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With the recent attacks on Apache servers and the rise of PHP-based worms, (such as Santy.E, or the renamed version PhpIncludeWorm), which attempt to exploit any PHP script they find it's suddenly much more important to protect your Web server. For popular sites probes constitute considerable part of hits and necessitate efforts to protect from them using mod_rewrtite in Apache and similar mechanisms in other webservers.
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A good start is to avoid displaying the software versions you are using.
Let me explain. When somebody request a page to a HTTP server, this one respond with headers such as Content-Type, Content-Length... as well as Server.
People don't usually see those headers, but if someone wants to hack your box, they might be looking for it. Why? Because known exploits usually work on specific software version.
Lets look at default HTTP headers on my ubuntu dapper box:
~$ telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
HEAD / HTTP/1.0HTTP/1.1 200 OK
Date: Tue, 25 Jul 2006 10:47:13 GMT
Server: Apache/2.0.55 (Ubuntu) PHP/5.1.4-1.dotdeb.2
Last-Modified: Mon, 20 Mar 2006 09:51:25 GMT
ETag: "3057-1f8-1a0f4140"
Accept-Ranges: bytes
Content-Length: 504
Connection: close
Content-Type: text/html; charset=ISO-8859-1Connection closed by foreign host.
As you can see from this excerpt, my box is running Apache 2.0.55 on an Ubuntu box and php-5.1.4 is used. This is perfect, if I want to hack that box, I simply have to look for known exploit for apache 2.0.55 or php 5.1.4 or even ubuntu.
The idea is to avoid telling too much, so we are going to make apache be less verbose.
2. Apache Configuration File:
In Apache, the ServerTokens directive allow the system administrator to set different type of Server HTTP response header:
- ServerTokens Prod[uctOnly] : this is the most restrictive, in our example, apache will respond:
Server: Apache- ServerTokens Major
response -> Server: Apache/2- ServerTokens Minor
response -> Server: Apache/2.0- ServerTokens Min[imal]
response -> Server: Apache/2.0.55- ServerTokens Os
response -> Server: Apache/2.0.55 (Ubuntu)- ServerTokens Full
response -> Server: Apache/2.0.55 (Ubuntu) PHP/5.1.4-1.dotdeb.2 mymod1/X.Y mymod2/W.ZBy default, ServerTokens is set to Full, on my dapper box at least. To change that value, edit /etc/apache2/apache2.conf and look for the line containing ServerTokens.
Nota: On my ubuntu dapper box, ServerTokens was not set and was therefore taking the default value (Full), in that case, simply add this directive to apache2.conf.
I would recommend setting ServerTokens to Prod by adding this to apache2.conf:
ServerTokens Prod
Reload apache:
$sudo /etc/init.d/apache2 reload
and check for the new headers. Here are the headers sent back by my local server after setting ServerTokens to Prod:
$ telnet localhost 80
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
HEAD / HTTP/1.0HTTP/1.1 200 OK
Date: Tue, 25 Jul 2006 11:33:09 GMT
Server: Apache
Last-Modified: Mon, 20 Mar 2006 09:51:25 GMT
ETag: "3057-1f8-1a0f4140"
Accept-Ranges: bytes
Content-Length: 504
Connection: close
Content-Type: text/html; charset=ISO-8859-1Connection closed by foreign host.
As you can see, apache does not tell anymore which version and modules are running
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3. PHP:
Another way to hide which PHP version you are running can be achieved through php.ini.
Php as a directive of its own in order not to be too verbose, this is the variable called expose_php. Turning this one to Off will avoid php telling that it is running. In the following output, I had ServerTokens set to Full and expose_php to Off:
"Mod_security 1.7 has been released.
Mod_security is an open source intrusion detection and prevention engine for web applications. It operates embedded into the web server, acting as a powerful umbrella - shielding applications from attacks. The latest release adds output scanning to Apache 2.x; the ability to analyze cookies; functionality to change the identity of the web server; several new actions for rule grouping; new null-byte attack anti-evasion code."
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***** NSA Guide to the Secure Configuration and Administration of iPlanet Web Server, Enterprise Edition 4.1 (425KB)
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