In the past days I worked intensively on Antivirus’s Device Drivers bugs, at the actual state of art the major part of well known AVs suffer of basical and more hidden bugs. The totality of AVs that I’ve checked presents defects that could be maliciously used to takeover an Antivirus Infrastructure and in some case the entire Operating System with attacks like DoS and/or Remote/Local Privilege Escalation.
I want to make a precisation here, exists an important difference between Bug and Vulnerability, simply bugs does not affects the integrity of a system and does not constitute a true danger. Vulnerabilities constitutes an effective risk for systems integrity, included informations contained inside it. When we are dealing with applications specifically devoted to security, every bug could be considered a vulnerability, because an attacker could block/kill overcome checks performed by the application itself and propagate in system and produce damages. Just think to a basical crash that could affect an Antivirus could be implemented into a malicious application that checks the presence of AVs and induces the bug.
In this little post we are going to see some defects of last device drivers used by Avast, I’m precisely talking of
Build Number: 4.8.1351.0
Avast loads the following drivers:
- Aasvmker4.sys
- aswMon2.sys
- aswRdr.sys
- aswSP.sys
Avast loads the following Drivers could be tested by fuzzing IOCTLs, for this task could be used IOCTL Fuzzer and Kartoffel. Let’s disassemble the first driver, Aavmker4.sys that from DeviceIoControl hook appears to be heavy used. This is the DriverEntry()drivers
Code:
00010748 mov eax, [ebp+DriverObject]
0001074B push offset NotifyRoutine ; NotifyRoutine
00010750 mov dword ptr [eax+70h], offset sub_1098C ; DriverObject->MajorFunction[14] = (PDRIVER_DISPATCH)sub_1098C;
00010757 call PsSetCreateProcessNotifyRoutine
sub_1098C contains the switch statement to handle various IOCTL notifications, essentially IOCTL check is structured in form of nested If and Switches.
Code:
001098C ; int __stdcall sub_1098C(int, PIRP Irp)
000109C4 mov ecx, 0B2D6002Ch
000109C9 cmp eax, ecx
000109CB ja loc_10D12
000109D1 jz loc_10CE9
Checks if IOCTL is less or equal to 0×0B2D6002C, if condition is true checks if IOCTL is exactly 0×0B2D6002C a certain task is performed by the device driver and finally ends with a classical
epilogue:
Code:
IofCompleteRequest(X, 0);
return value;
By monitoring Aavmker4.sys activity, with a DeviceIoControl hook emerges that the most used IOCTLs are:
Now we have two possibilities the first is to fuzz these IOCTLs and check crash dump if happens and after check code for more details, the second possibility is to invert the check order.
This the xml configuration to test Aavmker4.sys
Code:
<allow>
<drivers>
<entry>Aavmker4.sys</entry>
</drivers>
<devices>
<entry>\Device\AavmKer4</entry>
</devices>
<ioctls>
<entry>0xb2d60030</entry>
<entry>0xb2d60034</entry>
</ioctls>
<processes>
<entry>ashServ.exe</entry>
</processes>
</allow>
launch fuzzer and Avast Scan, as you can see Driver resists to Fuzzing attempts, its time to see code referred to
0xB2D60030 and
0xB2D60034.
0xB2D60030
Code:
00010D25 cmp eax, 0B2D60030h
00010D2A jz short loc_10DA8
00010D2C cmp eax, 0B2D60034h
00010D31 jz short loc_10D5B
00010DC5 mov edi, [ebx+0Ch]
00010DC8 cmp esi, 878h
00010DCE jz short loc_10DDA ;Check buffer size
00010DD0 push offset aIoctl_aavm_sta ; “******* IOCTL_AAVM_START_REQUEST_AND_SE”…
00010DD5 jmp loc_10ABA ;Jump to Io Completion
If buffer size differs from 878h Dbg Prints an error message, else supplied buffer is correctly sanitized via
MmUserProbeAddress,
MmIsAddressValid. We can say that this IOCTL is correctly handled.
0xB2D60034:
Code:
00010D5B cmp esi, 8
00010D5E jnz loc_10AC0 ;If differs from 8 return STATUS_INVALID_PARAMETER
00010D64 call PsGetCurrentProcessId
Now let’s test aswSP.sys. In Device Driver vulnerabilty research it’s fundamental to have a complete log of all activities of a driver, this can be obtained by correctly planning a battery of test unit. Each test should correspond to a primitive logic operation performed by an application that makes use of driver. I usually build several mini logs for each activity and finally a complete log. Here a little list of monitoring primitives:
- On Load
- On apparent Idle
- On Working
- On Shutdown
- Various, like On Surprise Stop
This will give us a complete report of all activities and involved IOCTL. In the specific case of aswMon2.sys we can isolate the following:
- * 0xb2c80018
- * 0xb2c80014
- * 0xb2c80020
- * 0xB2c800C0
- * 0xB2c800C4
- * 0xB2c800C8
From IOCTL Logger we can see that 0xB2c800C0 is heavly used, time to locate Ioctl Dispatcher:
Code:
0001178B and dword ptr [ebx+34h], 0
0001178F mov dword ptr [ebx+6Ch], offset sub_11FB6
00011796 mov dword ptr [ebx+28h], offset off_18988
C like:
Code:
v2->DriverUnload = 0;
v2->MajorFunction[13] = (PDRIVER_DISPATCH)sub_11FB6;
v2->FastIoDispatch = (PFAST_IO_DISPATCH)&unk_18988;
with a bit of research we land to sub_10B82 that contains the switch for Ioctls.
