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Creating the LMS environment

Installing ModSecurity

This tutorial is going to show you how to install and use ModSecurity with Apache on Debian/Ubuntu servers. ModSecurity is the most well-known open-source web application firewall (WAF), providing comprehensive protection for your web applications (like

WordPress⁠

Nextcloud⁠

Ghost⁠

etc) against a wide range of Layer 7 (HTTP) attacks, such as SQL injection, cross-site scripting, and local file inclusion.

Web applications are inherently insecure. If you are a WordPress admin, you probably hear news of hackers exploiting vulnerabilities in WordPress plugins and themes every once in a while. It’s essential that you deploy a WAF on your web server, especially when you have old applications that don’t receive updates. ModSecurity is originally created by Ivan Ristić in 2002, currently maintained by Trustwave SpiderLabs. It’s the world’s most widely deployed WAF, used by over a million websites. cPanel, the most widely used hosting control panel, includes ModSecurity as its WAF.

You may have heard other host-based firewalls like iptables,

UFW⁠

, and Firewalld, etc. The difference is that they work on layer 3 and 4 of the OSI model and take actions based on IP address and port number. ModSecurity, or web application firewalls in general, is specialized to focus on HTTP traffic (layer 7 of the OSI model) and takes action based on the content of HTTP request and response.

ModSecurity 3

ModSecurity was originally designed for Apache web server. It could work with Nginx before version 3.0 but suffered from poor performance. ModSecurity 3.0 (aka libmodsecurity) was released in 2017. It’s a milestone release, particularly for Nginx users, as it’s the first version to work natively with Nginx. The caveat of ModSecurity 3 is that it doesn’t yet have all the features as in the previous version (2.9), though each new release will add some of the missing features. Nginx users should use ModSecurity 3. However, if you use Apache, it’s recommended to continue using the 2.9 branch for the time being.

Step 1: Install ModSecurity with Apache on Debian/Ubuntu

The ModSecurity module for Apache is included in the default Debian/Ubuntu repository. To install it, run

sudo apt install libapache2-mod-security2

Then enable this module.

sudo a2enmod security2

Restart Apache for the change to take effect.

sudo systemctl restart apache2

Step 2: Configure ModSecurity

In the /etc/apache2/mods-enabled/security2.conf configuration file, you can find the following line.

IncludeOptional /etc/modsecurity/*.conf

⁠

This means Apache will include all the *.conf files in /etc/modsecurity/ directory. We need to rename the modsecurity.conf-recommended file to make it work.

sudo mv /etc/modsecurity/modsecurity.conf-recommended /etc/modsecurity/modsecurity.conf

Then edit this file with a command-line text editor like Nano.

sudo nano /etc/modsecurity/modsecurity.conf

Find the following line.

SecRuleEngine DetectionOnly

This config tells ModSecurity to log HTTP transactions, but takes no action when an attack is detected. Change it to the following, so ModSecurity will detect and block web attacks.

SecRuleEngine On

Then find the following line (line 186), which tells ModSecurity what information should be included in the audit log.

SecAuditLogParts ABDEFHIJZ

However, the default setting is wrong. You will know why later when I explain how to understand ModSecurity logs. The setting should be changed to the following.

SecAuditLogParts ABCEFHJKZ

Save and close the file. Then restart Apache for the change to take effect. (Reloding the web server isn’t enough.)

sudo systemctl restart apache2

Step 3: Install the OWASP Core Rule Set (CRS)

To make ModSecurity protect your web applications, you need to define rules to detect malicious actors and block them. For beginners, it’s a good idea to install existing rule sets, so you can get started quickly and then learn the nitty-gritty down the road. There are several free rule sets for ModSecurity. The OWASP Core Rule Set (CRS) is the standard rule set used with ModSecurity.

It’s free, community-maintained and the most widely used rule set that provides a sold default configuration for ModSecurity.

It contains rules to help stop common attack vectors, including SQL injection (SQLi), cross-site scripting (XSS), and many others.

