Category: Linux

A web application firewall (WAF) filters HTTP traffic. By integrating this in your web server, you can make sure potentially dangerous requests are blocked before they arrive to your web application or sensitive data leaks out of your web server. This way you add an extra defensive layer potentially offering extra protection against zero-day vulnerabilities in your web server or web applications. In this blog post, I give a tutorial how to install and configure ModSecurity web application firewall and the Core Rule Set on Debian. With some minor adaptions you can also use this guide for setting up ModSecurity on Ubuntu or other distributions.

ModSecurity is the most well-known open source web application firewall. The future of ModSecurity does not look too bright but fortunately with Coraza WAF an alternative which is completely compatible with ModSecurity is in development. At this moment Coraza only integrates with the Caddy web server, and does not have a connector for Apache or NGinx so for that reason it is currently not yet usable as a replacement for ModSecurity.

While ModSecurity provides the framework for filtering HTTP traffic, you also need rules which define what to bloc and that’s where the Core Rule Set (CRS) comes in. CRS is a set of generic rules winch offer protection to a various range of common attacks via HTTP, such as SQL injection, code injection and cross-site scripting (XSS) attacks.

Install ModSecurity and the Core Rule Set on Debian

I install the Apache module for ModSecurity, the geoip-database, which can be used for blocking all requests from certain countries, and modsecurity-crs, which contains the Core Rule Set. I take this package from testing, because it has a newer version (version 3.3.2 at the time of writing). There is no risk in taking this package from testing, because it only contains the rules and does not depend on any other packages from testing/unstable. If you prefer faster updates, you can also use unstable.

# apt install libapache2-mod-security2 geoip-database
# apt install -t testing modsecurity-crs

Configuring ModSecurity

In order to load the ModSecurity module in Apache, run this command:

# a2enmod security2

Then copy the example ModSecurity configuration file to /etc/modsecurity/modsecurity.conf:

cp /etc/modsecurity/modsecurity.conf-recommended /etc/modsecurity/modsecurity.conf

Now edit /etc/modsecurity/modsecurity.conf. I highlight some of the options:

SecRuleEngine on
SecRequestBodyLimit 536870912
SecRequestBodyNoFilesLimit 131072
SecAuditLog /var/log/apache2/modsec_audit.log
#SecRule MULTIPART_UNMATCHED_BOUNDARY "!@eq 0" \
#"id:'200004',phase:2,t:none,log,deny,msg:'Multipart parser detected a possible unmatched boundary.'"
SecPcreMatchLimit 500000
SecPcreMatchLimitRecursion 500000
SecStatusEngine Off

The SecRuleEngine option controls whether rules should be processed. If set to Off, you completely disable all rules, with On you enable them and it will block malicious actions. If set to DetectionOnly, ModSecurity will only log potential malicious activity flagged by your rules, but will not block them. DetectionOnly can be useful for temporary trying out the rules in order to find false positives before you really start blocking potential malicious activity.

The SecAuditLog option defines a file which contains audit logs. This file will contain detailed logs about every request triggering a ModSecurity rule.

The SecPcreMatchLimit and SecPcreMatchLimitRecursion set the match limit and match limit recursion for the regular expression library PCRE. Setting this high enough will prevent errors that the PCRE limits were exceeded while analyzing data, but setting it too high can make ModSecurity vulnerable to a Denial of Service (DoS) attack. A Core Rule Set developer recommends a value of 50000 so that’s what I use here.

I change SecRequestBodyLimit to a higher value to allow large file uploads.

I disable the rule 200004 because it is known to cause false positives.

Set SecStatusEngine to Off to prevent ModSecurity sending version information back its developers.

After changing any configuration related to ModSecurity or the Core Rule Set, reload your Apache web server:

# systemctl reload apache2

Configuring the Core Rule Set

The Core Rule Set can be configured via the file /etc/modsecurity/crs/crs-setup.conf.

Anomaly Scoring

By default the Core Rule Set is using anomaly scoring mode. This means that individual rules add to a so called anomaly score, which at the end is evaluated. If the anomaly score exceeds a certain threshold, then the traffic is blocked. You can read more about this configuration in crs-setup.conf but the default configuration should be fine for most people.

