateHeapInternal+0x00000000000005cb
    00007fff9b90be42 vrfcore!VfCoreRtlAllocateHeap+0x0000000000000022
    00007fffcca398f0 msvcrt!malloc+0x0000000000000070
    00007fffc086fd1e FONTSUB!Mem_Alloc+0x0000000000000012
    00007fffc0869011 FONTSUB!MakeFormat12MergedGlyphList+0x00000000000000fd
    00007fffc08691ee FONTSUB!MergeFormat12Cmap+0x000000000000011e
    00007fffc08696f8 FONTSUB!MergeCmapTables+0x0000000000000444
    00007fffc086b046 FONTSUB!MergeFonts+0x00000000000005a6
    00007fffc086baac FONTSUB!MergeDeltaTTF+0x00000000000003ec
    00007fffc08614b2 FONTSUB!MergeFontPackage+0x0000000000000132
0:000> k
 # Child-SP          RetAddr           Call Site
00 0000000c`1c7dd310 00007fff`c08691ee FONTSUB!MakeFormat12MergedGlyphList+0x176
01 0000000c`1c7dd350 00007fff`c08696f8 FONTSUB!MergeFormat12Cmap+0x11e
02 0000000c`1c7dd420 00007fff`c086b046 FONTSUB!MergeCmapTables+0x444
03 0000000c`1c7dd500 00007fff`c086baac FONTSUB!MergeFonts+0x5a6
04 0000000c`1c7dd6b0 00007fff`c08614b2 FONTSUB!MergeDeltaTTF+0x3ec
05 0000000c`1c7dd7f0 00007ff6`1a8a8a30 FONTSUB!MergeFontPackage+0x132
--- cut ---

The issue reproduces on a fully updated Windows 10 1709; we haven't tested earlier versions of the system. It could be potentially used to execute arbitrary code in the context of the FontSub client process. It is easiest to reproduce with PageHeap enabled, but it is also possible to observe a crash in a default system configuration. Attached are 2 proof of concept malformed font files which trigger the crash.

Proof of Concept:
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