It can integrate with Project Honeypot.

It also contains rules to detect bots and scanners.

It has been tuned through wide exposure to have very few false positives.

When installing ModSecurity from the default Debian/Ubuntu repository, the modsecurity-crs package is also installed as a dependency. This package contains the OWASP core rule set version 3.x. However, it can become out of date. If you care about security, you should use the latest version of core rule set.

Download the OWASP CRS from GitHub (Jul 23, 2023)

wget https://github.com/coreruleset/coreruleset/archive/v3.3.5.tar.gz

Extract the file.

tar xvf v3.3.5.tar.gz

Create a directory to store CRS files.

sudo mkdir /etc/apache2/modsecurity-crs/

Move the extracted directory to /etc/apache2/modsecurity-crs/.

sudo mv coreruleset-3.3.5/ /etc/apache2/modsecurity-crs/

Go to that directory.

cd /etc/apache2/modsecurity-crs/coreruleset-3.3.5/

Rename the crs-setup.conf.example file.

sudo mv crs-setup.conf.example crs-setup.conf

Edit the /etc/apache2/mods-enabled/security2.conf file.

sudo nano /etc/apache2/mods-enabled/security2.conf

Find the following line, which loads the default CRS files.

IncludeOptional /usr/share/modsecurity-crs/*.load

Change it to the following, so the latest OWASP CRS will be used.

IncludeOptional /etc/apache2/modsecurity-crs/coreruleset-3.3.5/crs-setup.conf

IncludeOptional /etc/apache2/modsecurity-crs/coreruleset-3.3.5/rules/*.conf

⁠

Save and close the file. Then test Apache configuration.

sudo apache2ctl -t

If the syntax is OK, then restart Apache.

sudo systemctl restart apache2

Step 4: Learn How OWASP CRS Works

Let’s take a look at the CRS config file, which provides you with good documentation on how CRS works.

sudo nano /etc/apache2/modsecurity-crs/coreruleset-3.3.5/crs-setup.conf

You can see that OWASP CRS can run in two modes:

self-contained mode. This is the traditional mode used in CRS v2.x. If an HTTP request matches a rule, ModSecurity will block the HTTP request immediately and stop evaluating remaining rules.

anomaly scoring mode. This is the default mode used in CRS v3.x. ModSecurity will check an HTTP request against all rules, and add a score to each matching rule. If a threshold is reached, then the HTTP request is considered an attack and will be blocked. The default score for inbound requests is 5 and for outbound response is 4.

⁠

When running in anomaly scoring mode, there are 4 paranoia levels.

Paranoia level 1 (default)

Paranoia level 2

Paranoia level 3

Paranoia level 4

With each paranoia level increase, the CRS enables additional rules giving you a higher level of security. However, higher paranoia levels also increase the possibility of blocking some legitimate traffic due to false alarms.

⁠

modsecurity-Paranoia-Level-Initialization-debian-ubuntu.jpg

⁠

These are the two basic concepts you need to understand before using the CRS. Now we can close the file. The individual CRS rules are stored in /etc/apache2/modsecurity-crs/coreruleset-3.3.0/rules/ directory. Each matching rule will increase the anomaly score.

Step 5: Testing

To check if ModSecurity is working, you can launch a simple SQL injection attack on your own website. (Please note that it’s illegal to do security testing on other people’s websites without authorization.) Enter the following URL in your web browser.

https://yourdomain.com/?id=3 or 'a'='a'

If ModSecurity is working properly, your Apache web server should return a 403 forbidden error message.

⁠

And in the audit log (/var/log/apache2/modsec_audit.log), you can see the following line in section H, which means ModSecurity detected and blocked this SQL injection attack by using OWASP CRS v3.3.0.

Action: Intercepted (phase 2)

⁠

When ModSecurity runs in DetectionOnly mode, it won’t block this SQL injection attack.

