Critical ModSecurity Flaw Exposes Web Servers to Devastating DoS Attacks: Here’s What You Need to Know

Introduction: Why This ModSecurity Bug Is a Big Deal

If you’re running a website protected by ModSecurity one of the most widely used web application firewalls (WAFs) you need to pay attention. A newly disclosed vulnerability (CVE-2025-47947) allows attackers to send a single malicious request that can crash your server by exhausting its memory, leading to a complete denial-of-service (DoS) condition.

This isn’t just theoretical. Security researchers have confirmed that the exploit is easy to execute, requires minimal effort, and affects millions of web servers running ModSecurity’s Apache module (mod_security2). The vulnerability has been lurking in the code for years, and while a patch is now available, many systems remain exposed.

In this deep dive, we’ll explore:

• How the vulnerability works (in simple terms)

• Who’s at risk and how bad the damage could be

• Step-by-step mitigation strategies

• Why upgrading to libmodsecurity3 might be the best long-term fix

Let’s break it down.

What Is CVE-2025-47947? The ModSecurity DoS Vulnerability Explained

The Basics: What Went Wrong?

ModSecurity is an open-source WAF designed to protect web apps from attacks like SQL injection and cross-site scripting (XSS). However, researchers discovered a critical flaw in how it processes JSON payloads when certain rules are enabled.

The vulnerability triggers when:

1. A request has Content-Type: application/json

2. At least one active rule uses sanitiseMatchedBytes or sanitiseMatched (used for masking sensitive data in logs)

When these conditions are met, ModSecurity goes into a memory-eating frenzy, storing exponentially more data than it should. A single malicious JSON payload can blow up memory usage, slowing down or crashing the server.

The Technical Root Cause

Here’s where things get messy. Normally, sanitiseMatchedBytes replaces sensitive data (like credit card numbers) with asterisks (****) in logs. But when processing JSON, ModSecurity mishandles memory allocation, creating a self-replicating memory bloat.

• If a JSON payload has 1,000 elements, ModSecurity evaluates each one 1,000 times.

• Instead of storing 1,000 items, it stores 1,000 x 1,000 = 1,000,000 variables in memory.

• Apache’s APR table (where this data is stored) keeps expanding until the server runs out of RAM.

Proof of Concept: How Easy Is It to Exploit?

Security researchers demonstrated the attack using a simple Python command to generate a malicious JSON file:

python3 -c "print('[%s]' % ','.join(['1234567890123456'] * 1000))" > payload.json

Then, sending this file via a single HTTP POST request was enough to trigger severe memory exhaustion. No fancy hacking skills required just a basic HTTP request.

Who’s Affected? (And Who’s Safe?)

At-Risk Systems

• ModSecurity 2.x (mod_security2) All versions up to 2.9.8

• Apache web servers using ModSecurity (since the flaw is in the Apache module)

• Websites with JSON APIs that accept application/json content

Not Affected

• ModSecurity 3.x (libmodsecurity3)  The newer implementation doesn’t have this bug

• Nginx & other non-Apache setups (unless using mod_security2 in a non-standard way)

How to Fix It: Patch, Mitigate, or Upgrade?

1. Immediate Patch (Recommended Fix)

The ModSecurity team released version 2.9.9, which fixes the issue. Update immediately if you’re running mod_security2:

# For Debian/Ubuntu  
sudo apt update && sudo apt upgrade modsecurity  

# For CentOS/RHEL  
sudo yum update mod_security

2. Temporary Mitigations (If You Can’t Patch Yet)

If upgrading isn’t an option, consider:

• Disabling rules with sanitiseMatchedBytes (weakens logging but stops the exploit)

• Blocking malformed JSON payloads at the load balancer or CDN level

• Disabling ModSecurity’s audit engine (last resort loses security logs)

3. Long-Term Solution: Migrate to libmodsecurity3

Since ModSecurity 2.x is aging, switching to libmodsecurity3 (v3.x) is the best move. It’s faster, more efficient, and not vulnerable to this bug.

The Bigger Picture: What This Means for Web Security

This flaw highlights three critical lessons for security teams:

1. Open-source security tools need constant scrutiny: Even widely used projects like ModSecurity can have hidden flaws.

2. JSON parsing is a common weak spot:  Many WAFs struggle with complex JSON structures.

3. Memory exhaustion attacks are underrated: Unlike ransomware or data theft, DoS flaws often get ignored until it’s too late.

Final Thoughts: Act Now Before Attackers Do

CVE-2025-47947 is a silent killer it doesn’t steal data, but it can take your website offline with one request. If you’re using ModSecurity 2.x, patch immediately or apply mitigations.

For those still on mod_security2, this is your wake-up call: Upgrade to libmodsecurity3 or risk being an easy target.