Microsoft Font Subsetting – DLL Heap Corruption in MakeFormat12MergedGlyphList
-----=====[ 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. FONTSUB!MakeFormat12MergedGlyphList+0x176: 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: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/47268.zip |