Code:
00010BBD mov eax, 0B2C80018h
00010BC2 cmp ecx, eax
00010BC4 push edi
00010BC5 ja loc_11066
00010BCB jz loc_10F70
00010BD1 cmp ecx, 0B2C80008h
00010BD7 jz short loc_10C3C
00010BD9 cmp ecx, 0B2C8000Ch
00010BDF jz short loc_10C16
00010BE1 cmp ecx, 0B2C80010h
00010BE7 jz short loc_10BFF
00010BE9 cmp ecx, 0B2C80014h
00010BEF jnz loc_111AC
00010BF5 call sub_108BC
From logs emerged that the most frequently used is 0B2C8000C so it’s obvious that we will study this for first:
0xB2C8000C:
Code:
00010C16 cmp [ebp+arg_C], 1448h
00010C1D jnz loc_111AC ;check len
00010C23 mov esi, [ebp+SourceString]
00010C26 mov ecx, 512h
00010C2B mov edi, offset dword_18A58
00010C30 rep movsd
00010C32 call sub_108F0
00010C37 jmp loc_112C1 ;go out
In this case user supplied input is correctly sanitized, so
0xB2C8000C can be excluded from fuzz testing. From the log On Shutdown emerged the massive presence of 0xB2c80018, so let’s fuzz it. Here the configuration for IOCTL Fuzzer:
Code:
<?xml version=”1.0″ encoding=”windows-1251″?>
<cfg>
<log_file>C:\ioctls.txt</log_file>
<hex_dump>true</hex_dump>
<log_requests>true</log_requests>
<debug_log_requests>true</debug_log_requests>
<fuze_requests>true</fuze_requests>
<fuze_size>true</fuze_size>
<allow>
<drivers>
<entry>aswMon2.SYS</entry>
</drivers>
<devices>
<entry>\Device\aswMon</entry>
</devices>
<ioctls>
<entry>0xb2c80018</entry>
</ioctls>
<processes>
<entry>ashServ.exe</entry>
</processes>
</allow>
<deny>
<drivers>
<entry>aswSP.SYS</entry>
<entry>Aavmker4.SYS</entry>
<entry>aswTDI.SYS</entry>
</drivers>
<ioctls>
<entry>0xb2c8000c</entry>
<entry>0xb2c80014</entry>
<entry>0xb2c80020</entry>
</ioctls>
</deny>
</cfg>
The config script allows only 0xB2c80018 sent from aswMon, other drivers are locked. Obviously fuzzing need to follow the log unit that evidenced out IOCTL, so run fuzzer and stop all Avast services.
Bang..a BSOD, discovered an Avast vulnerability into aswMon2.sys
From crashdump:
kd> !analyze -v
Quote:
UNEXPECTED_KERNEL_MODE_TRAP_M
Arguments:
Arg1: 00000008, EXCEPTION_DOUBLE_FAULT
Arg2: 80042000
Arg3: 00000000
Arg4: 00000000_KERNEL_MODE_TRAP_M (1000007f)
Quote:
STACK_TEXT:
WARNING: Stack unwind information not available. Following frames may be wrong.
f76f3234 8053d251 f76f3250 00000000 f76f32a4 nt+0×600fa
f76f32a4 8052c712 badb0d00 20a0a0a1 f76f5658 nt+0×66251
f76f3328 8052c793 41414141 00000000 f76f377c nt+0×55712
f76f33a4 804fc700 f76f377c f76f3478 05050505 nt+0×55793
f76f3760 8053d251 f76f377c 00000000 f76f37d0 nt+0×25700
f76f37d0 8052c712 badb0d00 20a0a0a1 f76f5658 nt+0×66251
f76f3854 8052c793 41414141 00000000 f76f3ca8 nt+0×55712
f76f38d0 804fc700 f76f3ca8 f76f39a4 05050505 nt+0×55793
f76f3c8c 8053d251 f76f3ca8 00000000 f76f3cfc nt+0×25700
f76f3cfc 8052c712 badb0d00 20a0a0a1 f76f5658 nt+0×66251
f76f3d80 8052c793 41414141 00000000 f76f41d4 nt+0×55712
f76f3dfc 804fc700 f76f41d4 f76f3ed0 05050505 nt+0×55793
f76f41b8 8053d251 f76f41d4 00000000 f76f4228 nt+0×25700
f76f4228 8052c712 badb0d00 20a0a0a1 f76f5658 nt+0×66251
f76f42ac 8052c793
...
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