Microsoft Font Subsetting – DLL Heap Corruption in ComputeFormat4CmapData
-----=====[ 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!ComputeFormat4CmapData: --- cut --- (284c.42b4): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling. This exception may be expected and handled. FONTSUB!ComputeFormat4CmapData+0x1e5: 00007fff`aa44d295 41897cc304 mov dword ptr [r11+rax*8+4],edi ds:0000013d`775e8003=???????? 0:000> ? r11 Evaluate expression: 1363507314687 = 0000013d`775e7fff 0:000> ? rax Evaluate expression: 0 = 00000000`00000000 0:000> ? edi Evaluate expression: 1 = 00000000`00000001 0:000> !heap -p -a r11 address 0000013d775e7fff found in _DPH_HEAP_ROOT @ 13d77571000 in busy allocation ( DPH_HEAP_BLOCK: UserAddr UserSize - VirtAddr VirtSize) 13d77572e38: 13d775e7fff 1 - 13d775e7000 2000 00007fffcf6530df ntdll!RtlDebugAllocateHeap+0x000000000000003f 00007fffcf60b52c ntdll!RtlpAllocateHeap+0x0000000000077d7c 00007fffcf59143b ntdll!RtlpAllocateHeapInternal+0x00000000000005cb 00007fff9b90be42 vrfcore!VfCoreRtlAllocateHeap+0x0000000000000022 00007fffcca398f0 msvcrt!malloc+0x0000000000000070 00007fffaa44fd1e FONTSUB!Mem_Alloc+0x0000000000000012 00007fffaa448d1d FONTSUB!MergeFormat4Cmap+0x0000000000000261 00007fffaa449788 FONTSUB!MergeCmapTables+0x00000000000004d4 00007fffaa44b046 FONTSUB!MergeFonts+0x00000000000005a6 00007fffaa44baac FONTSUB!MergeDeltaTTF+0x00000000000003ec 00007fffaa4414b2 FONTSUB!MergeFontPackage+0x0000000000000132 [...] 0:000> k # Child-SP RetAddr Call Site 00 0000000c`654fd180 00007fff`aa448e11 FONTSUB!ComputeFormat4CmapData+0x1e5 01 0000000c`654fd1e0 00007fff`aa449788 FONTSUB!MergeFormat4Cmap+0x355 02 0000000c`654fd2e0 00007fff`aa44b046 FONTSUB!MergeCmapTables+0x4d4 03 0000000c`654fd3c0 00007fff`aa44baac FONTSUB!MergeFonts+0x5a6 04 0000000c`654fd570 00007fff`aa4414b2 FONTSUB!MergeDeltaTTF+0x3ec 05 0000000c`654fd6b0 00007ff6`1a8a8a30 FONTSUB!MergeFontPackage+0x132 [...] --- cut --- The root cause of the crash seems to be the fact that the MergeFormat4Cmap() function may allocate a 0-sized buffer and pass it to ComputeFormat4CmapData() in the second argument, but the ComputeFormat4CmapData() function assumes that the buffer is at least 8 bytes long, and unconditionally writes two 32-bit values of -1 and 1 into it. The issue reproduces on a fully updated Windows 10 1709; we haven't tested earlier versions of the system. In order to observe the crash, the PageHeap feature must be enabled in Application Verifier for the FontSub client process, preferably with the "/unaligned" and "/size 0 1" options. Attached are 3 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/47113.zip |