Setting the paranoia level

The paranoia level is a number from 1 to 4 which determines which rules are active and contribute to the anomaly scoring. The higher the paranoia level, the more rules are activated and hence the more aggressive the Core Rule Set is, offering more protection but potentially also causing more false positives. By default the paranoia level is set to 1. If you work with sensitive data, it is recommended to increase the paranoia level.

The executing paranoia level defines the rules which will be executed but their score will not be added to the anomaly scoring. When HTTP traffic hits rules of the executing paranoia level, this traffic will only be logged but not be blocked. It is a especially useful to prepare for increasing the paranoia level and finding false positives on this higher level, without causing any disruption for your users.

To set the paranoia level to 1 and the executing paranoia level to 2, make sure you have these rules set in crs-setup.conf:

SecAction \
  "id:900000,\
   phase:1,\
   nolog,\
   pass,\
   t:none,\
   setvar:tx.paranoia_level=1"
SecAction \
  "id:900001,\
   phase:1,\
   nolog,\
   pass,\
   t:none,\
   setvar:tx.executing_paranoia_level=2"

Once you have fixed all false positives, you can raise the paranoia level to 2 to increase security.

Defining the allowed HTTP methods

By default the Core Rule Set only allows the GET, HEAD, POST and OPTIONS HTTP methods. For many standard sites this will be enough but if your web applications also use restful APIs or WebDAV, then you will need to add the required methods. Change rule 900200, and add the HTTP methods mentioned in the comments in crs-setup.conf.

SecAction \
 "id:900200,\
  phase:1,\
  nolog,\
  pass,\
  t:none,\
  setvar:'tx.allowed_methods=GET HEAD POST OPTIONS'"

Disallowing old HTTP versions

There is a rule which determines which HTTP versions you allow in HTTP requests. I uncomment it and modify it to only allow HTTP versions 1.1 and 2.0. Legitimate browsers and bots always use one of these modern HTTP versions and older versions usually are a sign of malicious activity.

SecAction \
 "id:900230,\
  phase:1,\
  nolog,\
  pass,\
  t:none,\
  setvar:'tx.allowed_http_versions=HTTP/1.1 HTTP/2 HTTP/2.0'"

Blocking specific countries

Personally I’m not a fan of completely blocking all traffic from a whole country, because you will also block legitimate visitors to your site, but in case you want to this, you can configure this in crs-setup.conf:

SecGeoLookupDB /usr/share/GeoIP/GeoIP.dat
SecAction \
 "id:900600,\
  phase:1,\
  nolog,\
  pass,\
  t:none,\
  setvar:'tx.high_risk_country_codes='"

Add the two-letter country codes you want to block to the last line (before the two quotes), multiple country codes separated by a space.

Make sure you have the package geoip-database installed.

Core Rule Set Exclusion rules for well-known web applications

The Core Rule Set contains some rule exclusions for some well-known web applications like WordPress, Drupal and NextCloud which reduces the number of false positives. I add the following section to crs-setup.conf which will allow me to enable the exclusions in the Apache configuration by setting the WEBAPPID variable in the Apache configuration whenever I need them.

SecRule WEBAPPID '@beginsWith wordpress' 'id:20000,phase:1,nolog,pass,t:none setvar:tx.crs_exclusions_wordpress=1'
SecRule WEBAPPID '@beginsWith drupal' 'id:20001,phase:1,nolog,pass,t:none setvar:tx.crs_exclusions_drupal=1'
SecRule WEBAPPID '@beginsWith dokuwiki' 'id:20002,phase:1,nolog,pass,t:none setvar:tx.crs_exclusions_dokuwiki=1'
SecRule WEBAPPID '@beginsWith nextcloud' 'id:20003,phase:1,nolog,pass,t:none setvar:tx.crs_exclusions_nextcloud=1'
SecRule WEBAPPID '@beginsWith cpanel' 'id:20004,phase:1,nolog,pass,t:none setvar:tx.crs_exclusions_cpanel=1'
SecRule WEBAPPID '@beginsWith xenforo' 'id:20005,phase:1,nolog,pass,t:none setvar:tx.crs_exclusions_xenforo=1'

Adding rules for Log4Shell and Spring4Shell detection

At the end of 2021 a critical vulnerability CVE-2021-44228, named Log4Shell, was detected in Log4j, which allows remote attackers to run code on a server with the vulnerable Log4j version. While the Core Rule Set offered some mitigation of this vulnerability out of the box, this protection was not complete. New improved detection rules against Log4Shell were developed. Because of the severity of this bug and the fact that it’s being exploited in the wild, I strongly recommend adding this protection manually when using ModSecurity version 3.3.2 (or older). Newer, not yet released versions, should have complete protection out of the box.

