Apache 1.3.x shared memory scoreboard vulnerabilities

I. BACKGROUND

The Apache Software Foundation's HTTP Server is an effort to develop and maintain an open-source HTTP server for modern operating systems including Unix and Windows NT. The goal of this project is to provide a secure, efficient and extensible server that provides HTTP services in sync with the current HTTP standards. More detail about it
is available at http://httpd.apache.org.

II. DESCRIPTION

Apache HTTP Server contains a vulnerability in its shared memory scoreboard. Attackers who can execute commands under the Apache UID can either send a (SIGUSR1) signal to any process as root, in most cases killing the process, or launch a local denial of service (DoS) attack.

III. ANALYSIS

Exploitation requires execute permission under the Apache UID. This can be obtained by any local user with a legitimate Apache scripting resource (ie: PHP, Perl), exploiting a vulnerability in web-based applications written in the above example languages, or through the use of some other local/remote Apache exploit.

Once such a status is attained, the attacker can then attach to the httpd daemon's 'scoreboard', which is stored in a shared memory segment owned by Apache. The attacker can then cause a DoS condition on the system by continuously filling the table with null values and causing the server to spawn new children.

The attacker also has the ability to send any process a SIGUSR1 signal as root. This is accomplished by continuously overwriting the parent[].pid and parent[].last_rtime segments within the scoreboard to the pid of the target process and a time in the past. When the target pid receives the signal SIGUSR1, it will react according to how it
is designed to manage the signal. According to the man page (man 7 signal), if the signal is un-handled then the default action is to terminate:

     ...
     SIGUSR1 30,10,16 A User-defined signal 1
     ...
     The letters in the "Action" column have the following meanings:

     A Default action is to terminate the process.
     ...

iDEFENSE successfully terminated arbitrary processes, including those that "kicked" people off the system.

IV. DETECTION

Apache HTTP Server 1.3.x, running on all applicable Unix platforms, is affected.

V. VENDOR FIX/RESPONSE

Apache HTTP Server 1.3.27 fixes this problem. It should be available on October 3 at http://www.apache.org/dist/httpd/.

VI. CVE INFORMATION

The Mitre Corp.'s Common Vulnerabilities and Exposures (CVE) Project assigned the identification number CAN-2002-0839 to this issue.

VII. DISCLOSURE TIMELINE

8/27/2002	Issue disclosed to iDEFENSE
9/18/2002	Vendor notified at security@apache.org
9/18/2002	iDEFENSE clients notified
9/19/2002	Response received from Mark J Cox (mark@awe.com)
10/3/2002	Coordinated public disclosure

VIII. CREDIT

zen-parse (zen-parse@gmx.net) is credited with discovering this vulnerability.