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Apache httpd: vulnerability with chunked encoding
From: Mark J Cox <mjc () apache org>
Date: Mon, 17 Jun 2002 18:21:15 +0100 (BST)

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Date: June 17, 2002
Product: Apache Web Server
Versions: Apache 1.3 all versions including 1.3.24, Apache 2 all versions
up to 2.0.39

Introduction:

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 a remote exploit vulnerability.  

We were also notified today by ISS that they had published the same issue
which has forced the early release of this advisory.

The Common Vulnerabilities and Exposures project (cve.mitre.org) has
assigned the name CAN-2002-0392 to this issue.

Description:

Versions of the Apache web server up to and including 1.3.24 and 2.0 up to
and including 2.0.36 and 2.0.36-dev versions contain a bug in the routines
which deal with invalid requests which are encoded using chunked encoding.
This bug can be triggered remotely by sending a carefully crafted invalid
request. 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 execure arbitrary code on the server. However
platforms could be using a multithreaded model of 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 causes a stack overflow.  Due to the nature of the
overflow on 32-bit Unix platforms this will cause a segmentation violation
and the child will terminate.  However on 64-bit platforms the overflow
can be controlled and so for platforms that store return addresses on the
stack it is likely that it is further exploitable. This could allow
arbitrary code to be run on the server as the user the Apache children are
set to run as.

We have been made aware that Apache 1.3 on Windows is exploitable in this
way.

Please note that the patch provided by ISS does not correct this
vulnerability.

The Apache Software Foundation are currently working on new releases that
fix this issue, please see http://httpd.apache.org/ for updated
versions.










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