First modify /etc/apache2/mods-enabled/security2.conf so that it looks like this:

<IfModule security2_module>
        # Default Debian dir for modsecurity's persistent data
        SecDataDir /var/cache/modsecurity

        # Include all the *.conf files in /etc/modsecurity.
        # Keeping your local configuration in that directory
        # will allow for an easy upgrade of THIS file and
        # make your life easier
        IncludeOptional /etc/modsecurity/*.conf

        # Include OWASP ModSecurity CRS rules if installed
        IncludeOptional /usr/share/modsecurity-crs/*.load
        SecRuleUpdateTargetById 932130 "REQUEST_HEADERS"
</IfModule>

Then create the file /etc/modsecurity/99-CVE-2021-44228.conf with this content:

# Generic rule against CVE-2021-44228 (Log4j / Log4Shell)
# See https://coreruleset.org/20211213/crs-and-log4j-log4shell-cve-2021-44228/
SecRule REQUEST_LINE|ARGS|ARGS_NAMES|REQUEST_COOKIES|REQUEST_COOKIES_NAMES|REQUEST_HEADERS|XML://*|XML://@* "@rx (?:\${[^}]{0,4}\${|\${(?:jndi|ctx))" \
    "id:1005,\
    phase:2,\
    block,\
    t:none,t:urlDecodeUni,t:cmdline,\
    log,\
    msg:'Potential Remote Command Execution: Log4j CVE-2021-44228', \
    tag:'application-multi',\
    tag:'language-java',\
    tag:'platform-multi',\
    tag:'attack-rce',\
    tag:'OWASP_CRS',\
    tag:'capec/1000/152/137/6',\
    tag:'PCI/6.5.2',\
    tag:'paranoia-level/1',\
    ver:'OWASP_CRS/3.4.0-dev',\
    severity:'CRITICAL',\
    setvar:'tx.rce_score=+%{tx.critical_anomaly_score}',\
    setvar:'tx.anomaly_score_pl1=+%{tx.critical_anomaly_score}'"

In March 2022 CVE-2022-22963, another remote code execution (RCE) vulnerability was published in the Spring framework was published. The Core Rule Set developed a new rule to protect against this vulnerability which will be included in the next version, but the rule can be added manually if you are running the Core Rule Set version 3.3.2 or older.

To do so, create the file /etc/modsecurity/99-CVE-2022-22963.conf with this content:

# This rule is also triggered by the following exploit(s):
# - https://www.rapid7.com/blog/post/2022/03/30/spring4shell-zero-day-vulnerability-in-spring-framework/
# - https://www.ironcastle.net/possible-new-java-spring-framework-vulnerability-wed-mar-30th/
#
SecRule ARGS|ARGS_NAMES|REQUEST_COOKIES|!REQUEST_COOKIES:/__utm/|REQUEST_COOKIES_NAMES|REQUEST_BODY|REQUEST_HEADERS|XML:/*|XML://@* \
    "@rx (?:class\.module\.classLoader\.resources\.context\.parent\.pipeline|springframework\.context\.support\.FileSystemXmlApplicationContext)" \
    "id:1006,\
    phase:2,\
    block,\
    t:urlDecodeUni,\
    msg:'Remote Command Execution: Malicious class-loading payload',\
    logdata:'Matched Data: %{MATCHED_VAR} found within %{MATCHED_VAR_NAME}',\
    tag:'application-multi',\
    tag:'language-java',\
    tag:'platform-multi',\
    tag:'attack-rce',\
    tag:'OWASP_CRS',\
    tag:'capec/1000/152/248',\
    tag:'PCI/6.5.2',\
    tag:'paranoia-level/2',\
    ver:'OWASP_CRS/3.4.0-dev',\
    severity:'CRITICAL',\
    setvar:'tx.rce_score=+%{tx.critical_anomaly_score}',\
    setvar:'tx.anomaly_score_pl2=+%{tx.critical_anomaly_score}'"

Don’t forget to reload your Apache configuration after adding these rules.

