Apache is the most popular open source web server available for modern Linux servers. It offers flexible configuration allowing for a wide variety of uses, from serving basic HTML sites, to complex PHP/Passenger applications, to proxying requests as a reverse proxy gateway. Given its popularity and easy of use, it’s essential to install and maintain a secure environment for Apache installations.
Keep Apache Updated
Apache has a good security track record, and security bugs are seldom found within the web server itself. Still, it’s important to keep Apache updated in order to take advantage of the latest security, stability and features available. Generally, this is simply a matter of keeping the Apache package provided by the distro’s OS updated (e.g. via apt, yum, etc). It’s also recommended that Apache server operators following the Apache Server Announcements mailing list to stay abreast of the latest news from the Apache development team.
Computers are only as smart as the people using them. Apache is built to be stable and secure, but it will only be a secure as the user who configures it. Once Apache is built and installed, it’s important to configure the server to be as minimal as possible.
Run as an Unprivileged User
In security, the principle of least privilege states that an entity should be given no more permission than necessary to accomplish its goals within a given system. In the context of our web server, this means locking down Apache to run only with the permissions necessary to run. A first step in this process is to configure Apache to run as an unprivileged system user (e.g., not root). This is done via the User and Group directives in the Apache configuration file:
# configure a non-privileged user. this user must exist on your system User apache; Group apache;
Apache servers distributed as a common OS package may also use a user and group name such as www-data or nobody. Regardless of the choice of user’s name, make sure that the user/group selection has as few rights as necessary to run properly.
Disable Server Tokens
The HTTP spec recommends (but not requires) that web servers identify themselves via the Server header. Historically, web servers have included their version information as part of this header. Disclosing the version of Apache running can be undesirable, particularly in environments sensitive to information disclosure. Configure Apache not to display its version in Server header:
Disable .htaccess Files
.htaccess files are a powerful feature that allow Apache to have its configuration extended outside its main config file. While this may be convenient, it does present a security risk, as Apache will read any .htaccess file in its path- even ones placed by an attacker that could compromise the server. It may be desirable to lock down configuration by disabling .htaccess files entirely, via the AllowOverride directive:
Additionally, fine-grained control of which Apache directives can be used in .htaccess files can also be controlled by AllowOverride:
AllowOverride AuthConfig Indexes
In the example above, all directives that are neither in the group AuthConfig nor Indexes cause an internal server error.
Restrict Access by IP
Sensitive areas of websites, such as admin control panels, should have strict access controls placed on them. Access makes it easy to whitelist IP access to certain locations of your website and deny traffic to all other IP addresses:
<Directory "/wp-admin"> # allow access from one IP and an additional IP range, # and block everything else Order Allow,Deny Allow from 184.108.40.206 Allow from 192.168.0.0/24 </Directory>
In this example, the use of the Order directive instructs Apache to allow requests coming from IP addresses listed in the Allow blocks, and to deny all other traffic.
Restrict Access by Password
Access to certain locations can also be set via password-based credentials, using the htpasswd utility. First, create a file called .htpasswd to store the desired user/password combinations:
[root@server]# htpasswd -c /path/to/.htpasswd user1
The htpasswd command creates the file /path/to/.htpasswd if it doesn’t exist, and prompt for a password. To add another user, simply run the command, leaving out the -c argument:
[root@server]# htpasswd /path/to/.htpasswd user2
Once you’ve created the user, configure Apache to read the password file and control access to the desired directory:
<Directory "/wp-admin"> AuthType Basic AuthName "Restricted Content" AuthUserFile /path/to/.htpasswd Require valid-user </Directory>
Preventing DoS Attacks
The default model in which Apache processes requests, called prefork mode, is subject to an attack known as a Slowloris attack. A Slowloris attack is a form of DoS (Denial of Service) attack in which the Apache server is forced to wait on requests from malicious clients taking a long time to send traffic, thus forcing legitimate requests to time out or be ignored entirely. Thankfully, modern Apache servers are capable of mitigating this threat with a few additional configuration directives.
mod_reqtimeout is an Apache module designed to shut down connections from clients taking too long to send their request, such as is seen in a Slowloris attack. This module provides a directive that allows Apache to close the connection if it senses that the client is not sending data quickly enough. For example:
RequestReadTimeout header=10-20,MinRate=500 body=20,MinRate=500
In this example, Apache will close the connection if the client takes more than 10 seconds to send its HTTP headers, or if the client takes more than 20 seconds to send headers at a rate of 500 bytes per second. Apache will also close the connection if the client takes more than 20 seconds to send its request body, but will allow the request to continue as long as the client sends more than 500 bytes per second. This configuration allows clients will poor TCP connection quality (such as remote clients with high latency, or those on low-grade cellular or satellite networks) to send requests, while still protecting against known fingerprints of the Slowloris attack. RequestReadTimeout configurations can be complex; more information about this directive can be found at the module documentation page.
Apache excels at serving SSL/TLS traffic. Configuring a web server to provide secure SSL/TLS configurations for clients is essential to maintaining a secure connection.
