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Adobe Acrobat Reader DC for Windows - Use of Uninitialized Pointer due to Malformed OTF Font (CFF Table)


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We have observed the following access violation exception in the latest version of Adobe Acrobat Reader DC for Windows, when opening a malformed PDF file:

--- cut ---
(5708.4564): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=c0c0c0c0 ebx=00000000 ecx=6826e380 edx=00000000 esi=00000002 edi=00000006
eip=15e440e8 esp=047fc158 ebp=047fc1b8 iopl=0         nv up ei ng nz ac po cy
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00210293
15e440e8 f6403860        test    byte ptr [eax+38h],60h     ds:002b:c0c0c0f8=??

0:000> u @$scopeip-9
15e440df 8b4d08          mov     ecx,dword ptr [ebp+8]
15e440e2 8b7dc4          mov     edi,dword ptr [ebp-3Ch]
15e440e5 8b0481          mov     eax,dword ptr [ecx+eax*4]
15e440e8 f6403860        test    byte ptr [eax+38h],60h
15e440ec 0f851f010000    jne     CoolType!CTCleanup+0x25d11 (15e44211)
15e440f2 0fb7781a        movzx   edi,word ptr [eax+1Ah]
15e440f6 0fb7401e        movzx   eax,word ptr [eax+1Eh]
15e440fa 8bc8            mov     ecx,eax

0:000> dd ecx
6826e380  16063e80 16063e40 1605fd00 c0c0c0c0
6826e390  c0c0c0c0 c0c0c0c0 c0c0c0c0 c0c0c0c0
6826e3a0  c0c0c0c0 c0c0c0c0 c0c0c0c0 c0c0c0c0
6826e3b0  c0c0c0c0 c0c0c0c0 c0c0c0c0 c0c0c0c0
6826e3c0  c0c0c0c0 c0c0c0c0 c0c0c0c0 c0c0c0c0
6826e3d0  c0c0c0c0 c0c0c0c0 c0c0c0c0 c0c0c0c0
6826e3e0  c0c0c0c0 c0c0c0c0 c0c0c0c0 c0c0c0c0
6826e3f0  c0c0c0c0 c0c0c0c0 c0c0c0c0 c0c0c0c0

