• Posted by: Eng. Donya Bino
• September 11, 2025

XXE Injection Explained: The Overlooked Web Security Risk

When most people think of web security bugs, the usual suspects come up: SQL Injection, XSS, maybe CSRF. But there’s another one that doesn’t get nearly as much attention, yet it can be just as dangerous: XXE Injection.

If you’re not familiar with it, don’t worry—you’re not alone. I’ve run into plenty of developers and even sysadmins who had never heard of it until a penetration test flagged it. The scary part? XXE is often lurking in apps that look secure.

So, What Is XXE Anyway?

XXE stands for XML External Entity injection. In plain English, it’s when an attacker sneaks something malicious into an XML file or request, and the system ends up processing it in ways the developers never intended.

The “external entity” part basically means the XML parser (the thing that reads XML data) gets tricked into pulling in outside resources—like local files on your server.

Here’s the kicker: the parser is just doing its job, but the attacker is the one holding the puppet strings.

A Quick Example

Normal XML is pretty harmless:

<User>

  <Name>Sarah</Name>

</User>

But with XXE, an attacker can craft something like this:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE foo [

<!ENTITY secret SYSTEM "file:///etc/passwd">

]>

<User>

  <Name>&secret;</Name>

</User>

That little trick tells the parser: “Hey, instead of just putting a name here, go grab that sensitive file on the server and drop its contents in.”

Boom. Sensitive data exposed.

Why You Should Care

The dangerous thing about XXE is that it doesn’t always stop at reading files. Depending on how the app is set up, attackers can:

1. Make the server send requests to internal systems (a sneaky way of bypassing firewalls).
2. Leak API keys, config files, or even user data.
3. Sometimes even crash the system with denial-of-service payloads.

And here’s the thing: you might not even realize your app is vulnerable because XML parsing often happens behind the scenes, thanks to libraries or frameworks.

Real Talk: XXE in the Wild

I’ve seen XXE pop up most often in older enterprise applications—especially ones still using SOAP-based APIs. Financial systems, healthcare apps, even government platforms… you’d be surprised how many are still parsing XML like it’s 2005.

And attackers know this. It’s one of those “quiet” vulnerabilities that doesn’t make headlines as much as ransomware, but it’s just as effective for data theft.

How to Defend Against It

The good news is, XXE isn’t hard to fix once you know it’s there:

1. Turn off external entities in your XML parsers. (Seriously—disable them unless you absolutely need them.)
2. Keep libraries updated—older XML processors are much more likely to allow dangerous behavior by default.
3. Validate user input—never trust uploaded files or unfiltered XML.
4. Run security tests—automated tools and pentests can catch XXE before an attacker does.

Wrapping Up

XXE might not have the flashy reputation of other web bugs, but don’t underestimate it. A single misconfigured XML parser can open the door to sensitive files, data leaks, or worse.

If your systems still use XML (and a lot of them do, even if you don’t realize it), it’s worth taking the time to double-check your setup. A few configuration tweaks now can save you a massive headache later.