26th Sep 2002 [SBWID-5313]
Winamp code injection via an mp3 file header that causes a buffer
Nullsoft Winamp 2.79
o' \,=./ `o
Andreas Sandblad [[email protected]] (o o)
A mp3 file can contain the ID3v2 tag. It's a newer version of the ID3v1
tag and carries information like title, artist and album. The tag is
parsed by Winamp when a 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 occours when the
url string intended to be sent to the minibrowser is created. That
means the buffer overflow occours before any actual internet connection
to info.winamp.com is beeing 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
Access violation - code c0000005 (first chance)
eax=00000021 ebx=00000000 ecx=0012bd00 edx=00000032 esi=00000113
eip=32253132 esp=00129c08 ebp=25313225 iopl=0 nv up ei pl zr na po
cs=001b ss=0023 ds=0023 es=0023 fs=0038 gs=0000
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 stackpointer 0x129c04 to be in a dataspace we are in
control of. Seems like we are lucky! The url string is stored from
0x1298d8 to 0x129cd8, thus it seems like we can control the eip. And in
fact we can. 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 (opcode
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 can't insert 0x00 in the return adress
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
Once we control the eip we have to do something useful. Since we still
are limited in what kind of opcodes 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 unescaped. So 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 opcode we want. This can be done with opcodes like:
0x66335142 ("f3QB") XOR DX,[ECX+0x42]
0x4A ("J") DEC EDX
0x665A ("fZ") POP DX
0x6652 ("fR") PUSH DX
It's not an easy task to perform the above and on some OS it has to be
done differently, but still it's possible.
Upgrade to Winamp 2.80 or disable the minibrowser.