0:000> kb
 # ChildEBP RetAddr  Args to Child              
WARNING: Stack unwind information not available. Following frames may be wrong.
00 047fc1b8 15e434ea 6826e380 1605fce0 6826e388 CoolType!CTCleanup+0x25be8
01 047fc1d4 15e43f02 6826e380 1605fd00 6826e388 CoolType!CTCleanup+0x24fea
02 047fc1fc 15e4edc1 6936cff0 16063e40 1605fd00 CoolType!CTCleanup+0x25a02
03 047fc230 15deb53d 6936cbf0 047fcca4 00000f5c CoolType!CTCleanup+0x308c1
04 047fc94c 15de6251 6936cbf0 047fcbdc 047fcd5c CoolType!CTInit+0x483dd
05 047fca30 15e223fa 6936cbf0 047fcbdc 047fcd5c CoolType!CTInit+0x430f1
06 047fcb88 15e220be 6936cbf0 047fcd5c 047fcd2c CoolType!CTCleanup+0x3efa
07 047fcc04 15df972d 6936cbf0 16067080 047fcd5c CoolType!CTCleanup+0x3bbe
08 047fcdcc 15df8f00 047fcfc0 00000000 16067330 CoolType!CTInit+0x565cd
09 047fce9c 15df7d87 0b601000 00000001 00000001 CoolType!CTInit+0x55da0
0a 047fd268 15df7414 0000012c 86c0e9cc 00001aba CoolType!CTInit+0x54c27
0b 047fd2ac 15df63de 86c0e9c0 00000064 047fd344 CoolType!CTInit+0x542b4
0c 047fd41c 15df5eb9 047fd834 047fdbb0 0000044a CoolType!CTInit+0x5327e
0d 047fd470 16112a42 3ede4e60 047fd834 047fdbb0 CoolType!CTInit+0x52d59
0e 047fd7b8 16111888 8ec19b64 047fd834 047fdbb0 AGM!AGMInitialize+0x69bd2
0f 047fd918 160dc460 047fd980 8ec19b00 047fdc48 AGM!AGMInitialize+0x68a18
10 047fd9b4 160e469a 047fdb98 8ec19b00 047fdc48 AGM!AGMInitialize+0x335f0
11 047fdbe0 160e2ae0 3eb84ba0 67b69f70 8ec19b00 AGM!AGMInitialize+0x3b82a
12 047fddbc 160e186c 3eb84ba0 67b69f70 56375db9 AGM!AGMInitialize+0x39c70
13 047fde08 161107ff 3eb84ba0 67b69f70 68a8ad50 AGM!AGMInitialize+0x389fc
14 047fde2c 1611030e 00000301 1611044f 67b69f70 AGM!AGMInitialize+0x6798f
15 047fde34 1611044f 67b69f70 56375d11 68a8ad50 AGM!AGMInitialize+0x6749e
16 047fde6c 160b945b 047fdf40 1610f910 00000000 AGM!AGMInitialize+0x675df
17 047fdec0 5fdcd4ad 047fde00 5fdcd4b4 dd9e27c4 AGM!AGMInitialize+0x105eb
18 047fdec8 5fdcd4b4 dd9e27c4 68a8ad50 047fdeac AcroRd32!DllCanUnloadNow+0x18454d
19 047fdee8 5fddb77d 3ede4f64 7cb8ed90 047fdf00 AcroRd32!DllCanUnloadNow+0x184554
1a 047fdf04 5fddb274 553c0f84 dd9e2644 553c0f58 AcroRd32!DllCanUnloadNow+0x19281d
1b 047fdf6c 5fdeef36 dd9e2698 00000000 553c0f58 AcroRd32!DllCanUnloadNow+0x192314
1c 047fdfb0 5fddaa40 dd9e26d4 5e4a0f78 553c0f58 AcroRd32!CTJPEGDecoderRelease+0x3426
1d 047fdffc 5fdda902 dd9e196c 5e4a0f78 047fe0ec AcroRd32!DllCanUnloadNow+0x191ae0
1e 047fe044 5fdda7e3 047fe060 dd9e1998 047fe41c AcroRd32!DllCanUnloadNow+0x1919a2
1f 047fe0b0 5fdda677 047fe0ec 8ef46ff0 3fe7bc80 AcroRd32!DllCanUnloadNow+0x191883
20 047fe110 5fdd8aed 8ef46ff0 5fddbc70 047fe41c AcroRd32!DllCanUnloadNow+0x191717
21 047fe210 5fdd8542 047fe41c dd9e1b74 1a74ed88 AcroRd32!DllCanUnloadNow+0x18fb8d
22 047fe25c 5fdd79dd 047fe41c 047fe424 dd9e1df0 AcroRd32!DllCanUnloadNow+0x18f5e2
23 047fe4d8 5fdd77ee 00000002 81ffa4e2 dd9e1c1c AcroRd32!DllCanUnloadNow+0x18ea7d
24 047fe534 5fd9706a 00000002 81ffa4e2 dd9e1ec4 AcroRd32!DllCanUnloadNow+0x18e88e
25 047fe7ec 5fd95d98 5ee78ef8 047fe880 047fe8d0 AcroRd32!DllCanUnloadNow+0x14e10a
26 047fe8f0 5fd95175 5ee78ef8 047fea20 00000000 AcroRd32!DllCanUnloadNow+0x14ce38
27 047fea44 5fd942ba 5ee78ef8 047feb48 00000000 AcroRd32!DllCanUnloadNow+0x14c215
28 047feaa4 5fd9414d 5ee78ef8 047feb48 00000000 AcroRd32!DllCanUnloadNow+0x14b35a
29 047feac4 5fd92d3c 5ee78ef8 047feb48 00000000 AcroRd32!DllCanUnloadNow+0x14b1ed
2a 047feb7c 5fd92762 00000001 00000000 dd9e12fc AcroRd32!DllCanUnloadNow+0x149ddc
2b 047febd4 5fd9257a 7313eef0 00000001 dd9e1510 AcroRd32!DllCanUnloadNow+0x149802
2c 047fec38 5fd922ff 047fed2c dd9e1464 81ff8fa0 AcroRd32!DllCanUnloadNow+0x14961a
2d 047fed4c 5fc5687c 81ff8fa0 5fc567a0 00000000 AcroRd32!DllCanUnloadNow+0x14939f
2e 047fed64 5fc5678f 0000000f 00000000 00000000 AcroRd32!DllCanUnloadNow+0xd91c
2f 047fed80 745de0bb 03870c42 0000000f 00000000 AcroRd32!DllCanUnloadNow+0xd82f
30 047fedac 745e8849 5fc566d0 03870c42 0000000f USER32!_InternalCallWinProc+0x2b
31 047fedd0 745eb145 0000000f 00000000 00000000 USER32!InternalCallWinProc+0x20
32 047feea0 745d8503 5fc566d0 00000000 0000000f USER32!UserCallWinProcCheckWow+0x1be
33 047fef08 745d8aa0 13ff4e80 00000000 0000000f USER32!DispatchClientMessage+0x1b3
34 047fef50 77371a6d 047fef6c 00000020 047fefcc USER32!__fnDWORD+0x50
35 047fef88 745d91ee 047ff01c e165025c 18170dd8 ntdll!KiUserCallbackDispatcher+0x4d
36 047fefdc 745d8c20 e51aed80 047ff000 5fc6da6d USER32!DispatchMessageWorker+0x5be
37 047fefe8 5fc6da6d 047ff01c 18170dd8 18170dd8 USER32!DispatchMessageW+0x10
38 047ff000 5fc6d89e 047ff01c dd9e095c 18170dd8 AcroRd32!DllCanUnloadNow+0x24b0d
39 047ff074 5fc6d744 dd9e0984 18170dd8 00000000 AcroRd32!DllCanUnloadNow+0x2493e
3a 047ff0ac 5fbfc575 dd9e0834 16d7eff8 00000000 AcroRd32!DllCanUnloadNow+0x247e4
3b 047ff11c 5fbfbf81 5fbd0000 00110000 16d7eff8 AcroRd32!AcroWinMainSandbox+0x775
3c 047ff53c 0011783d 5fbd0000 00110000 16d7eff8 AcroRd32!AcroWinMainSandbox+0x181
3d 047ff908 002201aa 00110000 00000000 0b61b3f2 AcroRd32_exe+0x783d
3e 047ff954 76698674 0480b000 76698650 5ab919ba AcroRd32_exe!AcroRd32IsBrokerProcess+0x992da
3f 047ff968 77365e17 0480b000 666934db 00000000 KERNEL32!BaseThreadInitThunk+0x24
40 047ff9b0 77365de7 ffffffff 7738ad9b 00000000 ntdll!__RtlUserThreadStart+0x2f
41 047ff9c0 00000000 00111390 0480b000 00000000 ntdll!_RtlUserThreadStart+0x1b
--- cut ---


