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                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
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                               ADVISORY NOTICE

                        Unix NCSA httpd Vulnerability

February 14, 1995 1030 PST                                        Number F-11
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PROBLEM:       A vulnerability has been discovered in the NCSA WWW server
               software (httpd).
PLATFORMS:     Unix systems running NCSA httpd version 1.3.
DAMAGE:        Remote users may gain unauthorized access.
SOLUTION:      Implement workaround as described below.
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VULNERABILITY  This vulnerability, along with an automated exploitation
ASSESSMENT:    script, has been announced in public forums on the Internet.
               CIAC recommends that sites install the workaround on affected 
               systems as soon as possible.
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          Critical Information about the NCSA httpd Vulnerability

CIAC has learned of a serious vulnerability in the NCSA WWW server software,
httpd.  By sending a carefully constructed request to the WWW server, an
intruder can cause an internal buffer overflow and push arbitrary 
instructions onto the program stack.  These new instructions may allow the
intruder unauthorized access to the WWW server.

Until official patches are available from NCSA, CIAC recommends the following
temporary fix be installed.  In the file httpd.h, change the string length
definitions from:

      /* The default string lengths */
      #define MAX_STRING_LEN 256
      #define HUGE_STRING_LEN 8192

to:

      /* The default string lengths */
      #define HUGE_STRING_LEN 8192
      #define MAX_STRING_LEN  HUGE_STRING_LEN

Then rebuild, install, and restart the new httpd server.

It is likely that these attacks will generate unusual server log entries.  
The httpd access_log file should be examined for unusual requests, especially
those containing control characters.

Note that while this workaround addresses the vulnerability currently being
exploited, there are likely to be other similar vulnerabilities present in
this and other WWW server software.  To lessen the chance of compromise, it
is strongly recommended that WWW servers run as unprivileged users (e.g.
user "nobody") and that they be locked into a restricted filesystem via the
chroot() system call.  For more information, please see CIAC Document 2308, 
"Securing Internet Information Servers," which is available via anonymous
FTP from ciac.llnl.gov in the directory /pub/ciac/ciacdocs/.

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CIAC wishes to acknowledge the contributions of the DFN-CERT in the
construction of this bulletin.
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For emergencies and off-hour assistance, DOE and DOE contractor sites can
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To use this service, dial 1-510-422-8193 or 1-800-759-7243 (SKYPAGE). The
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_____________________________________________________________________________

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