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chmlib Buffer Overflow (Technical Details)
------------------------------------------------------------------------
SUMMARY
CHMLIB is "a library for dealing with Microsoft ITSS/CHM format files.
Right now, it is a very simple library, but sufficient for dealing with
all of the .chm files I've come across. Due to the fairly well-designed
indexing built into this particular file format, even a small library is
able to gain reasonably good performance indexing into ITSS archives".
chmlib has been found to be vulnerable to a buffer overflow, allowing
attacker to execute arbitrary code.
DETAILS
Vulnerable Systems:
* chmlib version 0.36 and prior.
Immune Systems:
* CHM lib version 0.37 (download
<http://morte.jedrea.com/%7Ejedwin/projects/chmlib/chmlib-0.37.2.tgz>
here)
* CHM lib version 0.37.2 (download
<http://morte.jedrea.com/%7Ejedwin/projects/chmlib/chmlib-0.37.tgz> here)
0. chm_lib.c: _chm_decompress_block(struct chmFile *h, UInt64 block,UChar
**ubuffer)
1. see cmpLen and cbuffer declaration
2. call to _chm_get_cmpblock_bounds(h, block, &cmpStart, &cmpLen) to set
cmpLen
3. cmpLen is used to offset write operations in cbuffer
(_chm_fetch_bytes(h, cbuffer, cmpStart, cmpLen))
4. if cmpLen > h->reset_table.block_len + 6144 the buffer overflows
5. as we can supply the fd data, exploitation is possible and an attacker
can execute arbitrary code.
Vulnerable code:
/* decompress the block. must have lzx_mutex. */
static Int64 _chm_decompress_block(struct chmFile *h,
UInt64 block,
UChar **ubuffer)
{
..
UChar cbuffer[h->reset_table.block_len + 6144]; /* compressed
buffer */
Int64 cmpLen; /* compressed
len */
..
..
..
..
if (! _chm_get_cmpblock_bounds(h, block, &cmpStart, &cmpLen)
||
_chm_fetch_bytes(h, cbuffer, cmpStart, cmpLen) != cmpLen
||
LZXdecompress(h->lzx_state, cbuffer, lbuffer, (int)cmpLen,
(int)h->reset_table.block_len) != DECR_OK)
{
..
-----------------------------
/* get the bounds of a compressed block. return 0 on failure */
static int _chm_get_cmpblock_bounds(struct chmFile *h,
UInt64 block,
UInt64 *start,
Int64 *len)
{
UChar buffer[8], *dummy;
unsigned int remain;
/* for all but the last block, use the reset table */
if (block < h->reset_table.block_count-1)
{
...
...
/* unpack the end address */
dummy = buffer;
remain = 8;
if (_chm_fetch_bytes(h, buffer,
(UInt64)h->data_offset
+ (UInt64)h->rt_unit.start
+ (UInt64)h->reset_table.table_offset
+ (UInt64)block*8 + 8,
remain) != remain
||
!_unmarshal_int64(&dummy, &remain, len))
return 0;
}
/* for the last block, use the span in addition to the reset table */
else
{
...
}
/* compute the length and absolute start address */
*len -= *start;
*start += h->data_offset + h->cn_unit.start;
return 1;
}
--------------
/*
* dest(len) is read out of the fd
*/
static int _unmarshal_int64(unsigned char **pData,
unsigned int *pLenRemain,
Int64 *dest)
{
Int64 temp;
int i;
if (8 > *pLenRemain)
return 0;
temp=0;
for(i=8; i>0; i--)
{
temp <<= 8;
temp |= (*pData)[i-1];
}
*dest = temp;
*pData += 8;
*pLenRemain -= 8;
return 1;
}
---------------
/* utility function to handle differences between {pread,read}(64)? */
static Int64 _chm_fetch_bytes(struct chmFile *h,
UChar *buf,
UInt64 os,
Int64 len)
{
Int64 readLen=0, oldOs=0;
if (h->fd == CHM_NULL_FD)
return readLen;
CHM_ACQUIRE_LOCK(h->mutex);
..
readLen = pread(h->fd, buf, (long)len, (unsigned int)os);
..
CHM_RELEASE_LOCK(h->mutex);
return readLen;
}
Disclosure Timeline:
24.10.05 - Issue found by Sven Tantau
25.10.05 - Contacted chmlib maintainer
25.10.05 - Quick reaction with confirmation
26.10.05 - New release of chmlib and public disclosure
Vendor Status:
chmlib maintainer Jed Wing released a new version 0.37
ADDITIONAL INFORMATION
The information has been provided by <mailto:sven@sven-tantau.de> Sven
Tantau.
The original article can be found at:
<http://www.sven-tantau.de/public_files/chmlib/chmlib_20051126.txt>
http://www.sven-tantau.de/public_files/chmlib/chmlib_20051126.txt
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