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Security: Apache (2 of 3) Print E-mail
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Features This is the second installation of a 3 part article on LAMP (Linux Apache MySQL PHP). Apache is the most widely used HTTP-server in the world today.

It is also the most used web server for a Linux system. A web server like Apache, in its simplest fun ction, is software that displays and serves HTML pages hosted on a server to a client browser that understands the HTML code.

You must always be careful in knowing the web services that run. Run:

[root@aquarius /]# netstat -a | grep LISTEN

to scan your ports. A secure site should have only ports 22 (SSH), 80 (HTTP) and 443 (SSL) exposed.

Log files are another useful utility for monitoring attacks on your server. One must set up a centarlised secure log server so that hackers will not be able to remov e traces of their intrusion so easily. Various logfile analyzers like analog, webaliser help in keeping track of the web server access by people. By installing and configuring a good logfile analyser one can know details about the total traffic across the network and the various files and directories accessed,mod ified,deleted or any such activity. It will also tell you the pages that were visited and by whom. In addition to that are all the resources that are busy with respect to apache.

Maintaining Logfiels is such an important task that one must follow in order to keep track of his system's activities.Apache web server logfiles are httpd.log,error _log and access_log These files log all the attempts by a user in order to perform a task,it can be an attempt for compromising the system The daemon syslog must be enabled which is responsible for logging activity. Care must be taken that logging is on for mail and auth privileges in /etc/syslog.conf

In typical operation, Apache is started by the root user, and it switches to the user defined by the User directive to serve hits. As is the case with any command th at root executes, you must take care that it is protected from modification by non-root users. Not only must the files themselves be writeable only by root, but so mu st the directories, and parents of all directories. For example, if you choose to place ServerRoot in /usr/local/apache then it is suggested that you create that directory as root, with commands like these:

    mkdir /usr/local/apache
    cd /usr/local/apache
    mkdir bin conf logs
    chown 0 . bin conf logs
    chgrp 0 . bin conf logs
    chmod 755 . bin conf logs

It is assumed that /, /usr, and /usr/local are only modifiable by root. When you install the httpd executable, you should ensure that it is similarly protected:

    cp httpd /usr/local/apache/bin
    chown 0 /usr/local/apache/bin/httpd
    chgrp 0 /usr/local/apache/bin/httpd
    chmod 511 /usr/local/apache/bin/httpd

You can create an htdocs subdirectory which is modifiable by other users -- since root never executes any files out of there, and shouldn't be creating files in there.

If you allow non-root users to modify any files that root either executes or writes on then you open your system to rootcompromises. For example, someone could repla ce the httpd binary so that the next time you start it, it will execute some arbitrary code. If the logs directory is writeable (by a non-root user), someone could repl ace a log file with a symlink to some other system file, and then root might overwrite that file with arbitrary data. If the log files themselves are writeable (by a no n-root user), then someone may be able to overwrite the log itself with bogus data.

Server Side Includes

Server Side Includes (SSI) present a server administrator with several potential security risks. The first risk is the increased load on the server. All SSI-enabled files have to be parsed by Apache, whether or not there are any SSI directives included within the f iles. While this load increase is minor, in a shared server environment it can become significant.SSI files also pose the same risks that are associated with CGI script s in general. Using the "exec cmd" element,SSI-enabled files can execute any CGI script or program under the permissions of the user and group Apache runs as, as config ured in httpd.conf. That should definitely give server administrators pause.

There are ways to enhance the security of SSI files while still taking advantage of the benefits they provide. To isolate the damage a wayward SSI file can cause, a server administrator can enable suexec as described in the CGI in General section. Enabling SSI for files with .html or .htm extensions can be dangerous. This is especially true in a shared, or high traffic, server environment. SSI-enabled files shoul d have a separate extension, such as the conventional .shtml. This helps keep server load at a minimum and allows for easier management of risk.

Another solution is to disable the ability to run scripts and programs from SSI pages. To do this replace Includes with IncludesNOEXEC in the Options directive. Note that users may still use <--#include virtual="..." --> to execute CGI scripts if these scripts are in directories desginated by a ScriptAlias directive.

Non Script Aliased CGI

Allowing users to execute CGI scripts in any directory should only be considered if:

  • You trust your users not to write scripts which will deliberately or accidentally expose your system to an attack.

  • You consider security at your site to be so feeble in other areas, as to make one more potential hole irrelevant.

  • You have no users, and nobody ever visits your server.

Script Aliased CGI

Limiting CGI to special directories gives the admin control over what goes into those directories. This is inevitably more secure than non script aliased CGI, but only if users with write access to the directories are trusted or the admin is willing to test each new CGI script/program for potential security holes. Most sites choose this option over the non script aliased CGI approach.

CGI in General

Always remember that you must trust the writers of the CGI script/programs or your ability to spot potential security holes in CGI, whether they were deliberate or accidental.

All the CGI scripts will run as the same user, so they have potential to conflict (accidentally or deliberately) with other scripts e.g. User A hates User B, so he writes a script to trash User B's CGI database. One program which can be used to allow scripts to run as different users is suEXEC which is included with Apache as of 1. 2 and is called from special hooks in the Apache server code. Another popular way of doing this is with CGIWrap.

Protecting System Settings

To run a really tight ship, you'll want to stop users from setting up .htaccess files which can override security features you've configured. Here's one way to do it.

In the server configuration file, put


AllowOverride None

This prevents the use of .htaccess files in all directories apart from those specifically enabled.

Protect Server Files by Default

One aspect of Apache which is occasionally misunderstood is the feature of default access. That is, unless you take steps to change it, if the server can find its wa y to a file through normal URL mapping rules, it can serve it to clients.

For instance, consider the following example:

# cd /; ln -s / public_html

If a client accessed http://localhost/~root/, this would allow them to walk through the entire filesystem. To work around this, add the following block to your serve r's configuration:

 
     Order Deny,Allow
     Deny from all
  

This will forbid default access to filesystem locations. Add appropriate blocks to allow access only in those areas you wish. For example,

 
     Order Deny,Allow
     Allow from all
 
 
     Order Deny,Allow
     Allow from all
 

Pay particular attention to the interactions of and directives; for instance, even if denies access, a directive might overturn it.

htpasswd authentication

With apache,you can secure your files and directories in a more simplest way by using any of the authentication methods like basic,digest etc. By using htpasswd one can allow only specific users to access a particular file or a directory like this.

  1. Create a file called users and list all the names of the users u want to give access and place it in a location like /etc/httpd/

  2. Use the following command:
    root# htpasswd -cm /etc/httpd/users username
    
    • -c is only used the first time
    • For other users you need not use the -c option

  3. type the password on prompt

You can notice the file users containing the passwords encrypted

Now add this in httpd.conf file at the end o fthe file


AuthType "Basic"
AuthUserFile /etc/httpd/users
AuthName "Authorisation Required"
require valid-user

Whenever a user tries to access /var/www/html/test directory he will be prompted for the username and password and if he is allowed to access only then he will be pe rmitted to enter into the directory and access fiels otherwise he will not be allowed. This is just one way of securing your files with apache.


Roopa Rannorey

Roopa has been in the IT field in Karnataka, India for about three plus years. Her interests include Linux Security and Networking and she has been at them for a while.

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