A buffer overrun vulnerability in the Apache HTTP server included with many popular Web servers enables an attacker to execute code on vulnerable machines.

The flaw lies in the way that the server handles data transmissions of unknown size. Typically, these transmissions are broken into "chunks" for easier handling. But Apaches HTTP server misinterprets the size of the chunks, which leads to an overrun of the heap memory, according to an advisory published Monday by Internet Security Systems Inc.s X-Force research team.

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The vulnerability can be exploited remotely by way of a carefully crafted invalid request to the server, and the flawed functionality is enabled by default. Exploiting the flaw could either lead to a denial of service on the machine or the execution of malicious code.

An attacker would only be able to execute code on 64-bit Unix systems and Windows machines, according to Apache. Exploiting the vulnerability on 32-bit Unix machines would crash the Apache HTTP server.

The Apache Software Foundations Apache Server Project, which maintains the open-source HTTP server, also issued a bulletin warning that all versions of Apache 1.3 are vulnerable, as are copies of version 2 up to 2.0.39.

However, versions 2.0 and later are not vulnerable to the remote execution of code, Apaches advisory said.

Apache is by far the most widely deployed Web server on the Internet, running on more than 50 percent of machines surveyed by Netcraft Ltd. Its HTTP server is included in products such as Oracle Corp.s 9i Application Server and IBM Corp.s WebSphere.

The Foundation said it was forced to release its advisory early—and without an available patch—because of ISS decision to publish its bulletin.

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