Trustwave Unveils New Offerings to Maximize Value of Microsoft Security Investments. Learn More

Trustwave Unveils New Offerings to Maximize Value of Microsoft Security Investments. Learn More

Managed Detection & Response

Eliminate active threats with 24/7 threat detection, investigation, and response.

Co-Managed SOC (SIEM)

Maximize your SIEM investment, stop alert fatigue, and enhance your team with hybrid security operations support.

Advisory & Diagnostics

Advance your cybersecurity program and get expert guidance where you need it most.

Penetration Testing

Test your physical locations and IT infrastructure to shore up weaknesses before exploitation.

Database Security

Prevent unauthorized access and exceed compliance requirements.

Email Security

Stop email threats others miss and secure your organization against the #1 ransomware attack vector.

Digital Forensics & Incident Response

Prepare for the inevitable with 24/7 global breach response in-region and available on-site.

Firewall & Technology Management

Mitigate risk of a cyberattack with 24/7 incident and health monitoring and the latest threat intelligence.

Offensive Security
Solutions to maximize your security ROI
Microsoft Exchange Server Attacks
Stay protected against emerging threats
Rapidly Secure New Environments
Security for rapid response situations
Securing the Cloud
Safely navigate and stay protected
Securing the IoT Landscape
Test, monitor and secure network objects
Why Trustwave
About Us
Awards and Accolades
Trustwave SpiderLabs Team
Trustwave Fusion Security Operations Platform
Trustwave Security Colony
Technology Alliance Partners
Key alliances who align and support our ecosystem of security offerings
Trustwave PartnerOne Program
Join forces with Trustwave to protect against the most advance cybersecurity threats
SpiderLabs Blog

Using ModSecurity 2 Collections in Rules

A recent posting on the ModSecurity mailing list by K.C. Li is a very good excuse to discuss some major changes between ModSecurity version 1 and 2 and how to it influence rule writing. K. C. used the following rule in ModSecurity v1:

SecFilterSelective ARGS "(^|[^_])(comments?|story)=.*(href|http)"

This rule searched for the values "href" or "http" in a bunch of different parameters: story, _story, comment, comments, _comment and _comments. The rule replaces 6 rules, each one specific to a parameter. While the rule is very effective, as K.C. writes, it suffers from the following shortcomings:

  • It only detects these parameters if they appear first.
  • It searches for "href" and "http" everywhere, spanning to fields beyond the specific ones searched.
  • it might find href and http as part of longer words and not as separate tokens.

The rule can be corrected like this:

SecFilterSelective ARGS "(?:^|\&)_?(?:comments?|story)=[^\&]*\b(?:href|http)\b"

By adding checks for a "&" prior to the parameter name and ensuring "&" does not exists between the parameter name and the keyword, we make sure that we capture the parameters in any location in the request string and that the tokens are part of the value for this parameter only. The meta character "\b" is a regular expression meta character that matches a word boundary, ensuring that "href" and "http" are tokens. The construct "?:" at the beginning of each parentheses is a performance optimization which prevents the parentheses from capturing the value, a side effect that is not needed unless we use the capture action.

Well, but all this become very complex.

While in ModSecurity 1.x the ARGS location is simply a string that represented either QUERY_STRING or POST_PAYLOAD, in ModSecurity 2 ARGS is a collection that enables searching in individual parameters. Collections are fundamental in ModSecurity 2 and I suggest reading the relevant section in ModSecurity 2 reference guide.

You can still use the location QUERY_STRING|REQUEST_BODY to rewrite the rule for ModSecurity 2.0 but using the ARGS collection will make the rule much simpler. Using a regular expression to select the elements of the collection tested, the following rule will do the same in ModSecurity 2:

SecRule ARGS:'/(?:^|^_)(?:comments?|story)$/' "\b(?:href|http)\b"

The other rules that K.C. uses can also use collections and be converted to a single ModSecurity V2 rule. Instead of:

SecFilterSelective HTTP_x-aaaaaaaaa|HTTP_XAAAAAAAAA ".+"
SecFilterSelective HTTP_x-aaaaaaaaaaa|HTTP_XAAAAAAAAAAA ".+"
SecFilterSelective HTTP_x-aaaaaaaaaaaa|HTTP_X_AAAAAAAAAAAA ".+"

You can simply write:

SecRule "&REQUEST_HEADERS:'/^(?i)x[-_]a{9,12}$/'" "@gt 0"

This rule uses the "&" construct to count the number of elements in a collection, or a subset if a regular expression is used to select elements from the collection.

To complement the discussion, a word about actions in ModSecurity 2. Just as in ModSecurity 1, there is no need to explicitly state actions in each rule and the actions listed in SecDefaultAction will be used. However, due to the bigger role that actions now have in ModSecurity, it is advisable to add them to each rule. Especially important are:

  • The phase action. As ModSecurity 2 now has 4 phases, specifying the phase becomes very important
  • Meta information actions. As ModSecurity rules and rule sets are becoming bigger, it is important to maintain the meta information. And as stated in the manual it is recommended to use only numbers between 1 and 99999 for internally developed rule IDs
  • Anti evasion transformation functions are not explicit in ModSecurity 2.0, and should be set in either a SetDefaultAction directive or in the action list for the event. In this case I would use lowercase and urlDecodeUni.

So K.C. rules becomes:

SecRule "ARGS:'/(?:^|^_)(?:comments?|story)$/'" "\b(?:href|http)\b" \
"deny,log,status:403,phase:2,t:lowercase,t:urlDecodeUni,id:90004,severity:2,msg:'Comment Spam'"

SecRule "&REQUEST_HEADERS:'/^(?i)x[-_]a{9,12}$/'" "@gt 0" \
"deny,log,status:403,phase:2,t:lowercase,id:90005,severity:2,msg:'Comment Spam'"

Latest SpiderLabs Blogs

Fare Thee Well ModSecurity: End-of-Life and Last Commercial Rules Update for June 2024

A Fourteen-Year Journey Comes to an End In June 2010, Trustwave acquired Breach Security, which brought with it the popular Open-Source Web Application Firewall ModSecurity for Apache. At that time,...

Read More

Secure Access Service Edge: Another Multi-Tool for the SOC

Over the years, several security defense architectures have merged into a single solution. Endpoint detection tools can perform sophisticated detections and correlations that used to require a...

Read More

Search & Spoof: Abuse of Windows Search to Redirect to Malware

Trustwave SpiderLabs has detected a sophisticated malware campaign that leverages the Windows search functionality embedded in HTML code to deploy malware. We found the threat actors utilizing a...

Read More