Microsoft DirectWrite is a modern Windows API for high-quality text rendering. A majority of its code resides in the DWrite.dll user-mode library. It is used by a variety of widely used desktop programs (such as web browsers) and constitutes an attack surface for memory corruption bugs, as it performs the processing of untrusted font files and is written in C/C++.

Through fuzzing, we have discovered a crash caused by an invalid memory read in DWrite!sfac_GetSbitBitmap, while rasterizing the glyphs of a slightly malformed TrueType font. The problem reproduces in Microsoft Edge (supposedly not in Chrome and Firefox due to OpenType Sanitizer); below is a crash log from the Microsoft Edge renderer process, generated when trying to open a web page with the proof-of-concept font embedded:

--- cut ---
(4368.698c): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
DWrite!sfac_GetSbitBitmap+0x2ad:
00007ffe`b1ce47bd 410fb65500      movzx   edx,byte ptr [r13] ds:000001b9`94823000=??

0:036> u
DWrite!sfac_GetSbitBitmap+0x2ad:
00007ffe`b1ce47bd 410fb65500      movzx   edx,byte ptr [r13]
00007ffe`b1ce47c2 0811            or      byte ptr [rcx],dl
00007ffe`b1ce47c4 49ffc5          inc     r13
00007ffe`b1ce47c7 48ffc1          inc     rcx
00007ffe`b1ce47ca 66413bc3        cmp     ax,r11w
00007ffe`b1ce47ce 7471            je      DWrite!sfac_GetSbitBitmap+0x331 (00007ffe`b1ce4841)
00007ffe`b1ce47d0 66ffc0          inc     ax
00007ffe`b1ce47d3 ebd1            jmp     DWrite!sfac_GetSbitBitmap+0x296 (00007ffe`b1ce47a6)

