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TUCoPS :: Web :: Apache :: m-093.txt

Apache HTTP Server Chunk Encoding Vulnerability (CIAC M-093)


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

                Apache HTTP Server Chunk Encoding Vulnerability

June 19, 2002 21:00 GMT                                           Number M-093
[Revised 26 June 2002]
PROBLEM:       The Apache HTTP Server has a software flaw that misinterprets 
               invalid requests encoded using chunked encoding. This error can 
               be triggered remotely by sending certain invalid requests. 
PLATFORM:      Any systems running Apache web server 1.3.24 and 2.0 up to and 
               including 2.0.36. 
DAMAGE:        Successful exploitation may lead to modified Web content, 
               denial of service, or further compromise. 
SOLUTION:      Users of Apache 1.3 should upgrade to 1.3.26, and users of 
               Apache 2.0 should upgrade to 2.0.39, which contains a fix for 
               this issue. 
VULNERABILITY  The risk is HIGH. The vulnerability is in the default version 
ASSESSMENT:    of Apache HTTP Server described above. It is possible for 
               remote attackers to exploit this vulnerability to compromise 
               Apache Web servers. There are known exploits widely available 
               on the Internet. 
 ORIGINAL BULLETIN:                                                           
					 - 26 June, 2002 - Supersedes Apache security bulletin 20020617.
	                 - 32- and 64-bit platforms are exploitable.					 


[******  Start of Apache Bulletin ******]


Date: June 20, 2002
Product: Apache Web Server
Versions: Apache 1.3 all versions including 1.3.24; Apache 2.0 all versions
up to 2.0.36; Apache 1.2 all versions.

CAN-2002-0392 ( [CERT VU#944335]

- ----------------------------------------------------------
         ------------UPDATED ADVISORY------------
- ----------------------------------------------------------

While testing for Oracle vulnerabilities, Mark Litchfield discovered a
denial of service attack for Apache on Windows.  Investigation by the
Apache Software Foundation showed that this issue has a wider scope, which
on some platforms results in a denial of service vulnerability, while on
some other platforms presents a potential remote exploit vulnerability.  

This follow-up to our earlier advisory is to warn of known-exploitable
conditions related to this vulnerability on both 64-bit platforms and
32-bit platforms alike.  Though we previously reported that 32-bit
platforms were not remotely exploitable, it has since been proven by
Gobbles that certain conditions allowing exploitation do exist.

Successful exploitation of this vulnerability can lead to the execution of
arbitrary code on the server with the permissions of the web server child
process.  This can facilitate the further exploitation of vulnerabilities
unrelated to Apache on the local system, potentially allowing the intruder
root access.

Note that early patches for this issue released by ISS and others do not
address its full scope.

Due to the existence of exploits circulating in the wild for some platforms,
the risk is considered high.

The Apache Software Foundation has released versions 1.3.26 and 2.0.39
that address and fix this issue, and all users are urged to upgrade

As a reminder, we respectfully request that anyone who finds a potential
vulnerability in our software reports it to

- ----------------------------------------------------------
Full Description:

Versions of the Apache web server up to and including 1.3.24 and 2.0
up to and including 2.0.36 contain a bug in the routines that deal with 
requests encoded using chunked encoding.  This bug can be triggered
remotely, and this functionality is enabled by default.

In most cases the outcome of the invalid request is that the child process
dealing with the request will terminate.  At the least, this could help a
remote attacker launch a denial of service attack as the parent process
will eventually have to replace the terminated child process, and starting
new children uses non-trivial amounts of resources.

On the Windows and Netware platforms, Apache runs one multithreaded child
process to service requests.  The teardown and subsequent setup time to
replace the lost child process presents a significant interruption of
service.  As the Windows and Netware ports create a new process and reread
the configuration, rather than fork a child process, this delay is much
more pronounced than on other platforms.

In Apache 2.0, the error condition is correctly detected, so it will not
allow an attacker to execute arbitrary code on the server.  However,
platforms could be using a multithreaded model with multiple concurrent
requests per child process (although the default preference remains
multiple processes with a single thread and request per process, and most
multithreaded models continue to create multiple child processes).  Using
any multithreaded model, all concurrent requests currently served by the
affected child process will be lost.

In Apache 1.3, the issue should cause a stack overflow.  Due to the nature
of the overflow on 32-bit Unix platforms, this should cause a segmentation
violation and cause the child to terminate.  However, some 32-bit platforms
are indeed exploitable due to quirks in their implementation.  64-bit
platforms are also likely to be exploitable due to a data type conversion
that occurs within Apache.  We have been made aware that Apache 1.3 on
Windows is exploitable in a similar way as well.

[******  End of Apache Bulletin ******]


CIAC wishes to acknowledge the contributions of The Apache Software Foundation 
for the information contained in this bulletin.

CIAC, the Computer Incident Advisory Capability, is the computer
security incident response team for the U.S. Department of Energy
(DOE) and the emergency backup response team for the National
Institutes of Health (NIH). CIAC is located at the Lawrence Livermore
National Laboratory in Livermore, California. CIAC is also a founding
member of FIRST, the Forum of Incident Response and Security Teams, a
global organization established to foster cooperation and coordination
among computer security teams worldwide.

CIAC services are available to DOE, DOE contractors, and the NIH. CIAC
can be contacted at:
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    FAX:      +1 925-423-8002
    STU-III:  +1 925-423-2604

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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
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