ADVISORY NOTICE

F-11: Unix NCSA httpd Vulnerability

February 14, 1995 1030 PST


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

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/.

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

CIAC services are available to DOE, DOE Contractors, and the NIH. CIAC can be contacted at:

    Voice:          +1 925-422-8193 (7 x 24)
    FAX:            +1 925-423-8002
    STU-III:        +1 925-423-2604
    E-mail:          ciac@ciac.org
    World Wide Web:  http://www.ciac.org/
    Anonymous FTP:   ftp.ciac.org

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, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, 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 or the University of California, and shall not be used for advertising or product endorsement purposes.

UCRL-MI-119788
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