[H20020304]: Remotely exploitable format string vulnerability in ntop

[hologram <holo () brained org>]

        h o l o g r a m  |  s e c u r i t y  |  a d v i s o r y
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                       Advisory ID : H20020304
                          Software : ntop
                          Synopsis : Remotely exploitable format
                                     string vulnerability in ntop.
                            Vendor : Luca Deri <www.ntop.org>
                          Verified : Version 2.0
                            Author : hologram <holo () brained org>

_______________________________________________________________________


| Overview |-----------------------------------------------------------

ntop is a UNIX tool that shows the network usage, similar to what the
popular top UNIX command does on the system level. A format string
vulnerability has been discovered on the programmatic level and
is currently known to affect the UNIX version, however, the Windows
port of the program remains untested. The vulnerability allows
for remote arbitrary code execution.

| Vulnerability |------------------------------------------------------

The format string vulnerability lies within the traceEvent() function
which is declared as:

void traceEvent(int eventTraceLevel, char* file,
  int line, char * format, ...)

in the file util.c. The third argument, as is apparent, is a format
string to be later manipulated by the traceEvent() call.

Further into the code, the following is made visible:

...

  va_list va_ap;
  va_start (va_ap, format);

...

    char buf[BUF_SIZE];

...

#ifdef WIN32
      /* Windows lacks of vsnprintf */
      vsprintf(buf, format, va_ap);
#else
      vsnprintf(buf, BUF_SIZE-1, format, va_ap);
#endif

      if(!useSyslog) {  // syslog() logging is not enabled
        printf(buf);  // vulnerability

...

#ifndef WIN32
      else {  // syslog() logging is enabled
#if 0
        switch(traceLevel) {
        case 0:
          syslog(LOG_ERR, buf);  // vulnerability
          break;
        case 1:
          syslog(LOG_WARNING, buf);  // vulnerability
          break;
        case 2:
          syslog(LOG_NOTICE, buf);  // vulnerability
          break;
        default:
          syslog(LOG_INFO, buf);   // vulnerability
          break;
        }
#else
        syslog(LOG_ERR, buf);

...

Obviously, a call such as syslog(LOG_ERR, buf) should be replaced
with syslog(LOG_ERR, "%s", buf) to remove the insecurity.

The bug can be exploited whether or not syslog() logging is enabled
because of the erroneous printf(buf) call, as well.

One of the simplest points of entry I have determined is if the -w
option was specified when ntop was ran, which allows web access
to the ntop information. A HTTP request of the following:

GET /%s%s%s HTTP/1.0

will cause program termination (the HTTP deamon for ntop is normally
listening on port 3000).

The vulnerability does allow remote execution of arbitrary commands,
and if concerned, an appropriate fix should be quickly applied.


-------------------------------| Copyright 2002. All rights reserved. |

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