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March 13, 2013

Flash Player Unloading Vulnerability

Opera unloads Flash Player after two minutes of not being used. Flash Player can create dialog that can run even when the player itself is unloaded. When it's unloaded, the running dialog makes a call back to the unloaded module, in a time window.

If an attacker can exploit the time window between the unload and the dereference, it's possible to redirect the execution flow.

While experimenting, I used heap spray to inject 0xCC bytes to the deleted memory. I succeeded once of many attempts and this suggests it's practically possible to create an exploit that could inject malicious code within the time window.

When investigated this vulnerability, I wanted evidence to the question: what module unloads the Flash Player? I used Windbg to place breakpoint at unload events. I let the application to trigger the unload events, and when the debugger hit the breakpoint, printed the call stack.
0:014> sxe ud
0:014> g
Unload module C:\Windows\SysWOW64\Macromed\Flash\NPSWF32_11_4_402_287.dll at 54760000
eax=00000000 ebx=035f5584 ecx=00000000 edx=00000000 esi=035f5bc8 edi=00000000
eip=77b1fc72 esp=0037dab0 ebp=0037db30 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00200246
77b1fc72 83c404          add     esp,4
0:000> kc
WARNING: Stack unwind information not available. Following frames may be wrong.
I saw the Flash Player was freed by Opera indeed.

I needed the answer for the question also: what module created the thread that calls back to the unloaded module? Here are the series of Windbg commands used to get the answer.
sxn et
sxn ud
bp CreateRemoteThreadEx "kc; gu; !handle eax 8; gc"

WARNING: Stack unwind information not available. Following frames may be wrong.
Handle a20
  Object Specific Information
    Thread Id   88ac.1700
    Priority    10
    Base Priority 0
    Start Address 5497083d NPSWF32_11_4_402_287!native_ShockwaveFlash_TCallLabel
ModLoad: 6bda0000 6bdd0000   C:\Windows\SysWOW64\wdmaud.drv
ModLoad: 72cb0000 72cb4000   C:\Windows\SysWOW64\ksuser.dll
ModLoad: 6d9e0000 6d9e7000   C:\Windows\SysWOW64\AVRT.dll
ModLoad: 6cad0000 6cad8000   C:\Windows\SysWOW64\msacm32.drv
ModLoad: 6bd80000 6bd94000   C:\Windows\SysWOW64\MSACM32.dll
ModLoad: 6bd70000 6bd77000   C:\Windows\SysWOW64\midimap.dll
Exit thread 6:3f90, code 0
Exit thread 7:4074, code 0
Unload module C:\Windows\SysWOW64\Macromed\Flash\NPSWF32_11_4_402_287.dll at 54760000
Unload module C:\Windows\system32\WINSPOOL.DRV at 736f0000
(88ac.1700): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
eax=00000102 ebx=00000000 ecx=75a914d0 edx=00000000 esi=11c8d000 edi=11c8d470
eip=549706c0 esp=0aacfd38 ebp=0aacfd80 iopl=0         nv up ei pl zr na pe nc
cs=0023  ss=002b  ds=002b  es=002b  fs=0053  gs=002b             efl=00010246
549706c0 ??              ???
I saw that thread 88ac.1700 is created by the Flash Player. When the player is unloaded the thread is still running. Finally, the thread calls back to the unloaded module. Therefore, I knew Flash Player created the thread that calls back to its own module that has been unloaded.

And here is some interesting observation.

When started to look into triggering a dialog in Flash Player I created the following ActionScript that executes an infinite loop.
class InfiniteLoop {
static function main() {
           for (var i = 0; i < 1;) {}
I built the Flash file with the command below.
mtasc.exe -swf InfiniteLoop.swf -main -header 125:125:20
The Flash file triggers a slow script warning dialog. I was able to unload and to crash the Flash Player while the dialog was still running. I achieved this with version 11.4.402.278. When reported the problem to Opera they couldn't reproduce it. That time there was a Flash Player update, and I found it out that they used the latest version of Flash Player that was 11.4.402.287. With the latest version, I couldn't reproduce the problem either.

It got me thinking if Adobe fixed security problem either silently or "accidentally". When I checked the release notes of the latest version I didn't see indication of this kind of fix. I still don't know what had happened between the two releases but thought to investigate this issue further. I tried to trigger the bug via different code paths. I looked at my Flash file collection to dig out a file that triggers a dialog. And here it is, with the one I found, I could crash the player with the same call stack as before.

Initially, I thought Opera could fix this issue by leaving the module permanent in the address space, so I reported this to Opera. However, they notified Adobe after confirming it's not the problem in Opera (though they considered it as a stability issue). Adobe received the report at the end of November, 2012. The bug is now fixed in the security update that has a number of APSB13-09 at 12 March, 2013.

The preliminary version of this blog post was reviewed by Adobe. The preliminary version is same to the published version apart from some changes in the wording.
  This blog is written and maintained by Attila Suszter. Read in Feed Reader.