As a note, it’s strongly recommended that encrypted traffic use only newer TLS protocols, instead of legacy SSL. Both versions of SSL widely available today (SSLv2 and SSLv3) have severe security flaws, and should never be used in productions environments. Historically, the configurations associated with SSL/TLS configuration in Apache are prefixed with SSL; to promote the use of modern security protocols, in this tutorial the term ‘TLS’ is used when referencing encrypted (HTTPS) traffic, and ‘ssl’ when applicable to specific Apache configuration directives.
Turn TLS On
In order to serve encrypted traffic, SSL/TLS needs to be enabled in Apache. Enable secure communications with the SSLEngine directive:
<VirtualHost 192.168.1.1:443> SSLEngine on SSLCertificateFile /path/to/cert SSLCertificateKeyFile /path/to/key </VirtualHost>
Enable Strong TLS Ciphers
By default, Apache allows for a wide variety of cryptographic ciphers to be used in TLS connections. Some of these ciphers are legacy offerings that are weak or prone to attack, and shouldn’t be used. Dreamhost recommends using the Modern or Intermediate cipher suites outlined by Mozilla (the modern list of ciphers is stronger, but will cause connectivity problems for older platforms like Internet Explorer or Windows XP). Additionally, it’s recommended that the server prefer which cipher to be used:
SSLCipherSuite 'ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-SHA256:ECDHE-RSA-AES128-SHA256:ECDHE-ECDSA-AES128-SHA:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA:ECDHE-ECDSA-AES256-SHA384:ECDHE-ECDSA-AES256-SHA:ECDHE-RSA-AES256-SHA:DHE-RSA-AES128-SHA256:DHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA256:DHE-RSA-AES256-SHA:ECDHE-ECDSA-DES-CBC3-SHA:ECDHE-RSA-DES-CBC3-SHA:EDH-RSA-DES-CBC3-SHA:AES128-GCM-SHA256:AES256-GCM-SHA384:AES128-SHA256:AES256-SHA256:AES128-SHA:AES256-SHA:DES-CBC3-SHA:!DSS'; SSLHonorCipherOrder on SSLProtocol all -SSLv2 -SSLv3
Enable TLS Session Caching
Opening a new TLS connection to a server is very expensive as a result of the cryptographic protocols involved. To maintain a high-performance environment, cache existing TLS connections so that each new request from a client/browser does not need to perform the full TLS handshake:
SSLSessionCache shm:/path/to/session_cache(512000) SSLSessionCacheTimeout 300
Use Custom Diffie-Hellman Parameters
The Logjam attack, published in 2015, showed that it was possible for attackers (such as nation-state actors) to break the Diffie-Hellman key exchange, used to implement forward secrecy (essentially, another layer on top of existing encrypted messages). Mitigating this attack is possible in Apache by computing a unique set of Diffie-Hellman parameters and configuring Apache to use this value:
# build a 2048-bit DH prime [user@server]$ openssl dhparam 2048 > /path/to/dhparam
From here, add the params to the end of the file noted in the SSLCertificateFile directive:
[root@server]# cat /path/to/custom/dhparam >> /path/to/sslcertfile
For more information on the Logjam attack, see https://weakdh.org/
Force All Connections over TLS
Encrypted communications are only useful when actually in use. Apache can tell browsers to only use TLS connections for your site. This is accomplished with the Strict-Transport-Security header:
Header always set Strict-Transport-Security max-age=15768000;
For all plaintext connections, configure Apache to send a 301 redirect for requests to the TLS version of the site:
<VirtualHost 192.168.1.1:80> [...] ServerName example.com Redirect permanent / https://example.com/ </VirtualHost>
Additional Security Measures
Beyond the basics of installing a secure Apache binary, locking down access to sensitive areas of your site, and properly serving TLS connections, there are some additional steps that the extra security-conscious user can take:
Install a WAF
A WAF (web application firewall) is a piece of software designed to inspect HTTP/HTTPS traffic, deny malicious requests, and generally act as an additional layer of security in an HTTP web stack. A properly configured WAF can protect your site from SQLi, XSS, CSRF, and DDoS attacks, as well as provide brute force attack mitigation and zero-day threat patching. The most popular and stable WAF for Apache is ModSecurity; see the project’s GitHub page for more detail on installation and configuration.
Automated Log Analysis + Monitoring
Programs like Fail2Ban can be used to monitor Apache access and error logs, searching for attack patterns and taking actions against the attacking client (such as dropping IP addresses, reporting malicious behavior to the IP’s owner, etc). Fail2Ban is extensible, allowing for the creation of unique search patterns and response behaviors.
Limit Input Traffic via IPTables
Beyond securing Apache itself, it’s important to secure the host environment used to host the web server. Locking down access to things like SSH can greatly increase the security of the host by preventing intrusion attempts. A common approach is to whitelist known IPs that will access the host via SSH, and deny all other port 22 traffic, or to use a jump box that strictly filters shell access.