0:036> k
 # Child-SP          RetAddr           Call Site
00 000000a3`a00ec740 00007ffe`b1ce3aaa DWrite!sfac_GetSbitBitmap+0x2ad
01 000000a3`a00ec840 00007ffe`b1ce3954 DWrite!GetSbitComponent+0xfe
02 000000a3`a00ec950 00007ffe`b1d4cc66 DWrite!sbit_GetBitmap+0xd0
03 000000a3`a00eca10 00007ffe`b1d43dfe DWrite!fs_ContourScan+0x3b6
04 000000a3`a00ecaf0 00007ffe`b1d43e98 DWrite!TrueTypeRasterizer::Implementation::GetBitmapInternal+0xe6
05 000000a3`a00ecb40 00007ffe`b1d42c03 DWrite!TrueTypeRasterizer::Implementation::GetBitmap+0x30
06 000000a3`a00ecbb0 00007ffe`b1d11754 DWrite!GlyphBitmapRasterizationState::RasterizeGlyph+0x8b
07 000000a3`a00ecbf0 00007ffe`bf4de1ce DWrite!DWriteGlyphLookupCache::GetGlyphBitmapInfo+0x264
08 000000a3`a00ece60 00007ffe`bf4de95f d2d1!GlyphRunAnalyzer::AddCachedGlyph+0x62
09 000000a3`a00ecf10 00007ffe`bf4e60b0 d2d1!GlyphRunAnalyzer::GetGlyphs+0x18f
0a 000000a3`a00ecf60 00007ffe`bf4f572d d2d1!GlyphRunRenderer::InitForRendering+0x2c0
0b 000000a3`a00ed0a0 00007ffe`bf55ffe4 d2d1!CHwSurfaceRenderTarget::DrawGlyphRun+0x38d
0c 000000a3`a00ed3b0 00007ffe`bf4f379e d2d1!BrushRedirectionCompatibleCommand<CCommand_DrawGlyphRun,0>::Execute+0x134
0d 000000a3`a00ed4c0 00007ffe`bf50e7ef d2d1!CHwSurfaceRenderTarget::ProcessBatch+0x3ce
0e 000000a3`a00ed570 00007ffe`bf50a0ae d2d1!CBatchSerializer::FlushInternal+0x13f
0f 000000a3`a00ed600 00007ffe`bf50143b d2d1!DrawingContext::Flush+0x96
10 000000a3`a00ed660 00007ffe`99d3551e d2d1!D2DDeviceContextBase<ID2D1DeviceContext6,ID2D1DeviceContext6,null_type>::EndDraw+0x13b
11 000000a3`a00ed7a0 00007ffe`99bca704 edgehtml!CDXRenderTarget::EndDrawD2D+0x66
12 000000a3`a00ed7d0 00007ffe`99bca4e8 edgehtml!CDXRenderTarget::EnsureRenderMode+0x184
13 000000a3`a00ed800 00007ffe`99d2db85 edgehtml!CDXRenderTarget::EndDraw+0x38
14 000000a3`a00ed850 00007ffe`99d2da0b edgehtml!CDispSurface::EndLayerToRenderTarget+0x145
15 000000a3`a00ed8f0 00007ffe`99bb585f edgehtml!CDispNodeDestination::EndRender+0x6b
16 000000a3`a00ed960 00007ffe`99cf60c1 edgehtml!CDispNodeDestination::EndRect+0xaf
17 000000a3`a00ed9a0 00007ffe`99bacf83 edgehtml!CDispDestinationDrawHelper::EndRect+0x31
18 000000a3`a00ed9d0 00007ffe`99b6e055 edgehtml!CDispContainer::DrawSelfContent+0x583
19 000000a3`a00edb40 00007ffe`99b6f37e edgehtml!CDispContainer::DrawSelf+0x365
1a 000000a3`a00edcd0 00007ffe`99baee43 edgehtml!CDispNode::DrawInternal+0x7ce
1b 000000a3`a00ee060 00007ffe`99bad747 edgehtml!CDispNode::Draw+0x943
1c 000000a3`a00ee270 00007ffe`99bad297 edgehtml!CDispContainer::DrawChildren+0x227
1d 000000a3`a00ee330 00007ffe`99bacbc8 edgehtml!CDispContainer::DrawSelfContentFullStackingContext+0x127
1e 000000a3`a00ee420 00007ffe`99b6e055 edgehtml!CDispContainer::DrawSelfContent+0x1c8
1f 000000a3`a00ee590 00007ffe`99b6f37e edgehtml!CDispContainer::DrawSelf+0x365
20 000000a3`a00ee720 00007ffe`99baead3 edgehtml!CDispNode::DrawInternal+0x7ce
21 000000a3`a00eeab0 00007ffe`99bba8fc edgehtml!CDispNode::Draw+0x5d3
22 000000a3`a00eecc0 00007ffe`99bb9b68 edgehtml!CDispRoot::DrawIndependentCompositionLayerTree+0x5c
23 000000a3`a00eedb0 00007ffe`99bb97f5 edgehtml!CDispRoot::DrawRoot+0x1b8
24 000000a3`a00ef000 00007ffe`99c3452c edgehtml!CPaintHandler::RenderInternal+0x2b5
25 000000a3`a00ef580 00007ffe`99b54ac8 edgehtml!CPaintHandler::RenderIfNeeded+0x7c
26 000000a3`a00ef5f0 00007ffe`99d3a80d edgehtml!CRenderThread::ProcessRenderWork+0xdc
27 000000a3`a00ef650 00007ffe`99c5fdb9 edgehtml!CRenderTaskDrawInPlace::Execute+0xad
28 000000a3`a00ef6c0 00007ffe`99d77542 edgehtml!CRenderThread::RenderThread+0x229
29 000000a3`a00ef760 00007ffe`c32937e4 edgehtml!CRenderThread::StaticRenderThreadProc+0x42
2a 000000a3`a00ef790 00007ffe`c5e1cb81 KERNEL32!BaseThreadInitThunk+0x14
2b 000000a3`a00ef7c0 00000000`00000000 ntdll!RtlUserThreadStart+0x21
--- cut ---

We have minimized the test cases to a 1-byte difference in the EBLC table, and a 2-byte difference in the EBDT table in relation to the original files.

The issue reproduces on a fully updated Windows 10 1709; we haven't tested other versions of the system. It could be used to disclose sensitive data from the process address space, which is clearly visible when opening the PoC HTML files in Edge. In most cases, instead of crashing, the browser will display random chunks of heap memory residing after the glyph's bitmap allocation. As shown in 1/poc.html and 2/poc.html, the problems are related to glyphs corresponding to characters with codes 0xF0 and 0x2020, respectively.

Attached is a pair of minimized PoC fonts, original fonts, and HTML files to reproduce the bug in a browser.


Proof of Concept:
https://github.com/offensive-security/exploitdb-bin-sploits/raw/master/bin-sploits/47382.zip