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| ISS X-Force and Mark Litchfield have both 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. |
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Credit:
The information has been provided by Mark J Cox.
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Vulnerable systems:
Apache 1.3 all versions including 1.3.24, Apache 2 all versions up to 2.0.39
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 execute 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.
Vendor status:
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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