Further Securing Sessions
There are a few remaining PHP directives for controlling sessions, several of these have security implications. Firstly, the session name (set with session.name) should be changed from the default to avoid collisions, especially on servers with multiple users.
The session.cookie_path directive determines the default cookie path, the path for which cookies will be sent in an HTTP request. If you have a forum at somedomain.com/forum, and somedomain.com/ does not require session management, you can change session.cookie_path as shown below.
php_value session.cookie_path /forum/
This prevents sections of your site which do not require the session cookie from being sent it, and limits exposure of the session IDs to those parts of a site where sessions are actually being used. This is especially important if some sections of your site have pages provided by other users, who could use those pages to steal session IDs from your visitors.
Setting session.use_only_cookies to true disables the passing of session IDs in URLs, at the cost of losing sessions support for users with cookies disabled, or on browsers not supporting cookies. Setting session.cookie_domain to the most restrictive domain name possible (e.g. forum.somesite.com instead of somesite.com) also helps to minimise exposure of session IDs. Of course, if you have a single login for an entire range of subdomains, you will have to set the domain as somedomain.com to ensure that the sessions are correctly managed across all of the subdomains.
Finally, it is possible to set the hash function used when creating session IDs. The default is to use MD5 (hash function 0), but SHA1 may also be used (hash function 1). SHA1 is a 160-bit hash function, whereas MD5 is only a 128-bit hash function, so using SHA1 for session hashes improves security slightly over using MD5. You can set the hash function using This setting was introduced in PHP 5.
php_value session.hash_function 1
Beyond PHP Security
Everything I've covered so far has been directly related to PHP and SQL security. The best situation we can manage here is PHP Safe Mode, which uses self-imposed restrictions to improve security. That this is the best we can achieve is due to the server architecture currently in use. There are, however, a few options for taking security a little further, and imposing the restrictions at a lower level than PHP itself. To conclude this series, I'll mention some of these briefly here.
Chroot changes the "root" directory that a process can see. This effectively locks it into a certain directory structure within the overall filesystem. With this approach, you can lock a web server into some directory such as /home/www and it will not be able to access anything outside of that structure.
There are several advantages to doing this. The first is that the web server, PHP, any user scripts, and also any attackers, will be contained within this chroot "jail", unable to access files outside of it. Furthermore, you can remove all but the most essential software from the chroot environment. Removing any shells from the environment prevents a large number of exploits which attempt to invoke a remote shell. The minimal environment inside a chroot makes life very difficult for attackers, no matter whether their method of attack is through a vulnerability in your PHP code, or a vulnerability in the underlying web server.
Apache mod_security and mod_chroot
mod_security and mod_chroot are extension modules specifically for the Apache web server. These two modules provide chroot support for Apache without externally applying a chroot technique. mod_security also provides several other security features. Further information is available at http://www.modsecurity.org/ for mod_security and at http://core.segfault.pl/~hobbit/mod_chroot/ for mod_chroot.
suEXEC and Multiple Server Instances
Using a chroot to lock your web server into a restricted environment helps to prevent some security problems, but one of the big issues is shared hosting. Running multiple websites on the same server requires that the web server process has access to each user's files. If the web server has access, so do the other users (subject to PHP Safe Mode restrictions, of course). There are two ways around this, one which is Apache specific, and one which may be deployed on any server environment.
suEXEC, specific to Apache, switches an Apache process to be owned by the same user as the script it is executing, losing any escalated permissions. This locks that Apache instance into the permissions held by that user, rather than the permissions held by the master web server process itself. This mechanism allows a return to the more traditional permissions system, and each user can be reasonably sure his or her files are protected. The cost of this is that an Apache process may not then be promoted back to regain permissions and switch user again to serve a different user's files. This system works best when there will be many requests for pages owned by the same user. suEXEC is explained in more detail at http://httpd.apache.org/docs/1.3/suexec.html
The alternative is to use multiple instances of the web server, each one running with the permissions of a different user. Each server then only has the permissions it needs to serve a single website, so a reverse proxy must be used as a front to all of these server instances, redirecting requests for a virtually hosted website to the Apache instance responsible for actually serving that site. This solution is the most secure, but also the most resource-hungry. Information about using Apache as a reverse proxy is available at http://httpd.apache.org/docs/1.3/mod/mod_proxy.html
How to check for PHP vulnerabilities
The best way to check whether your web site & applications are vulnerable to PHP security attacks is by using a Web Vulnerability Scanner. A Web Vulnerability Scanner crawls your entire website and automatically checks for vulnerabilities to PHP attacks. It will indicate which scripts are vulnerable so that you can fix the vulnerability easily. Besides PHP security vulnerabilities, a web application scanner will also check for SQL injection, Cross site scripting & other web vulnerabilities.
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This concludes the tour of PHP and SQL security. There is much more information available on the web about each security issue covered in this series, and many other issues also. In particular, several Apache modules exist for improving security through chrooting, reverse proxies, or simply checking requests for attacks such as directory traversal.
The PHP manual over at php.net points out any security concerns relating to most functions, and the comments posted by users below the reference pages often contain useful security hints and code samples