Step 6: Understanding the ModSecurity Logs

It’s important to analyze the ModSecurity logs, so you will know what kind of attacks are directed to your web applications and take better actions to defend against threat actors. There are mainly two kinds of logs in ModSecurity:

debug log: disabled by default.

audit log: /var/log/apache2/modsec_audit.log

To understand ModSecurity audit logs, you need to know the 5 processing phases in ModSecurity, which are:

Phase 1: Inspect request headers

Phase 2: Inspect request body

Phase 3: Inspect response headers

Phase 4: Inspect response body

Phase 5: Action (logging/blocking malicious requests)

They are also two types of logging file.

Serial: one file for all logs. This is the default.

Concurrent: multiple files for logging. This can provide better write performance. If you can notice your web pages slowing down after enabling ModSecurity, you can choose to use the concurrent logging type.

Events in the log are divided into several sections.

section A: audit log header

section B: request header

section C: request body

section D: reserved

section E: intermediary response body

section F: final response headers

section G: reserved

section H: audit log trailer

section I: compact request body alternative, which excludes files

section J: information on uploaded files

section K: every rule matched by an event, in order of match

section Z: final boundary

If you run a high traffic website, the ModSecurity audit log can get too large very quickly, so we need to configure log rotation for the ModSecurity audit log. Create a logrotate configuration file for ModSecurity.

sudo nano /etc/logrotate.d/modsecurity

Add the following lines to this file.

/var/log/apache2/modsec_audit.log

{

rotate 14

daily

missingok

compress

delaycompress

notifempty

}

This will rotate the log file every day (daily), compressing old versions (compress). The previous 14 log files will be kept (rotate 14), and no rotation will occur if the file is empty (notifempty). Save and close the file.

Step 7: Handling False Positives

ModSecurity is a generic web application firewall and not designed for a specific web application. The OWASP core rule set is also a generic rule set with no particular application in mind, so it’s likely that you will see false positives after enabling ModSecurity and OWASP CRS. If you increase the paranoia level in the CRS, there will be more false positives.

For example, by default, the CRS forbids Unix command injection like entering sudo on a web page, which is rather common on my blog. To eliminate false positives, you need to add rule exclusions to the CRS.

Application-Specific Rule Exclusions

There are some prebuilt, application-specific exclusions shipped with OWASP CRS. Edit the crs-setup.conf file.

sudo nano /etc/apache2/modsecurity-crs/coreruleset-3.3.0/crs-setup.conf

Go to the Application Specific Rule Exclusions section and find the following lines.

#SecAction \

# "id:900130,\

# phase:1,\

# nolog,\

# pass,\

# t:none,\

# setvar:tx.crs_exclusions_cpanel=1,\

# setvar:tx.crs_exclusions_drupal=1,\

# setvar:tx.crs_exclusions_dokuwiki=1,\

# setvar:tx.crs_exclusions_nextcloud=1,\

# setvar:tx.crs_exclusions_wordpress=1,\

# setvar:tx.crs_exclusions_xenforo=1"

For instance, If I want to enable WordPress exclusions, the above lines should be changed to the following. Please be careful about the syntax. There should be no comments between t:none,\ and setvar:tx.crs_exclusions_wordpress=1". (The backslash \ character at the end indicates the next line is a continuation of the current line.)

SecAction \

"id:900130,\

phase:1,\

nolog,\

pass,\

t:none,\

setvar:tx.crs_exclusions_wordpress=1"

# setvar:tx.crs_exclusions_cpanel=1,\

# setvar:tx.crs_exclusions_drupal=1,\

# setvar:tx.crs_exclusions_dokuwiki=1,\

# setvar:tx.crs_exclusions_nextcloud=1,\

# setvar:tx.crs_exclusions_xenforo=1"

Save and close the file. Then test Apache configurations.

sudo apache2ctl -t

If the test is successful, restart Apache for the change to take effect.

