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Home : Advisories : NCSA HTTP Daemon for UNIX Vulnerability

Title: NCSA HTTP Daemon for UNIX Vulnerability
Released by: CERT
Date: 17th February 1995
Printable version: Click here

Hash: SHA1


CERT(*) Advisory CA-95:04

Original issue date:  February 17, 1995

Last revised: September 23, 1997

                Updated Copyright Statement

Topic: 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 contact NCSA

(Elizabeth Frank, efrank@ncsa.uiuc.edu).

We will update this advisory as we receive additional information.

Please check advisory files regularly for updates that relate to your site.

- -----------------------------------------------------------------------------

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

     Review the text provided by NSCA (#1 below) and install the patch

     provided under #2 below.

     1. Read the following text, which was provided by the HTTPD Team at SDG

        at NCSA. This text replaces Step 1 in the original version of the

        advisory. The NCSA text can also be found at


                   Beginning of Text Provided by NCSA


                  NCSA httpd Patch for Buffer Overflow

A vulnerability was recently discovered in the NCSA httpd. A program which will

break into an HP system running the precompiled httpd has been published, along

with step by step instructions. The program overflows a buffer into program

space which then gets executed.

If you are running a precompiled NCSA httpd, please ftp a new copy of the

binary. If you have compiled your own source code, we recommend applying the

following Patch to fix the vulnerability in the NCSA HTTP Daemon V.1.3 for

UNIX. It modifies the strsubfirst subroutine in util.c.

We believe that earlier versions of the server are vulnerable to a similar

attack, and strsubfirst should be modified for all releases of the server.

[The original version of] Cert Advisory CA-95:04 describes the problem and

includes two suggested steps. We do not recommend taking step 1, which

increases MAX_STRING_LEN to 8192.  There are 154 occurrences of variables using

MAX_STRING_LEN and changing them from 256 to 8192 bytes is going to expand the

memory needed to run httpd tremendously! On top of that, httpd forks a new

process (a complete copy of the parent) for each connection, which if your

site gets hit a lot will use unnecessarily large amounts of memory. We have

already had reports from admins who have made the change saying they are

experiencing performance degradation due to swapping. Step 2, applying the

patch to util.c, should be sufficient to fix the problem. There is

significantly less forking in Release 1.4 of the NCSA HTTP Daemon which will

be released soon.

Detecting a Break-in

If the access log contains control characters, there is a chance that someone

has tried to break into your system. If your server has died recently, they

failed at least one attempt. And, if your server has not crashed and there are

control characters in the access log you should assume your system has been


In this case, servers which currently use the User Directive to run the server

as "nobody", have limited the potential damage of an intruder to those commands

which "nobody" may execute.

Control Characters in the Access Log

You've discovered control characters in your access log. How do you tell if was

an intruder?

If the beginning of the line containing the control characters begins sensibly

(eg. machine name, and date (the GET periodically gets clobbered)) and ends

with a series of control characters, it is a break-in attempt. If the beginning

of the line starts with control characters (often nulls), this is a symptom of

a collision problem that occurs when two children try to write to the access

log simultaneously. This problem has only been seen with moderately to heavily

loaded servers. (We are working to fix this in Release 1.4.)

Other ways to Make Your Server More Secure

A tutorial about running a secure server is available. We also recommend that

the User Directive be used to run the server as "nobody".

Patched Source and Binaries

The patched source and precompiled binaries are available. We will also be

correcting the source for previous releases, but we will NOT be generating

binaries for previous releases.

Elizabeth Frank



                        End of Text Provided by NCSA

     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))=src_len;i--)

!       dest[i] = dest[i-src_len+start];

!     for (i=0;ihttp://www.cert.org/legal_stuff.html and http://ftp.cert.org/pub/legal_stuff .

If you do not have FTP or web access, send mail to cert@cert.org with

"copyright" in the subject line.

CERT is registered in the U.S. Patent and Trademark Office.


Revision history

Sep. 23, 1997  Updated copyright statement

Aug. 07, 1996  Information previously in the README was inserted

                         into the advisory.

Mar. 15, 1995  Sec. III - Replaced original Step 1 with text from NCSA.

                          Updated NCSA contact information.


Version: PGP for Personal Privacy 5.0

Charset: noconv





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