On Tue, May 13, 2003 at 01:36:24PM -0400, David Riley wrote:
> On Tuesday, May 13, 2003, at 12:25 PM, Dave McKinney wrote:
>
> >We'll kick this off with the first challenge, which was devised by
> >Aaron
> >Adams:
> >
> >// vulndev-1.c
[ snip ]
> I'll start by saying that I like this idea... it'll give me a chance to
> brush up on my skills in this area.
>
> Now, the only error I see in this program is that the for() loop checks
> for i <= SIZE rather than i < SIZE. However, this doesn't seem to
> affect much... when I run the compiled program on my OS X machine with
> these args:
>
> ./vuln `perl -e 'print "a" x 2000'` `perl -e 'print "b" x 2000'`
>
> It exits cleanly. I imagine that it might overwrite a byte somewhere,
> but it's not really doing much for me.
It "works" for me on a Debian Linux box:
ben@Fenrir:/tmp$ ./vulndev-1 `perl -we'print "A" x 253'` a
Segmentation fault
Note that it does _not_ crash in the reverse case (large argv[2]),
although I'm not sure why. However, this might be indicative (pardon my
rusty-as-can-be C skills):
------------------------------------------------------------------------
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define SIZE 252
int
main(int argc, char *argv[])
{
int i, l1, l2, s1, s2;
char *p1, *p2;
char *buf1 = malloc(SIZE);
char *buf2 = malloc(SIZE);
if (argc != 3)
exit(1);
p1 = argv[1], p2 = argv[2];
strncpy(buf2, p2, SIZE);
for (i = 0; i <= SIZE && p1[i] != '\0'; i++)
buf1[i] = p1[i];
l1 = strlen(p1);
s1 = sizeof(p1);
l2 = strlen(p2);
s2 = sizeof(p2);
free(buf1);
free(buf2);
printf("strlen1: %i sizeof1: %i\n", l1, s1);
printf("strlen2: %i sizeof2: %i\n", l2, s2);
return 0;
}
------------------------------------------------------------------------
Now, note the output of this:
ben@Fenrir:/tmp/vuln-dev$ ./tst `perl -we'print "A" x 252'` a
strlen1: 252 sizeof1: 4
strlen2: 1 sizeof2: 4
ben@Fenrir:/tmp/vuln-dev$ ./tst `perl -we'print "A" x 253'` a
Segmentation fault
ben@Fenrir:/tmp/vuln-dev$ ./tst a `perl -we'print "A" x 253'`
strlen1: 1 sizeof1: 4
strlen2: 253 sizeof2: 4
ben@Fenrir:/tmp/vuln-dev$ ./tst a `perl -we'print "A" x 300'`
strlen1: 1 sizeof1: 4
strlen2: 300 sizeof2: 4
ben@Fenrir:/tmp/vuln-dev$ ./tst a `perl -we'print "A" x 1000'`
strlen1: 1 sizeof1: 4
strlen2: 1000 sizeof2: 4
Seems like "argv[2]" is copied no matter what the "strncpy"'s SIZE is.
Hmm. Anybody have an idea?
Ben Okopnik
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
The reason the government thinks you're just a number
is because it's just a machine.
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