This tutorial is going to show you how to install and use ModSecurity with Nginx 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.
Note: This tutorial works on all current Debian and Ubuntu versions, including Debian 10, Ubuntu 18.04, Ubuntu 20.04 and Ubuntu 20.10.
Web applications are inherently insecure. If you are a WordPress admin, you will 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.
Table of Contents
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.
Step 1: Install the Latest Version of Nginx on Debian/Ubuntu
ModSecurity integrates with Nginx as a dynamic module, which allows you to compile source code of individual modules without compiling Nginx itself.
The Nginx binary needs to be compiled with the –with-compat argument, which will make dynamic modules binary-compatible with your existing Nginx binary. However, not every Nginx binary shipped from the default Debian/Ubuntu repository is compiled with the –with-compat argument. To make things easier, we can install the latest version of Nginx from the ondrej/nginx-mainline PPA, which is maintained by a Debian developer.
Note: The nginx.org repository also provides the latest version of Nginx. However, the ondrej/nginx-mainline PPA provides extra dynamic modules that you might find useful, such as the Brotli module.
Ubuntu
If you use Ubuntu 16.04, 18.04, 20.04, or 20.10, run the following commands to install the latest version of Nginx.
sudo add-apt-repository ppa:ondrej/nginx-mainline -y sudo apt update sudo apt install nginx-core nginx-common nginx nginx-full
During the installation process, it may tell you that the package distributor has shipped an updated version of the main configuration file. It’s recommended to press n to keep your current version. You can always examine the difference later.
By default, only the binary repository is enabled. We also need to enable the source code repository in order to download Nginx source code. Edit the Nginx mainline repository file.
sudo nano /etc/apt/sources.list.d/ondrej-ubuntu-nginx-mainline-*.list
Find the line that begins with # deb-src.
# deb-src http://ppa.launchpad.net/ondrej/nginx-mainline/ubuntu/ focal main
Remove the # character to enable this source code repository.
deb-src http://ppa.launchpad.net/ondrej/nginx-mainline/ubuntu/ focal main
Save and close the file. Then update repository index.
sudo apt update
Debian
If you use Debian 10 or Debian 11, run the following commands to install the latest version of Nginx.
curl -sSL https://packages.sury.org/nginx-mainline/README.txt | sudo bash -x sudo apt update sudo apt install nginx-core nginx-common nginx nginx-full
During the installation process, it may tell you that the package distributor has shipped an updated version of the main configuration file. It’s recommended to press n to keep your current version. You can always examine the difference later.
By default, only the binary repository is enabled. We also need to enable the source code repository in order to download Nginx source code. Edit the Nginx mainline repository file.
sudo nano /etc/apt/sources.list.d/nginx-mainline.list
You should find a single line that enables the Nginx binary repository. Add the following line to enable the Nginx source code repository on Debian 10.
deb-src https://packages.sury.org/nginx-mainline/ buster main
If you use Debian 11, add the following line instead.
deb-src https://packages.sury.org/nginx-mainline/ bullseye main
Save and close the file. Then update repository index.
sudo apt update
Checking Nginx Configure Arguments
Now check the configure arguments of Nginx with the following command:
sudo nginx -V
All Nginx binaries in the PPA are compiled with the –with-compat argument.
Step 2: Download Nginx Source Package
Although we don’t need to compile Nginx itself, we need the Nginx source code package in order to compile the ModSecurity dynamic module. Run the following command to make a nginx directory under /usr/local/src/ to store the Nginx source code package. Replace username with your real username.
sudo chown username:username /usr/local/src/ -R mkdir -p /usr/local/src/nginx
cd into the Nginx source directory.
cd /usr/local/src/nginx/
Download Nginx source package with the commands below:
sudo apt install dpkg-dev apt source nginx
If you see the following warning message, you can safely ignore it.
W: Download is performed unsandboxed as root as file 'nginx_1.19.5.orig.tar.gz' couldn't be accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)
Check out the downloaded source code files.
ls
Sample output:
nginx-1.19.5 nginx_1.19.5-3+ubuntu20.04.1+deb.sury.org+1.debian.tar.xz nginx_1.19.5-3+ubuntu20.04.1+deb.sury.org+1.dsc nginx_1.19.5.orig.tar.gz
Make sure the source code version is the same as your Nginx binary version (sudo nginx -v).
