CKEditor4 is a popular open source WYSIWYG (“what you see is what you get”) HTML editor used on thousands of websites since 2012. But in versions prior to 4.18., a subtle vulnerability lurked inside its dialog plugin—a bug that could let an attacker freeze a user’s browser tab through a carefully-crafted input. Let’s break down what went wrong, how it works, and what you should do to stay safe.
Understanding the Vulnerability
The core issue with CVE-2022-24729 stems from how the dialog plugin handles input validation. Specifically, CKEditor4's dialog fields often use regular expressions to check if input matches expected patterns.
However, one of those regular expressions was inefficient, and in certain cases, it could get stuck trying to process very long and maliciously-crafted input—causing extreme slowdowns, or even freezing the browser tab altogether.
What Makes Regex Dangerous?
Regular expressions (“regex”) are powerful, but certain patterns—especially if they mix repetition and optional elements—can get stuck analyzing complex input, a problem called Regular Expression Denial of Service (ReDoS).
The Vulnerable Code
Here’s what such a problematic validator pattern might look like in CKEditor4 dialog input (source):
// Inside dialog definition
{
type: 'text',
id: 'validatorField',
label: 'Your Input',
validate: CKEDITOR.dialog.validate.regex( /^[a-zA-Z-9\s]*$/ , 'Only letters, numbers, and spaces are allowed.' )
}
At first glance, this regex looks simple, but with enough input, especially if the regex is more complex, processing could grind to a halt.
But in CKEditor4, the real issue came from more complex validator expressions, sometimes used for input like URLs or email addresses.
How an Attack Could Work
An attacker could place a text input pattern—such as a long, repetitive string designed to confuse the regex—into a vulnerable CKEditor dialog field. By pasting or sending this input, the regular expression engine in JavaScript tries to evaluate the input, but gets stuck, and the browser tab becomes unresponsive.
Example Exploit Input
aaaaaaaaaaaaaaaaaaaaaaa...a@a.aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa...a
If the validator is a faulty email regex like
/^\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}$/
a long string with lots of "a"s and dangerous repetition will cause the browser to hang.
Here’s a minimal reproducible example
<!-- Minimal dialog using CKEditor4 plugins -->
<script src="https://cdn.ckeditor.com/4.17.2/standard/ckeditor.js"></script>;
<textarea id="editor1"></textarea>
<script>
CKEDITOR.replace('editor1', {
on: {
instanceReady: function(evt) {
evt.editor.addCommand("openDialog", {
exec: function(editor) {
editor.openDialog('testDialog');
}
});
}
},
extraPlugins: 'dialog'
});
CKEDITOR.dialog.add('testDialog', function(editor) {
return {
title: 'Vulnerable Validator',
minWidth: 400,
minHeight: 200,
contents: [
{
id: 'tab1',
label: 'First Tab',
elements: [
{
type: 'text',
id: 'address',
label: 'Email Address',
validate: CKEDITOR.dialog.validate.regex(
/^\w+@[a-zA-Z_]+?\.[a-zA-Z]{2,3}$/,
'Please enter a valid email address.'
)
}
]
}
]
};
});
</script>
<button onclick="CKEDITOR.instances.editor1.execCommand('openDialog')">Open Dialog</button>
With a maliciously long input, your browser will likely freeze once validation triggers.
The Fix
The CKEditor team patched this vulnerability in version 4.18. (release notes). They improved the problematic regular expressions and input validation logic, making them resistant to ReDoS.
Patched Regex:
The new expressions no longer allow catastrophic backtracking.
No Workarounds:
The maintainers have reported that there are currently no practical workarounds short of upgrading.
What Should You Do?
If you use CKEditor4:
Upgrade immediately to at least version 4.18.
References and Further Reading
- National Vulnerability Database: CVE-2022-24729
- CKEditor4 Security Advisory
- CKEditor4 GitHub Issue #5124
- Short intro to Regular Expression Denial of Service
Conclusion
CVE-2022-24729 is a real-world reminder that even seemingly innocent input validation code can hide dangerous bugs, especially when regex is involved. If you rely on CKEditor4—directly or in another web app—be sure to upgrade right away. Leaving this bug unpatched makes your users vulnerable to a simple trick that could crash their browser or disrupt their workflow.
Timeline
Published on: 03/16/2022 17:15:00 UTC
Last modified on: 07/25/2022 18:21:00 UTC