59 lines
No EOL
3.3 KiB
Text
59 lines
No EOL
3.3 KiB
Text
-----=====[ Background ]=====-----
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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.
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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.
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-----=====[ Description ]=====-----
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We have encountered the following crash in fontsub!ReadAllocFormat12CharGlyphMapList:
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--- cut ---
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(5a30.397c): Access violation - code c0000005 (first chance)
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First chance exceptions are reported before any exception handling.
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This exception may be expected and handled.
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FONTSUB!ReadAllocFormat12CharGlyphMapList+0x13d:
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00007fff`c086cf8d 448904c8 mov dword ptr [rax+rcx*8],r8d ds:00000225`050b9000=????????
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0:000> ? rax
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Evaluate expression: 2358021689232 = 00000225`050b8f90
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0:000> ? rcx
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Evaluate expression: 14 = 00000000`0000000e
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0:000> ? r8d
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Evaluate expression: 4294967286 = 00000000`fffffff6
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0:000> !heap -p -a rax
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address 00000225050b8f90 found in
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_DPH_HEAP_ROOT @ 22505011000
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in busy allocation ( DPH_HEAP_BLOCK: UserAddr UserSize - VirtAddr VirtSize)
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22505012478: 225050b8f90 68 - 225050b8000 2000
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00007fffcf6530df ntdll!RtlDebugAllocateHeap+0x000000000000003f
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00007fffcf60b52c ntdll!RtlpAllocateHeap+0x0000000000077d7c
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00007fffcf59143b ntdll!RtlpAllocateHeapInternal+0x00000000000005cb
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00007fff9b90be42 vrfcore!VfCoreRtlAllocateHeap+0x0000000000000022
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00007fffcca398f0 msvcrt!malloc+0x0000000000000070
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00007fffc086fd1e FONTSUB!Mem_Alloc+0x0000000000000012
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00007fffc086cf24 FONTSUB!ReadAllocFormat12CharGlyphMapList+0x00000000000000d4
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00007fffc08706cd FONTSUB!ModCmap+0x0000000000000459
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00007fffc0866eab FONTSUB!CreateDeltaTTFEx+0x0000000000000413
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00007fffc0866a63 FONTSUB!CreateDeltaTTF+0x00000000000002cb
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00007fffc086132a FONTSUB!CreateFontPackage+0x000000000000015a
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[...]
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0:000> k
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# Child-SP RetAddr Call Site
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00 000000ad`62cfd4b0 00007fff`c08706cd FONTSUB!ReadAllocFormat12CharGlyphMapList+0x13d
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01 000000ad`62cfd520 00007fff`c0866eab FONTSUB!ModCmap+0x459
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02 000000ad`62cfd660 00007fff`c0866a63 FONTSUB!CreateDeltaTTFEx+0x413
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03 000000ad`62cfd780 00007fff`c086132a FONTSUB!CreateDeltaTTF+0x2cb
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04 000000ad`62cfd8c0 00007ff6`1a8a85d1 FONTSUB!CreateFontPackage+0x15a
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[...]
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--- cut ---
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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 3 proof of concept malformed font files which trigger the crash.
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Proof of Concept:
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https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/47266.zip |