62 lines
No EOL
3.4 KiB
Text
62 lines
No EOL
3.4 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!FixSbitSubTables:
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--- cut ---
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(8ec.5f20): 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!FixSbitSubTables+0x9a9:
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00007fff`b4841371 6644892446 mov word ptr [rsi+rax*2],r12w ds:0000023c`a6765000=????
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0:000> ? rsi
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Evaluate expression: 2459514064800 = 0000023c`a6764fa0
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0:000> ? rax
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Evaluate expression: 48 = 00000000`00000030
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0:000> ? r12w
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Evaluate expression: 0 = 00000000`00000000
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0:000> !heap -p -a rsi
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address 0000023ca6764fa0 found in
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_DPH_HEAP_ROOT @ 23ca6681000
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in busy allocation ( DPH_HEAP_BLOCK: UserAddr UserSize - VirtAddr VirtSize)
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23ca6683888: 23ca6764fa0 60 - 23ca6764000 2000
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unknown!printable
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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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00007fffb483fd1e FONTSUB!Mem_Alloc+0x0000000000000012
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00007fffb48412ac FONTSUB!FixSbitSubTables+0x00000000000008e4
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00007fffb4841a5a FONTSUB!FixSbitSubTableArray+0x00000000000001f6
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00007fffb4842460 FONTSUB!ModSbit+0x0000000000000520
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00007fffb48370aa FONTSUB!CreateDeltaTTFEx+0x0000000000000612
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00007fffb4836a63 FONTSUB!CreateDeltaTTF+0x00000000000002cb
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00007fffb483132a 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 000000b1`2ccfd580 00007fff`b4841a5a FONTSUB!FixSbitSubTables+0x9a9
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01 000000b1`2ccfd6c0 00007fff`b4842460 FONTSUB!FixSbitSubTableArray+0x1f6
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02 000000b1`2ccfd7e0 00007fff`b48370aa FONTSUB!ModSbit+0x520
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03 000000b1`2ccfd920 00007fff`b4836a63 FONTSUB!CreateDeltaTTFEx+0x612
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04 000000b1`2ccfda40 00007fff`b483132a FONTSUB!CreateDeltaTTF+0x2cb
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05 000000b1`2ccfdb80 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 4 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/47264.zip |