Step 3: Install libmodsecurity3
libmodsecurrity is the ModSecurity library that actually does the HTTP filtering for your web applications. On Debian 10 and Ubuntu 20.04, 20.10, you can install it with sudo apt install libmodsecurity3, but I recommend you compile it from the source.
To compile libmodsecurity, first clone the source code from Github.
sudo apt install git git clone --depth 1 -b v3/master --single-branch https://github.com/SpiderLabs/ModSecurity /usr/local/src/ModSecurity/ cd /usr/local/src/ModSecurity/
Install build dependencies.
sudo apt install gcc make build-essential autoconf automake libtool libcurl4-openssl-dev liblua5.3-dev libfuzzy-dev ssdeep gettext pkg-config libpcre3 libpcre3-dev libxml2 libxml2-dev libcurl4 libgeoip-dev libyajl-dev doxygen
Install required submodules.
git submodule init git submodule update
Configure the build environment.
./build.sh ./configure
If you see the following error, you can ignore it.
fatal: No names found, cannot describe anything.
Compile the source code with the following command. By default, make will use only one CPU core. I have 4 CPU cores on my server, so I specify 4 jobs (-j4) for make to speed up the compilation process. Change 4 to your own number of CPU cores.
make -j4
After the make command finished without errors, install the binary.
sudo make install
It will be installed under /usr/local/modsecurity/ directory.
Troubleshooting Compile Errors
error #1
If you see the following error when running the make command, it’s probably because your server is out of RAM.
g++: internal compiler error: Killed (program cc1plus)
The following lines in /var/log/syslog file indicates the server is out of memory, so consider upgrading the server specs.
Alternatively, you can create swap space to solve out-of-memory problem. However, it should be only used as a temporary measure. Using swap space on SSD servers can be detrimental to system performance.
error #2
If you see the following error when compiling ModSecurity,
utils/geo_lookup.cc: In member function ‘bool modsecurity::Utils::GeoLookup::lookup(const string&, modsecurity::Transaction*, std::function<bool(int, const std::__cxx11::basic_string<char>&)>) const’: utils/geo_lookup.cc:124:32: error: invalid conversion from ‘const MMDB_s*’ to ‘MMDB_s*’ [-fpermissive] r = MMDB_lookup_string(&mmdb, target.c_str(), &gai_error, &mmdb_error);
It’s probably because you installed an outdated version of libmaxminddb-dev. You can remove this package.
sudo apt remove libmaxminddb-dev
And install libgeoip-dev, which is an alternative to libmaxminddb-dev.
sudo apt install libgeoip-dev
Then re-compile ModSecurity.
make clean ./configure make -j4
Install the binary.
sudo make install
Step 4: Download and Compile ModSecurity v3 Nginx Connector Source Code
The ModSecurity Nginx Connector links libmodsecurity to the Nginx web server. Clone the ModSecurity v3 Nginx Connector Git repository.
git clone --depth 1 https://github.com/SpiderLabs/ModSecurity-nginx.git /usr/local/src/ModSecurity-nginx/
Make sure you are in the Nginx source directory.
cd /usr/local/src/nginx/nginx-1.19.5/
Install build dependencies for Nginx.
sudo apt build-dep nginx sudo apt install uuid-dev
Next, configure the environment with the following command. We will not compile Nginx itself, but compile the ModSecurity Nginx Connector module only. The –with-compat flag will make the module binary-compatible with your existing Nginx binary.
