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Mastering OWASP ModSecurity Core Rule Set(6): Unmasking Threats with Rule Validation and Scoring | Christian Folini - All Articles - CISO Platform
Mastering OWASP ModSecurity Core Rule Set(6): Unmasking Threats with Rule Validation and Scoring | Christian Folini

Unmasking Threats with Rule Validation and Scoring

Think about securing your home. You’ve installed surveillance cameras, sensors, and even a guard dog. But what happens when someone manages to sneak in? Wouldn’t it be great to have a system that not only detects the intruder but also evaluates the level of threat?

That’s where OWASP ModSecurity Core Rule Set (CRS) Part 6 steps in. It digs into the error logs, interprets patterns, and scores threats to decide whether to block or allow requests.

 

 

Error Logs: The Hidden Story

The real action happens behind the scenes—in the error log. Every suspicious request triggers a rule, and these rules write details to the error log. But reading through raw logs is like looking for a needle in a haystack.

Here’s where unique request IDs come into play:

  • Unique Identifiers: Every request is assigned a unique ID that ties the access log and the error log together.

  • Pattern Matching: Alerts appear in the error log, giving insights into what triggered the rule.

  • Deep Analysis: You can search for this unique ID in the error log to uncover what ModSecurity flagged and why.

Think of the error log as a detective’s notebook. Every clue points to something bigger.

 

Rule Matching and Scoring

In this part of the CRS journey, we encounter the concept of scoring and rule matching. Imagine a system where each suspicious activity adds points to an overall score. When the score exceeds a defined threshold, action is taken.

 

1. Paranoia Levels and Strictness

CRS operates at different paranoia levels. The higher the level, the stricter the rule enforcement.

  • Paranoia Level 1: Basic protection, fewer false positives.

  • Paranoia Level 4: Ultra-strict, catching everything but often leading to higher false positives.

In this scenario, a request triggered rules at Paranoia Level 4, where strict character sets are enforced. Even a single invalid byte can trigger an alert.

 

Rule IDs and Their Messages

Each alert comes with a rule ID that points to the exact rule that was triggered. Here’s a quick breakdown:

  • Rule 920273: Invalid characters in the request. This rule enforces strict ASCII character checks, flagging any deviation.

  • OS File Access Attempt: CRS knows when a sensitive file path is requested (like /etc/passwd). This triggers a critical alert.

  • Remote Command Execution (RCE): Certain patterns hint at potential RCE attempts, adding more points to the score.

These rules not only alert you but also score the request. A higher score increases the likelihood that the request is malicious.

 

The Scoring Game: Block or Pass?

ModSecurity works like a scoring system in a video game. Each violation adds points. If the score crosses a certain threshold, the request is blocked.

  • Threshold Set to 5: Initially, the threshold was relaxed, allowing most requests to pass.

  • Threshold Lowered Back to 5: After analysis, the threshold was returned to its original value. The system started blocking suspicious requests again.

When the threshold is low, even minor violations get blocked. But if the threshold is high, only severe attacks trigger a block.

 

Validating Rules in Action

Here’s where things get interesting. After detecting a suspicious request, the analyst dives into the error log.

  • Error Log Analysis: Searching for the unique request ID reveals a list of triggered rules.

  • Rule Patterns Identified: Complex patterns and parameter matches expose the potential attack.

  • Scoring Validation: Each rule adds to the overall score, ultimately deciding whether to allow or block the request.

In one instance, a search query included a suspicious string:

bash

CopyEdit

cat /etc/passwd

 

This immediately triggered an OS File Access Attempt rule. But the magic happens when multiple rules combine and push the score past the threshold.

 

Real-World Threat Validation

Security teams often run security scanners like Burp Suite or Nikto to validate CRS configurations.

 

1. Nikto – The Fast but Weak Scanner

Nikto is fast, but not the smartest scanner. It runs through 3,296 requests in seconds, generating alerts for 7 rules per request.

  • 41,000 Entries in the Log: Almost all requests triggered ModSecurity rules.

  • Blocked with Access Denied (403): CRS aggressively blocked Nikto’s requests, marking them as suspicious.

2. ModSecurity Alias for Easier Analysis

To make log analysis easier, security experts often create shell aliases that group and format error logs. This allows them to:

 Quickly search and grep for relevant request IDs.
Extract and categorize rule messages.
  Count alerts and analyze blocking behavior.

 

Access Denied: The Final Block

When a request crosses the score threshold, Rule 949110 steps in.

  • Access Denied with Code 403: This rule blocks the request, marking it as malicious.

  • No More Warnings: Unlike previous warnings, this rule ensures that suspicious traffic is denied access.

Without this block, an attacker could bypass security controls and access sensitive data.

 

Lessons for Cybersecurity Leaders

For CISOs, CIOs, and Security Managers, understanding CRS Part 6 is essential for fine-tuning security rules.

  • Log Analysis is Key: Reviewing error logs helps uncover rule patterns and improve configurations.

  • Adjusting Paranoia Levels: Tuning paranoia levels can balance security and usability.

  • Threshold Management: Setting the right threshold ensures that false positives don’t disrupt operations.

Testing and Fine-Tuning CRS

Testing CRS is not a one-time effort. Security teams should:

Regularly Run Security Scans: Use scanners like Nikto and Burp Suite to evaluate CRS effectiveness.
Analyze Logs for Insights: Search for unique IDs to correlate request logs and error logs.
Optimize Rule Sets: Adjust thresholds and fine-tune rules based on real-world traffic.

 

Final Thoughts

Mastering the OWASP ModSecurity Core Rule Set isn’t just about setting it up and forgetting it. It’s about constantly validating, adjusting, and fine-tuning the system.

CRS Part 6 empowers security teams to:

 Analyze error logs for patterns.
  Identify suspicious requests through scoring.
  Block malicious traffic before it causes damage.

Stay vigilant. Fine-tune your CRS. Block the threats before they breach your digital walls.

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By: Christian Folini (Teacher and Security Engineer, Partner, Netnea.com)

 

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