Microsoft Windows - 'USP10!SubstituteNtoM' Uniscribe Font Processing Out-of-Bounds Memory Read

EDB-ID: 42236
Author: Google Security Research
Published: 2017-06-23
CVE: CVE-2017-0285
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

We have encountered a crash in the Windows Uniscribe user-mode library, in the USP10!SubstituteNtoM function, while trying to display text using a corrupted TTF font file:

(69c.164): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=035fc256 ebx=00000000 ecx=035fc256 edx=00000019 esi=035cc1d0 edi=00000001
eip=7750a774 esp=0014eb38 ebp=0014eb64 iopl=0 nv up ei pl nz na pe nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010206
7750a774 668b08 mov cx,word ptr [eax] ds:0023:035fc256=????
0:000> kb
# ChildEBP RetAddr Args to Child
00 0014eb64 7750994d 0014ef40 0014ef64 035fc256 USP10!SubstituteNtoM+0x2c4
01 0014eba8 7750142a 0014ef40 0014ef64 0014ee54 USP10!otlMultiSubstLookup::apply+0x10d
02 0014ec10 775039f1 00000000 0014ef40 00001c3a USP10!ApplyLookup+0x1aa
03 0014ee14 774fefcf 42555347 0014ef7c 0014ef40 USP10!ApplyFeatures+0x481
04 0014ee60 774ff920 00000000 035f3ffe 035f3c90 USP10!SubstituteOtlGlyphs+0x1bf
05 0014ee9c 774f1500 0014ef18 0014ef50 0014ef7c USP10!SubstituteOtlChars+0x220
06 0014f118 774e55da 0014f224 0014f250 0014f238 USP10!HebrewEngineGetGlyphs+0x690
07 0014f1d8 774e273f 0014f224 0014f250 0014f238 USP10!ShapingGetGlyphs+0x36a
08 0014f2c4 774b5c6f a30105a2 035c6124 035c6318 USP10!ShlShape+0x2ef
09 0014f308 774c174a a30105a2 035c6124 035c6318 USP10!ScriptShape+0x15f
0a 0014f368 774c2bd4 00000000 00000000 0014f3e8 USP10!RenderItemNoFallback+0xfa
0b 0014f394 774c2e62 00000000 00000000 0014f3e8 USP10!RenderItemWithFallback+0x104
0c 0014f3b8 774c43f9 00000000 0014f3e8 035c6124 USP10!RenderItem+0x22
0d 0014f3fc 774b7a04 000004a0 00000400 a30105a2 USP10!ScriptStringAnalyzeGlyphs+0x1e9
0e 0014f414 760a1736 a30105a2 035c6040 0000000a USP10!ScriptStringAnalyse+0x284
0f 0014f460 760a18c1 a30105a2 0014f8e4 0000000a LPK!LpkStringAnalyse+0xe5
10 0014f55c 760a17b4 a30105a2 00000000 00000000 LPK!LpkCharsetDraw+0x332
11 0014f590 77df56a9 a30105a2 00000000 00000000 LPK!LpkDrawTextEx+0x40
12 0014f5d0 77df5a64 a30105a2 00000728 00000000 USER32!DT_DrawStr+0x13c
13 0014f61c 77df580f a30105a2 0014f8e4 0014f8f8 USER32!DT_GetLineBreak+0x78
14 0014f6c8 77df5882 a30105a2 00000000 0000000a USER32!DrawTextExWorker+0x250
15 0014f6ec 77df5b68 a30105a2 0014f8e4 ffffffff USER32!DrawTextExW+0x1e

The issue reproduces on Windows 7, and could be potentially used to disclose sensitive data from the process heap. It is easiest to reproduce with PageHeap enabled, but it is also possible to observe a crash in a default system configuration. In order to reproduce the problem with the provided samples, it might be necessary to use a custom program which displays all of the font's glyphs at various point sizes.

Attached are 2 proof of concept malformed font files which trigger the crash.

Proof of Concept:

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