./configure --with-compat --add-dynamic-module=/usr/local/src/ModSecurity-nginx
Build the ModSecurity Nginx Connector module.
make modules
The module will be save as objs/ngx_http_modsecurity_module.so. Copy it to the /usr/share/nginx/modules/ directory.
sudo cp objs/ngx_http_modsecurity_module.so /usr/share/nginx/modules/
Step 5: Load the ModSecurity v3 Nginx Connector Module
Edit the main Nginx configuration file.
sudo nano /etc/nginx/nginx.conf
Add the following line at the beginning of the file.
load_module modules/ngx_http_modsecurity_module.so;
Also, add the following two lines in the http {…} section, so ModSecurity will be enabled for all Nginx virtual hosts.
modsecurity on; modsecurity_rules_file /etc/nginx/modsec/main.conf;
Save and close the file. Next, create the /etc/nginx/modsec/ directory to store ModSecurity configuration
sudo mkdir /etc/nginx/modsec/
Then copy the ModSecurity configuration file.
sudo cp /usr/local/src/ModSecurity/modsecurity.conf-recommended /etc/nginx/modsec/modsecurity.conf
Edit the file.
sudo nano /etc/nginx/modsec/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 224), which tells ModSecurity what information should be included in the audit log.
SecAuditLogParts ABIJDEFHZ
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
If you have a coding website, you might want to disable response body inspection, otherwise, you might get 403 forbidden errors just by loading a web page with lots of code content.
SecResponseBodyAccess Off
Save and close the file. Next, create the /etc/nginx/modsec/main.conf file.
sudo nano /etc/nginx/modsec/main.conf
Add the following line to include the /etc/nginx/modsec/modsecurity.conf file.
Include /etc/nginx/modsec/modsecurity.conf
Save and close the file. We also need to copy the Unicode mapping file.
sudo cp /usr/local/src/ModSecurity/unicode.mapping /etc/nginx/modsec/
Then test Nginx configuration.
sudo nginx -t
If the test is successful, restart Nginx.
sudo systemctl restart nginx
Step 6: Enable OWASP Core Rule Set
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.
Download the latest OWASP CRS from GitHub.
wget https://github.com/coreruleset/coreruleset/archive/v3.3.0.tar.gz
Extract the file.
tar xvf v3.3.0.tar.gz
Move the directory to /etc/nginx/modsec/.
sudo mv coreruleset-3.3.0/ /etc/nginx/modsec/
Rename the crs-setup.conf.example file.
sudo mv /etc/nginx/modsec/coreruleset-3.3.0/crs-setup.conf.example /etc/nginx/modsec/coreruleset-3.3.0/crs-setup.conf
Then edit the main configuration file.
sudo nano /etc/nginx/modsec/main.conf
Add the following two lines, which will make Nginx include the CRS config file and individual rules.
Include /etc/nginx/modsec/coreruleset-3.3.0/crs-setup.conf Include /etc/nginx/modsec/coreruleset-3.3.0/rules/*.conf
Save and close the file. Then test Nginx configuration.
sudo nginx -t
If the test is successful, restart Nginx.
sudo systemctl restart nginx
Step 7: 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/nginx/modsec/coreruleset-3.3.0/crs-setup.conf
You can see that OWASP CRS can run in two modes:
When running in anomaly scoring mode, there are 4 paranoia levels.
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.
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/nginx/modsec/coreruleset-3.3.0/rules/ directory. Each matching rule will increase the anomaly score.
Step 8: 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 followign URL in your web browser.
https://yourdomain.com/?id=3 or 'a'='a'
If ModSecurity is working properly, your Nginx web server should return a 403 forbidden error message.
Firefox browser may fail to show the 403 error message. You can press Ctrl+Shift+I to open the developer tools window and select the network tab. Then press F5 to reload the web page. You should now see the 403 error code in Firefox.
If ModSecurity runs in DetectionOnly mode, it won’t block this SQL injection attack.
Step 9: 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:
To understand ModSecurity audit logs, you need to know the 5 processing phases in ModSecurity, which are:
They are also two types of logging file.
Events in the log are divided into several sections.
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/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.
