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TUCoPS :: Unix :: General :: ciacf011.txt

Unix NCSA Httpd

                       The U.S. Department of Energy
                    Computer Incident Advisory Capability
                           ___  __ __    _     ___
                          /       |     /_\   /
                          \___  __|__  /   \  \___

                               ADVISORY NOTICE

                        Unix NCSA httpd Vulnerability

February 14, 1995 1030 PST                                        Number F-11

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.

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.

          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


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

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 in the directory /pub/ciac/ciacdocs/.


CIAC wishes to acknowledge the contributions of the DFN-CERT in the
construction of this bulletin.

For emergencies and off-hour assistance, DOE and DOE contractor sites can
contact CIAC 24-hours a day via an integrated voicemail and SKYPAGE number.
To use this service, dial 1-510-422-8193 or 1-800-759-7243 (SKYPAGE). The
primary SKYPAGE PIN number, 8550070 is for the CIAC duty person. A second
PIN, 8550074 is for the CIAC Project Leader.  CIAC's FAX number is
510-423-8002, and the STU-III number is 510-423-2604.  Send E-mail to

Previous CIAC notices, anti-virus software, and other information are
available on the Internet via anonymous FTP from (IP address

CIAC has several self-subscribing mailing lists for electronic publications:
1.  CIAC-BULLETIN for Advisories, highest priority - time critical
    information, and Bulletins, important computer security information;
2.  CIAC-NOTES for Notes, a collection of computer security articles;
3.  SPI-ANNOUNCE for official news about Security Profile Inspector (SPI)
    software updates, new features, distribution and availability;
4.  SPI-NOTES, for discussion of problems and solutions regarding the use of
    SPI products.

Our mailing lists are managed by a public domain software package called
ListProcessor, which ignores E-mail header subject lines. To subscribe (add
yourself) to one of our mailing lists, send requests of the following form:

subscribe list-name LastName, FirstName PhoneNumber

as the E-mail message body, substituting CIAC-BULLETIN, CIAC-NOTES,
SPI-ANNOUNCE or SPI-NOTES for "list-name" and valid information for
"LastName" "FirstName" and "PhoneNumber."  Send to:
not to:

subscribe ciac-notes O'Hara, Scarlett 404-555-1212 x36
subscribe ciac-bulletin O'Hara, Scarlett 404-555-1212 x36

You will receive an acknowledgment containing address and initial PIN, and
information on how to change either of them, cancel your subscription, or get

PLEASE NOTE: Many users outside of the DOE and ESnet computing communities
receive CIAC bulletins. If you are not part of these communities, please
contact your agency's response team to report incidents. Your agency's team
will coordinate with CIAC. The Forum of Incident Response and Security Teams
(FIRST) is a world-wide organization. A list of FIRST member organizations
and their constituencies can be obtained by sending E-mail to with an empty subject line and a message body
containing the line: send first-contacts.

This document was prepared as an account of work sponsored by an agency of
the United States Government. Neither the United States Government nor the
University of California nor any of their employees, makes any warranty,
expressed or implied, or assumes any legal liability or responsibility for
the accuracy, completeness, or usefulness of any information, product, or
process disclosed, or represents that its use would not infringe privately
owned rights. Reference herein to any specific commercial products, process,
or service by trade name, trademark manufacturer, or otherwise, does not
necessarily constitute or imply its endorsement, recommendation, or favoring
by the United States Government or the University of California. The views
and opinions of authors expressed herein do not necessarily state or reflect
those of the United States Government nor the University of California, and
shall not be used for advertising or product endorsement purposes.

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