sudo systemctl restart apache2

Note that if you have multiple applications such as (

WordPress⁠

Nextcloud⁠

Drupal⁠

, etc) installed on the same server, then the above rule exclusions will be applied to all applications. To minimize the security risks, you should enable a rule exclusion for one application only. To do that, go to the /etc/apache2/modsecurity-crs/coreruleset-3.3.0/rules/ directory.

cd /etc/apache2/modsecurity-crs/coreruleset-3.3.0/rules/

Rename the REQUEST-900-EXCLUSION-RULES-BEFORE-CRS file.

sudo mv REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf.example REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf

Then edit this file.

sudo nano REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf

Add the following line at the bottom of this file. If your WordPress is using the blog.yourdomain.com sub-domain and the request header sent from visitor’s browser contains this sub-domain, then ModSecurity will apply the rule exclusions for WordPress.

SecRule REQUEST_HEADERS:Host "@streq blog.yourdomain.com" "id:1000,phase:1,setvar:tx.crs_exclusions_wordpress=1"

If you have installed Nextcloud on the same server, then you can also add the following line in this file, so if a visitor is accessing your Nextcloud sub-domain, ModSecurity will apply the Nextcloud rule exclusions.

SecRule REQUEST_HEADERS:Host "@streq nextcloud.yourdomain.com" "id:1001,phase:1,setvar:tx.crs_exclusions_nextcloud=1"

Save and close this file. Then test Apache configurations.

sudo apache2ctl -t

If the test is successful, rstart Apache for the change to take effect.

sudo systemctl restart apache2

Custom Rule Exclusions

Enabling the prebuilt application-specific rule exclusions might not eliminate all false positives. If so, you need to examine the ModSecurity audit log (/var/log/apache2/modsec_audit.log), check which rule caused the false positive and add your custom rule exclusions in the REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf file.

Section H in the audit log tells you which rule is matched. For example, If I try to use the <code>...</code> HTML in the comment form, ModSecurity blocks my comment. The following log tells me that the HTTP request matched a rule in REQUEST-941-APPLICATION-ATTACK-XSS.conf (line 527). The rule ID is 941310. The request URI is /wp-comments-post.php.

⁠

It’s detected as malformed encoding XSS filter attack. However, I want users to be able to use the <code>...</code> and <pre>...</pre> HTML tag in the comment form, so I created a rule exclusion. Add the following line at the bottom of the REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf file.

SecRule REQUEST_URI "@streq /wp-comments-post.php" "id:1002,phase:1,ctl:ruleRemoveById=941310"

This line tells ModSecurity that if the request URI is /wp-comments-post.php, then don’t apply rule ID 941310. Save and close the file. Then test Apache configurations.

sudo apache2ctl -t

If the test is successful, restart Apache for the change to take effect.

sudo systemctl restart apache2

If a false positive matches multiple rule IDs, you can add rule exclusions in one line like so:

SecRule REQUEST_URI "@streq /wp-admin/post.php" "id:1003,phase:1,ctl:ruleRemoveById=941160,ctl:ruleRemoveById=941310,ctl:ruleRemoveById=942100"

Note: It’s not recommended to disable too many rules of level 1 in the OWASP CRS, as it will make your website be hacked much more easily. Only disable rules if you know what you are doing.

IP Whitelisting

If you want to disable ModSecurity for your own IP address, but leave it enabled for all other IP addresses, then add the following custom rule in the REQUEST-900-EXCLUSION-RULES-BEFORE-CRS.conf file. Replace 12.34.56.78 with your real IP address.

SecRule REMOTE_ADDR "^12\.34\.56\.78" "id:1004,phase:1,allow,ctl:ruleEngine=off"

To whitelist a subnet, use the following syntax, which will whitelist the 10.10.10.0/24 network.

SecRule REMOTE_ADDR "^10\.10\.10.*" "id:1005,phase:1,allow,ctl:ruleEngine=off"

Save and close the file. Then test Apache configurations.

sudo apache2ctl -t

If the test is successful, restart Apache for the change to take effect.

sudo systemctl restart apache2

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