Testing ModSecurity and checking the logs

We can now easily test ModSecurity by doing a request which tries to abuse a cross-site scripting (XSS) vulnerability:

$ curl -I "https://example.org/?search=<script>alert('CRS+Sandbox+Release')</script>"

This should return HTTP response 403 (Forbidden).

Whenever something hits your ModSecurity rules, this will be logged in your Apache error log. The above request has created these messages in the error log:

[Sat Apr 09 22:22:02.716558 2022] [:error] [pid 847584:tid 140613499016960] [client client-ip:49688] [client client-ip] ModSecurity: Warning. detected XSS using libinjection. [file "/usr/share/modsecurity-crs/rules/REQUEST-941-APPLICATION-ATTACK-XSS.conf"] [line "55"] [id "941100"] [msg "XSS Attack Detected via libinjection"] [data "Matched Data: XSS data found within ARGS:search: <script>alert('CRS Sandbox Release')</script>"] [severity "CRITICAL"] [ver "OWASP_CRS/3.3.2"] [tag "application-multi"] [tag "language-multi"] [tag "platform-multi"] [tag "attack-xss"] [tag "paranoia-level/1"] [tag "OWASP_CRS"] [tag "capec/1000/152/242"] [hostname "example.org"] [uri "/"] [unique_id "YlHq6gKxO9SgyEd0xH9N5gADLgA"]
[Sat Apr 09 22:22:02.716969 2022] [:error] [pid 847584:tid 140613499016960] [client client-ip:49688] [client client-ip] ModSecurity: Warning. Pattern match "(?i)<script[^>]*>[\\\\s\\\\S]*?" at ARGS:search. [file "/usr/share/modsecurity-crs/rules/REQUEST-941-APPLICATION-ATTACK-XSS.conf"] [line "82"] [id "941110"] [msg "XSS Filter - Category 1: Script Tag Vector"] [data "Matched Data: <script> found within ARGS:search: <script>alert('CRS Sandbox Release')</script>"] [severity "CRITICAL"] [ver "OWASP_CRS/3.3.2"] [tag "application-multi"] [tag "language-multi"] [tag "platform-multi"] [tag "attack-xss"] [tag "paranoia-level/1"] [tag "OWASP_CRS"] [tag "capec/1000/152/242"] [hostname "example.org"] [uri "/"] [unique_id "YlHq6gKxO9SgyEd0xH9N5gADLgA"]
[Sat Apr 09 22:22:02.717249 2022] [:error] [pid 847584:tid 140613499016960] [client client-ip:49688] [client client-ip] ModSecurity: Warning. Pattern match "(?i:(?:<\\\\w[\\\\s\\\\S]*[\\\\s\\\\/]|['\\"](?:[\\\\s\\\\S]*[\\\\s\\\\/])?)(?:on(?:d(?:e(?:vice(?:(?:orienta|mo)tion|proximity|found|light)|livery(?:success|error)|activate)|r(?:ag(?:e(?:n(?:ter|d)|xit)|(?:gestur|leav)e|start|drop|over)|op)|i(?:s(?:c(?:hargingtimechange ..." at ARGS:search. [file "/usr/share/modsecurity-crs/rules/REQUEST-941-APPLICATION-ATTACK-XSS.conf"] [line "199"] [id "941160"] [msg "NoScript XSS InjectionChecker: HTML Injection"] [data "Matched Data: <script found within ARGS:search: <script>alert('CRS Sandbox Release')</script>"] [severity "CRITICAL"] [ver "OWASP_CRS/3.3.2"] [tag "application-multi"] [tag "language-multi"] [tag "platform-multi"] [tag "attack-xss"] [tag "paranoia-level/1"] [tag "OWASP_CRS"] [tag "capec/1000/152/242"] [hostname "example.org"] [uri "/"] [unique_id "YlHq6gKxO9SgyEd0xH9N5gADLgA"]
[Sat Apr 09 22:22:02.718018 2022] [:error] [pid 847584:tid 140613499016960] [client client-ip:49688] [client client-ip] ModSecurity: Access denied with code 403 (phase 2). Operator GE matched 5 at TX:anomaly_score. [file "/usr/share/modsecurity-crs/rules/REQUEST-949-BLOCKING-EVALUATION.conf"] [line "93"] [id "949110"] [msg "Inbound Anomaly Score Exceeded (Total Score: 15)"] [severity "CRITICAL"] [ver "OWASP_CRS/3.3.2"] [tag "application-multi"] [tag "language-multi"] [tag "platform-multi"] [tag "attack-generic"] [hostname "example.org"] [uri "/"] [unique_id "YlHq6gKxO9SgyEd0xH9N5gADLgA"]
[Sat Apr 09 22:22:02.718596 2022] [:error] [pid 847584:tid 140613499016960] [client client-ip:49688] [client client-ip] ModSecurity: Warning. Operator GE matched 5 at TX:inbound_anomaly_score. [file "/usr/share/modsecurity-crs/rules/RESPONSE-980-CORRELATION.conf"] [line "91"] [id "980130"] [msg "Inbound Anomaly Score Exceeded (Total Inbound Score: 15 - SQLI=0,XSS=15,RFI=0,LFI=0,RCE=0,PHPI=0,HTTP=0,SESS=0): individual paranoia level scores: 15, 0, 0, 0"] [ver "OWASP_CRS/3.3.2"] [tag "event-correlation"] [hostname "example.org"] [uri "/"] [unique_id "YlHq6gKxO9SgyEd0xH9N5gADLgA"]