If your ModSecurity log is empty, perhaps you need to restart Nginx.
sudo systemctl restart nginx
Step 10: 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/nginx/modsec/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 Nginx configurations.
sudo nginx -t
If the test is successful, reload Nginx for the change to take effect.
sudo systemctl reload nginx
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/nginx/modsec/coreruleset-3.3.0/rules/ directory.
cd /etc/nginx/modsec/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 send 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 Nginx configurations.
sudo nginx -t
If the test is successful, reload Nginx for the change to take effect.
sudo systemctl reload nginx
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/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 tell 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 Nginx configuraion.
sudo nginx -t
If the test is successful, then reload Nginx.
sudo systemctl reload nginx
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 Nginx configurations.
sudo nginx -t
If the test is successful, restart Nginx for the change to take effect.
sudo systemctl restart nginx
Chaining Rules
If your Nginx has multiple virtual hosts, you may want to whitelist your IP address for a specific virtual host. You need to chain two rules like so:
SecRule REMOTE_ADDR "^12\.34\.56\.78" "id:1004,phase:1,allow,ctl:ruleEngine=off,chain" SecRule REQUEST_HEADERS:Host "@streq nextcloud.yourdomain.com" "t:none"
The chain keyword at the end of the first rule indicates that the ruleEngine=off action will only be taken if the condition in the next rule is also true.
(Optional) Integrate ModSecurity with Project Honeypot
Project Honeypot maintains a list of known malicious IP addresses, available free to the public. ModSecurity can integrates with Project Honeypot and block IP addresses on the Project Honeypot list.
Note that using Project Honeypot will make your website slower for new visitors, because your web server will need to send a query to Project Honeypot before it can send a response to the new visitor. However, once the IP reputation data is cached on your web server, the performance impact will be very minimal.
To use Project Honeypot, first create a free account on its website. Then go to your account dashboard and click the get one link to request an access key for the HTTP blacklist.
Next, edit the crs-setup.conf file.
sudo nano /etc/nginx/modsec/coreruleset-3.3.0/crs-setup.conf
Find the following lines.
#SecHttpBlKey XXXXXXXXXXXXXXXXX #SecAction "id:900500,\ # phase:1,\ # nolog,\ # pass,\ # t:none,\ # setvar:tx.block_search_ip=1,\ # setvar:tx.block_suspicious_ip=1,\ # setvar:tx.block_harvester_ip=1,\ # setvar:tx.block_spammer_ip=1"
Remove the beginning # characters to uncomment them, and add your HTTPBL API key obtained from Project Honeypot.
Note that block_search_ip should be set to 0 (disabled), as we don’t want to block search engine crawlers. Save and close the file. Then reload Nginx.
sudo systemctl reload nginx
Now ModSecurity will query Project Honeypot on all HTTP requests. To test if this would work, edit the /etc/nginx/modsec/main.conf file.
sudo nano /etc/nginx/modsec/main.conf
Add the following line at the end of this file. This allows us to pass an IP address in an URL. (Once the test is successful, you can remove this line from the file.)
SecRule ARGS:IP "@rbl dnsbl.httpbl.org" "phase:1,id:171,t:none,deny,nolog,auditlog,msg:'RBL Match for SPAM Source'
Save and close the file. Test Nginx configurations.
sudo nginx -t
Then reload Nginx.
sudo systemctl reload nginx
Go to Project Honeypot website and find a malicious IP address, for example 134.119.218.243. Run the following command to test the HTTP blacklist.
curl -i -s -k -X $'GET' 'https://yourdomain.com/?IP=134.119.218.243'
Your web server should return a 403 forbidden response, because the IP address on Project Honeypot.
How to Use Brotli with ModSecurity in Nginx
Traditionally, web pages are compressed with GZIP for faster loading speed. Developed by Google, Brotli is a new compression algorithm that provides better compression ratio. It’s supported by all major web browsers. To use Brotli in Nginx, first you need to install the Brotli Nginx module from the the ondrej/nginx-mainline PPA.
sudo apt install libnginx-mod-brotli
Then open the main Nginx configuration file.
sudo nano /etc/nginx/nginx.conf
Add the following lines in the http {…} context.
brotli on;
brotli_comp_level 6;
brotli_static on;
brotli_types application/atom+xml application/javascript application/json application/rss+xml
application/vnd.ms-fontobject application/x-font-opentype application/x-font-truetype
application/x-font-ttf application/x-javascript application/xhtml+xml application/xml
font/eot font/opentype font/otf font/truetype image/svg+xml image/vnd.microsoft.icon
image/x-icon image/x-win-bitmap text/css text/javascript text/plain text/xml;
Save and close the file, then test Nginx configurations.
sudo nginx -t
If the test is successful, restart Nginx.
sudo systemctl restart nginx
Now go to the home page your website, open the developer tools in your web browser (In Firefox you can press Ctrl+Alt+I to open it up). Select the Network tab, and press F5 to reload the web page. Click on the main HTML page.
