Threats 

The threats we need to protect against are:

1. An attacker exploiting an httpd or other service vulnerability to take over the apache account.
2. An attacker exploiting a web application vulnerability to take over a user account.
3. A user accessing privileged information belonging to another user, or changing data belonging to another user, or an attacker doing the same after succeeding at (2).
4. A user exploiting a local root vulnerability to take over the machine, or an attacker doing the same after succeeding at (1) or (2).

1. httpd exploit

By exploiting a remote vulnerability in httpd, an attacker could gain control of the apache account.  A similar attack might take place against any other service running on the machine, possibly resulting in immediate root access if the other service runs as root.

Such an attack would likely disrupt the functioning of the entire service while the vulnerability responsible for the attack is uncovered and the machine is reinstalled.  User data could be lost or corrupted and might need to be restored from backup, although the Apache account should not have access to most user content.

Mitigation: run as few services on the machine besides httpd as possible.  Be alert to publically known httpd vulnerabilities and patch them as quickly as possible.

 2. Web application exploits

Web applications are generally implemented in PHP or another scripting language and are not usually subject to buffer overflow attacks.  Nevertheless, they have traditionally contained a rich variety of vulnerabilities, generally of one of the following forms:

• Shell command injection: execution of shell commands containing unsanitized user input, allowing attackers to execute arbitrary shell code
• SQL injection: execution of SQL queries containing unsanitized user input, allowing attackers to execute arbitrary SQL code
• Inclusion vulnerabilities: inclusion of other scripting code via a variable path containing unsanitized user input, allowing attackers to execute arbitrary scripting code
• Cross-site scripting: display of unsanitized HTML provided by one user to another, allowing attackers to execute Javascript code on another user's browser in the security context of the web application

Moreover, these vulnerabilities are the subject of widespread and frequent exploits.  Mitigation of this risk will come in two forms:

1. Reduce the frequency of compromises.  To the extent that we foster development of custom-written web applications, educate developers about these attacks and how to prevent them.  For off-the shelf web applications, provide cookie-cutter installations, monitor security sites for known exploits, and have a framework for fixing the cookie-cutter installations users have performed.
2. Reduce the severity of compromises.  No matter what we do, user account compromises will be endemic to our service, so we need to provide good tools for recovering from attacks (frequent user-accessible backups, for example) and to ensure that attacks do not spread to other users of the service.

[To do: sections 3 and 4]