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CERT Advisory CA-95:04.NCSA.http.daemon.for.unix.vulnerability

  • From: CERT Advisory
  • Date: Fri Feb 17 17:42:46 1995
  • Address: Software Engineering Institute Carnegie Mellon University Pittsburgh, Pennsylvania 15213-3890
  • Fax: +1 412 268-6989
  • Phone: +1 412 268-7090

CA-95:04                         CERT Advisory
                               February 17, 1995
                     NCSA HTTP Daemon for UNIX Vulnerability

The CERT Coordination Center has received reports that there is a
vulnerability in the NCSA HTTP Daemon V.1.3 for UNIX. Because of this
vulnerability, the daemon can be tricked into executing shell commands.

If you have any questions regarding this vulnerability, please send
e-mail to Beth Frank at the NCSA,


I.   Description

     A vulnerability in the NCSA HTTP Daemon allows it to be tricked into
     executing shell commands.

II.  Impact

     Remote users may gain unauthorized access to the account (uid) under
     which the httpd process is running.

III. Solution

     The following solution was provided by the HTTPD Team at SDG at

     Step 1: 

       In the file httpd.h, change the string length definitions
                /* The default string lengths */
                #define MAX_STRING_LEN 256
                #define HUGE_STRING_LEN 8192
                /* The default string lengths */
                #define HUGE_STRING_LEN 8192
                #define MAX_STRING_LEN  HUGE_STRING_LEN

     Step 2:

        Install the following patch, which performs the functionality of
        strsubfirst (i.e., copy src followed by dest[start] into dest) without
        the use of a temporary buffer.  
----------------------------------cut here-------------------------------------

*** util.c.bak  Sat May  7 21:47:15 1994
--- util.c      Thu Feb 16 04:17:07 1995
*** 158,168 ****
  void strsubfirst(int start,char *dest, char *src)
!     char tmp[MAX_STRING_LEN];
!     strcpy(tmp,&dest[start]);
!     strcpy(dest,src);
!     strcpy(&dest[strlen(src)],tmp);
--- 158,174 ----
  void strsubfirst(int start,char *dest, char *src)
!   int src_len, dest_len, i;
!   if ((src_len=strlen(src))<start){  /** src "fits" in dest **/
!     for (i=0;dest[i]=src[i];i++);
!     for (i=src_len;dest[i]=dest[i-src_len+start];i++);
!   }
!   else {                             /** src doesn't fit in dest **/
!     for (dest_len=strlen(dest),i=dest_len+src_len-start;i>=src_len;i--)
!       dest[i] = dest[i-src_len+start];
!     for (i=0;i<src_len;i++) dest[i]=src[i];
!   }

---------------------------------cut here------------------------------------- 

After you apply this patch, recompile httpd, kill the current running process,
and restart the new httpd.

The CERT Coordination Center thanks Steve Weeber, Carlos Varela, and
Beth Frank for their support in responding to this problem.

If you believe that your system has been compromised, contact the CERT
Coordination Center or your representative in Forum of Incident
Response and Security Teams (FIRST).

If you wish to send sensitive incident or vulnerability information to
CERT staff by electronic mail, we strongly advise that the e-mail be
encrypted.  The CERT Coordination Center can support a shared DES key, PGP
(public key available via anonymous FTP on, or PEM (contact
CERT staff for details).

Internet E-mail:
Telephone: +1 412-268-7090 (24-hour hotline)
           CERT personnel answer 8:30 a.m.-5:00 p.m. EST(GMT-5)/EDT(GMT-4),
           and are on call for emergencies during other hours.
Fax: +1 412-268-6989

Postal address:  CERT Coordination Center
                 Software Engineering Institute
                 Carnegie Mellon University
                 Pittsburgh, PA 15213-3890

CERT advisories and bulletins are posted on the USENET newsgroup If you would like to have future advisories and
bulletins mailed to you or to a mail exploder at your site, please send mail

Past advisories, CERT bulletins, information about FIRST representatives, and
other information related to computer security are available for anonymous FTP

CERT is a service mark of Carnegie Mellon University.

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