Check the response header on the right sidebar. If the content-encoding is set to br, then you have successfully enabled Brotli compression in Nginx.
If the content-encoding is gzip, then your Nginx web server is still using GZIP.
Upgrading Nginx
ModSecurity integrates with Nginx as a dynamic module, so every time the Nginx binary is upgraded, you need to rebuild the ModSecurity module for Nginx. This will make your application offline for a few minutes.
If a newer version of Nginx is available in the repository, the sudo apt upgrade command will upgrade Nginx. The newer version of Nginx is not going to be compatible with the previously compiled ModSecurity module. If Nginx is upgraded by the sudo apt upgrade command, it will fail to restart as shown in the screenshot below.
And if you run sudo nginx -t command, it tells you that Nginx expects a new version of the ModSecurity module.
My advice is to prevent Nginx from being upgraded when you run sudo apt upgrade command. This can be achieved by the following command:
sudo apt-mark hold nginx
Now if you run sudo apt update;sudo apt upgrade, and the package manager tells you that the nginx package is held back from upgrading, then it means there’s a new nginx version available in the repository.
You should download the new Nginx source package and compile the ModSecurity module again. Move the newly-compiled ModSecurity module to /usr/share/nginx/modules/ directory. Basically that means you need to remove everything under /usr/local/src/ directory (sudo rm /usr/local/src/* -rf ) and go through step 2 and step 4 again.
Then unhold Nginx.
sudo apt-mark unhold nginx
And upgrade Nginx.
sudo apt upgrade nginx
Once the upgrade is complete, hold Nginx again.
sudo apt-mark hold nginx
To show what packages are held, run
apt-mark showhold
Nginx Plus
If you use the commercial Nginx Plus web server, then ModSecurity is included in the Nginx Plus binary. It’s known as the NGINX WAF.
If you don’t want to spend time re-compiling the ModSecurity source code, then you might want to purchase Nginx Plus, as the ModSecurity module is pre-compiled in the Nginx Plus binary. Benefits of Using ModSecurity 3.0 with NGINX Plus:
How to Disable ModSecurity for a Virtual Host
In this tutorial, I added the following line in the http {…} context.
modsecurity on;
This will enable ModSecurity for all Nginx server blocks (aka virtual hosts). If you want to disable ModSecurity for a specific server block, then edit the server block file (/etc/nginx/conf.d/example.com.conf) and add the following line to the server {…} context.
modsecurity off;
Reload Nginx for the change to take effect.
sudo systemctl reload nginx
FAQ
Static Module vs Dynamic Module in Nginx
What does Binary Compatible Mean?
No matter a module is static or dynamic, binary compatible or non binary compatible, if you upgrade the Nginx binary later, you need to compile the module again.
Upgrade Server RAM
ModSecurity can use a fair amount of RAM. If you can see the following error in your Nginx error log (/var/log/nginx/error.log), it means your server is short of RAM.
fork() failed while spawning "worker process" (12: Cannot allocate memory) sendmsg() failed (9: Bad file descriptor) sendmsg() failed (9: Bad file descriptor) sendmsg() failed (9: Bad file descriptor)
You need to restart Nginx and upgrade server RAM, then the above error is not going to happen again.
How to Upgrade OWASP CRS
Besides upgrading the ModSecurity Nginx module, you also need to upgrade the core rule set when a new version comes out. The process is straightforward.
Next, test Nginx configurations.
sudo nginx -t
If the test is successful, reload Nginx for the change to take effect.
sudo systemctl reload nginx
How do you know if the new version is working? Launch a simple SQL injection attack like in step 8 and check your server logs. It will show you the CRS version that’s preventing this attack.
Wrapping Up
I hope this tutorial helped you set up ModSecurity web application firewall with Nginx on Debian/Ubuntu. You may also want to check out other security tutorials.
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