- Reproduces on Adobe Acrobat Reader DC (2019.012.20036) on Windows 10, with and without PageHeap enabled, but most consistently with PageHeap (thanks to the allocation marker bytes).

- The crash occurs immediately after opening the PDF document, and is caused by dereferencing an uninitialized pointer from the heap. With PageHeap enabled, all new allocations are filled with the 0xc0c0c0... marker, which is visible in the crash log above.

- Attached samples: poc.pdf (crashing file), original.pdf (original file).

- We have minimized the difference between the original and mutated files down to three bytes at offsets 0x71a4, 0x71a5 and 0x71ba. They were changed from 0x1C, 0x14, 0x89 to 0xFF, 0xFF, 0x0E. When we analyzed it further, we determined that these bytes reside inside the "CFF " table of the embedded OpenType font. After extracting the font and decompiling it with the ttx tool from FontTools, we found that the difference is in the CharString of the "afii10091" glyph.

Original code:

--- cut ---
          cntrmask 00011100
          cntrmask 00000110
          32 hmoveto
          660 hlineto
          120 0 32 -22 15 -146 rrcurveto
          28 0 -13 203 -2 0 rlineto
--- cut ---

Mutated code:

--- cut ---
          cntrmask 11111111
          1707.08974 -99 hlineto
          120 0 32 -22 15 -146 rrcurveto
          28 0 -13 203 endchar
          0 rlineto
--- cut ---

Proof of Concept:
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