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Remote Compromise Vulnerability in Apache HTTP Server
18 June 2002
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Internet Security Systems Security Advisory

June 17, 2002


ISS X-Force has discovered a serious vulnerability in the default version of Apache HTTP Server. Apache is the most popular Web server and is used on over half of all Web servers on the Internet. It may be possible for remote attackers to exploit this vulnerability to compromise Apache Web servers. Successful exploitation may lead to modified Web content, denial of service, or further compromise.

Affected Versions:

Apache 1.x

Note: Many commercial Web Application Servers such as Oracle 9ias and IBM Websphere use Apache HTTP Server to process HTTP requests. Additional products that bundle Apache HTTP Server for Windows may be affected.


The Apache HTTP Server is maintained by the Apache Software Foundation. Apache is an extremely popular open-source Web server. Netcraft (http://www.netcraft.com) reports that as of May 2002, Apache accounts for over 63% of all active Web sites. Apacheís installed base is larger than all other Web servers combined.

The Apache Project is an open-source and volunteer collaboration aimed to create and maintain a free, feature-rich, powerful, and secure Web server implementation. Apache is well regarded as the best, freely available Web server.

Apache contains a flawed mechanism meant to calculate the size of "chunked" encoding. Chunked encoding is part of the HTTP Protocol Specification used for accepting data from Web users. When data is sent from the user, the Web server needs to allocate a memory buffer of a certain size to hold the submitted data. When the size of the data being submitted is unknown, the client or Web browser will communicate with the server by creating "chunks" of data of a negotiated size.

The Apache HTTP Server has a software flaw that misinterprets the size of incoming data chunks. This error may lead to a signal race, heap overflow, and to exploitation of malicious code.

X-Force has verified that this issue is exploitable on Apache for Windows (Win32) version 1.3.24. Apache 1.x for Unix contains the same source code, but X-Force believes that successful exploitation on most Unix platforms is unlikely.


Internet Scanner X-Press Update 6.12 includes a check, ApacheChunkedEncodingBo, to detect installations of Apache HTTP Server for Win32. XPU 6.12 is available from the ISS Download Center at: http://www.iss.net/download. For questions about downloading and installing this XPU, email support@iss.net.

Detection support for this attack will be included in future X-Press Updates for RealSecure Network Sensor 6.x and 7.0. These XPUs will be available from the ISS Download Center, and this alert will be updated when these updates become available.

ISS X-Force has developed a patch for this issue. Follow the instructions below, or contact your vendor for assistance:

To apply a source code patch to your Apache package:

1. Locate your source directory and navigate into the "main" sub- directory.
2. Verify that "http_protocol.c" is present in the current directory.
3. To update your http_protocol.c file, create a file named "apache_patch.diff", containing the following text:

- --- http_protocol.c.vuln Fri Jun 14 16:12:50 2002
+++ http_protocol.c Fri Jun 14 16:13:47 2002
@@ -2171,7 +2171,7 @@

/* Otherwise, we are in the midst of reading a chunk of data */

- - len_to_read = (r->remaining > bufsiz) ? bufsiz : r->remaining;
+ len_to_read = (r->remaining > (unsigned int)bufsiz) ? bufsiz : r->

len_read = ap_bread(r->connection->client, buffer, len_to_read);
if (len_read <= 0) {

4. Apply the source code update using the "patch" command, or a similar utility.
5. Build new binaries and reinstall.

The Apache Server Project has been notified and will make a formal patch available soon. Please refer to the Apache Server Projectís homepage for more information: http://httpd.apache.org/

Additional Information:



This vulnerability was discovered and researched by Neel Mehta of the ISS X-Force.


About Internet Security Systems (ISS)
Founded in 1994, Internet Security Systems (ISS) (Nasdaq: ISSX) is a pioneer and world leader in software and services that protect critical online resources from an ever-changing spectrum of threats and misuse. Internet Security Systems is headquartered in Atlanta, GA, with additional operations throughout the Americas, Asia, Australia, Europe and the Middle East.

Copyright (c) 2002 Internet Security Systems, Inc. All rights reserved worldwide.

Permission is hereby granted for the electronic redistribution of this document. It is not to be edited or altered in any way without the express written consent of the Internet Security Systems X-Force. If you wish to reprint the whole or any part of this document in any other medium excluding electronic media, please email xforce@iss.net for permission.

Disclaimer: The information within this paper may change without notice. Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties, implied or otherwise, with regard to this information or its use. Any use of this information is at the user's risk. In no event shall the author/distributor (Internet Security Systems X-Force) be held liable for any damages whatsoever arising out of or in connection with the use or spread of this information.

X-Force PGP Key available on MIT's PGP key server and PGP.com's key server, as well as at http://www.iss.net/security_center/sensitive.php

Please send suggestions, updates, and comments to: X-Force xforce@iss.net of Internet Security Systems, Inc.


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