‘MP3 Files can Cause Code Execution under Winamp’
‘It is possible to modify an existing MP3 file in such a way that it can carry a virus. The virus is activated when the MP3 file is played in Winamp and can then infect other MP3 files found on hard drives or network shares. In order to protect yourself you need to upgrade to Winamp 2.80 or disable the minibrowser.’
‘The information has been provided by Andreas Sandblad.’
Nullsoft Winamp version 2.79
Nullsoft Winamp version 2.80
An MP3 file can contain the ID3v2 tag. It is a newer version of the ID3v1 tag and carries information like title, artist, and album. The tag is parsed by Winamp when an MP3 file is loaded.
If the minibrowser is enabled, Winamp will try to query a script on http://info.winamp.com for extra information about the song, based on data from the ID3v2 tag. The buffer overflow condition occurs when the URL string intended to be sent to the minibrowser is created. That means the buffer overflow occurs before any actual internet connection to info.winamp.com is being made.
The easiest way to test the buffer overflow condition is to apply at least 159 ‘?’ characters in the title field of the ID3v2 tag. When playing the MP3 file in Winamp 2.79 the following error log was created:
Access violation – code c0000005 (first chance)
eax=00000021 ebx=00000000 ecx=0012bd00 edx=00000032 esi=00000113 edi=00000113
eip=32253132 esp=00129c08 ebp=25313225 iopl=0 nv up ei pl zr na po nc
cs=001b ss=0023 ds=0023 es=0023 fs=0038 gs=0000 efl=00010246
32253132 ?? ???
If we debug Winamp, we notice that the created URL to be sent to the minibrowser looks something like this:
We also understand that the buffer is overwritten by maximum one NULL character, making the register ESP change to 0x129c00. The ESP is then incremented by 0x4 and a RET is done, sending the program to the address found in 0x129c04.
We need the stack pointer 0x129c04 to be in a data space we are in control of. The URL string is stored from 0x1298d8 to 0x129cd8, thus it seems like we can control the EIP. Simply check the EIP of the error log. It displays EIP=0x32253132, that is hex for ‘2%12’. Seems like the stack pointer is located somewhere in our URL string containing ‘…%21%21%21%21…’. (Remember that memory addresses are retrieved backwards!)
So how do we exploit this thing? Well normal buffer overflow exploit is to return to anywhere in memory where we can find JMP ESP (op-code 0xFFE4) in order to get back to the string that caused the buffer overflow. This method is used because normal buffer overflows are only limited to the fact that they cannot insert 0x00 in the return address because of string operations.
The problem we face is we can only use characters a-z, A-Z, 0-9, ‘.’, because all others will be escaped to %HEX. That means the addresses containing the JMP ESP instruction we need to find are a bit limited (but of course in most cases not impossible to find as we only need one location). Since the versions of the system .DLL files Winamp import at launch is OS and system dependent, it really depends on the system if we are able to find an available address containing the JMP ESP assembler instruction.
Once we control the EIP, we have to do something useful. Since we still are limited in what kind of op-codes we can construct, it’s better to try to get somewhere in memory where our URL is not escaped (ex. ! to %21). When debugging we notice the register ECX is 0x12bd00 and points to a copy of our URL partly un-escaped. Therefore, if we somehow can increase the ECX and change the memory to do JMP ECX we are on a address where we can create any op-code we want. This can be done with op-codes like:
0x66335142 (‘f3QB’) XOR DX,[ECX+0x42]
0x4A (‘J’) DEC EDX
0x665A (‘fZ’) POP DX
0x6652 (‘fR’) PUSH DX
It is not an easy task to perform the above and on some OS, it has to be done differently, but still it is possible.’