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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.
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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.
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LINKS:
CIAC BULLETIN: http://www.ciac.org/ciac/bulletins/m-093.shtml
ORIGINAL BULLETIN:
http://httpd.apache.org/info/security_bulletin_20020617.txt
- 26 June, 2002 - Supersedes Apache security bulletin 20020617.
http://httpd.apache.org/info/security_bulletin_20020620.txt
- 32- and 64-bit platforms are exploitable.
UPGRADE: http://www.apache.org/dist/httpd/
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[****** Start of Apache Bulletin ******]
SUPERSEDES: http://httpd.apache.org/info/security_bulletin_20020617.txt
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 (mitre.org) [CERT VU#944335]
- ----------------------------------------------------------
------------UPDATED ADVISORY------------
- ----------------------------------------------------------
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 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
immediately.
As a reminder, we respectfully request that anyone who finds a potential
vulnerability in our software reports it to security@apache.org.
- ----------------------------------------------------------
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:
Voice: +1 925-422-8193 (7x24)
FAX: +1 925-423-8002
STU-III: +1 925-423-2604
E-mail: ciac@ciac.org
Previous CIAC notices, anti-virus software, and other information are
available from the CIAC Computer Security Archive.
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Anonymous FTP: ftp.ciac.org
PLEASE NOTE: Many users outside of the DOE, ESnet, and NIH computing
communities receive CIAC bulletins. If you are not part of these
communities, please contact your agency's response team to report
incidents. Your agency's team will coordinate with CIAC. The Forum of
Incident Response and Security Teams (FIRST) is a world-wide
organization. A list of FIRST member organizations and their
constituencies can be obtained via WWW at http://www.first.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
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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
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