In the first 3 lines we see that we hit different filters which check for XSS vulnerabilities, more specifically rules 941100, 941110 and 941160 all of them having the tag paranoia-level/1.

Then the fourth line shows that we hit rule 949110 which caused the web server to return the HTTP 403 Forbidden response because the inbound anomaly score, 15, is higher than 5. Then rule 980130 gives us some more information about the scoring: we hit a score of 15 at the paranoia level 1, while rules at the other paranoia levels rules contributed 0 to the total score. We also see the scores for individual types of attack: in this case all 15 points where scored by rules detecting XSS attacks. This is the meaning of the different abbreviations used:

More detailed logs about the traffic hitting the rules can be found in the file /var/log/apache2/modsec_audit.log.

Fixing false positives

First of all, in order to minimize the amount of false positives, you should set the WEBAPPID variable if you are using one of the known web applications for which the Core Rule Set has a default exclusion set. These web applications are currently WordPress, Drupal, Dokuwiki, Nextcloud, Xenforo and cPanel. You can do so by using the <a href="https://github.com/SpiderLabs/ModSecurity/wiki/Reference-Manual-(v2.x)#SecWebAppId">SecWebAppId</a> option in a VirtualHost of Location definition in the Apache configuration. For example if you have a VirtualHost which is used by Nextcloud, set this within the VirtualHost definition:

<Virtualhost nextcloud.example.org>
    ...OTHER OPTIONS HERE...
    <IfModule security2_module>
        SecWebAppId "nextcloud"
    </IfModule>
</VirtualHost>

If you have a WordPress installation in a subdirectory, then add SecWebAppId within Location tags.

<Location /wordpress>
    <IfModule security2_module>
        SecWebAppId "wordpress-mysite"
    </IfModule>
</Location>

If you have multiple WordPress sites, give each of them a unique WEBAPPID which name starts with wordpress. Add a different suffix for every instance so that each one run its in own application namespace in ModSecurity.

If you still encounter false positives, you can completely disable rules by using the configuration directive SecRuleRemoveById. I strongly recommend not disabling rules globally, but limiting its removal to the specific location from which you want them to be removed, for example by putting them with <Location> or <LocationMatch> tags in the Apache configuration. For example:

<LocationMatch ^/wp-admin/(admin-ajax|post)\.php>
    <IfModule security2_module>
        SecRuleRemoveById 941160 941100 941130 932105 932130 932100
    </IfModule>
</LocationMatch>

Pay attention not to disable any of the 949*, 959*, and 980* rules: disabling the 949* and 959* rules would disable all the blocking rules, while disabling the 980* rules would give you less information about what is happening in the logs.

Conclusion

ModSecurity and the Core Rule Set offer an additional security layer for web servers in your defence in depth strategy. I strongly recommend implementing this on your servers because it makes it harder to abuse security vulnerabilities.

Keep an eye on the Core Rule Set blog and Twitter account: sometimes they post new rules for specific new critical vulnerabilities, which can be worthwhile to add to your configuration.