Common Web Application Security Attacks: Real-World Lessons from the Field

Picture this: You spend months building the perfect web application, only to find out it’s leaking data like a sieve. Scary, right? But that’s exactly what many web applications do—unless you have a strong web application security program, they unknowingly invite hackers inside by leaving security flaws wide open.

How big is the risk? The 2024 Verizon Data Breach Investigations Report found that close to 1 in 10 data breaches were caused by a basic web app attack. With the dramatic increase in web application (web app) use, your organization may be using hundreds of web applications that may have security gaps. At LMG Security, we spend our days thinking like attackers so we can help organizations stay one step ahead. In a recent talk at SecDSM, Tom Pohl, our head of penetration testing, shared some jaw-dropping real-world stories of security blunders that could have led to major breaches if left unchecked.

In this blog, we’ll cover how web apps get hacked, common attack tactics, and web application security strategies to keep your organization safe. Let’s dive in!

Three Common Web Application Security Flaws Hackers Love to Exploit

Case #1: The Command Injection Jackpot

We recently worked with a company that had just acquired a new business. As part of the deal, they inherited a few web applications that they needed to assess for security risks. One of these apps was designed for analytics—users could upload log files, which were then processed through a machine-learning model.

“As soon as we heard ‘file upload’ alarm bells started ringing,” stated Pohl. “Why? Because improperly handled file uploads are like gift-wrapped exploits waiting to be unboxed.” Pohl continued, “Sure enough, we found that the web application wasn’t sanitizing file names. We crafted an innocent-looking filename that actually contained a hidden command: cat /etc/passwd. When the system processed the file, it obediently executed our command, giving us access to the server’s password file.”

Then we got ambitious. Instead of just listing files, we injected a reverse shell payload—tricking the system into giving us full command-line access to the server. From there, it was just a few steps to full system compromise. We even found a privilege escalation vulnerability that allowed us to become the root (admin) user.

Key Web Application Security Takeaways:

• Never trust user input. Validate and sanitize everything, especially filenames.
• Restrict back-end command execution. User-controlled inputs should never interact directly with system commands.
• Regular web application security testing is a must. This company dodged a bullet by conducting web app penetration testing before launching its new acquisition.

Case #2: The API That Created Admins

APIs are the lifeblood of modern web applications. But if they’re not properly secured, they become an open buffet for attackers. In this case, we were testing a job management platform. The client had set up an environment for us, but there was a hiccup—our login credentials weren’t ready yet. No problem! Who needs credentials when you have an exposed API?

“While analyzing the web app’s JavaScript, we found a hidden API endpoint: account/register,” Pohl shared. “Here’s the kicker—the app didn’t even offer user registration! The developers had hidden the feature from the front-end but had left the functionality available in the back-end.”

We sent a request to the account/register endpoint and created a new user. But when we tried logging in, we hit a roadblock: 403 Unauthorized. At first, it looked like we were stuck. But then we found another API endpoint: add_user. Using the session from our first account—authenticated but unauthorized—we made the add_user call. Unlike account/register, this endpoint allowed us to assign user roles, including Super Admin. One request later, we were fully authenticated as a top-level admin. From there, we had complete control over the system.

Key Web Application Security Takeaways:

• Secure your API endpoints. Just because a feature isn’t visible doesn’t mean it isn’t accessible. “The three primary causes of security flaws in APIs are: improperly restricted API calls, APIs that expose unintended or insecure functionality, and weak or missing authorization for high-value operations,” Pohl shared. We recommend checking your APIs for these flaws.
• Implement strict role-based access controls. Users should only be able to perform actions necessary for their position.
• Test your APIs regularly. Attackers will poke and prod your application, you should do it first with pen testing. Read our API pentesting blog for more details.

Case #3: The Debug Tool That Leaked Everything

This last case is a favorite because it’s an example of a vulnerability hiding in plain sight.

We were testing a Spring Boot web application when we discovered a little-known feature: the /heapdump endpoint. By default, this endpoint allows developers to download a snapshot of the application’s memory to debug issues. That’s right, it lets anyone download a full dump of the server’s working memory.

Curious, we grabbed the heap dump and started sifting through it. What did we find? Active directory credentials, user session tokens, and even plaintext passwords stored in memory. This single misconfiguration exposed the entire system to compromise.

Key Web Application Security Takeaways:

• Disable unnecessary debugging features in production. If you don’t need it, don’t expose it.
• Never store plaintext passwords in memory. Use proper authentication mechanisms and session handling.
• Test for default configurations. Many vulnerabilities come from settings that were never changed from their default state, so always check for this issue.

How Secure Are Your Web Apps?

These cases illustrate a harsh reality: many organizations don’t know how vulnerable their web applications really are. It’s easy to assume that because your app “works,” it’s secure. But security isn’t about function—it’s about resilience against attackers who think differently than you do.

If your organization hasn’t conducted a web application penetration test recently, now is the time. Attackers are always innovating, and what was secure yesterday might be vulnerable today. Want to learn more about how LMG Security can help protect your applications? Contact us for a security assessment—and let’s make sure your web app isn’t an easy target.