CVE-2024-21539 is a vulnerability found in the @eslint/plugin-kit NPM package, versions before .2.3. Simply put, a flaw in the way this library sanitized inputs led to it being vulnerable to a type of attack called Regular Expression Denial of Service (ReDoS). This post breaks down how the bug works, demonstrates exploitation with a code sample, and offers the latest fixes.

What Is ReDoS and Why Does It Matter?

Regular Expression Denial of Service (ReDoS) is a well-known attack that targets inefficient regex patterns. If a regular expression is poorly designed, certain inputs can cause it to take a very long time to process—occupying 100% CPU usage and even crashing applications or servers.

With packages like @eslint/plugin-kit—which is often used in development or CI pipelines—a single malformed input could hang or bring down automated builds, causing serious issues for teams.

Vulnerable Code Explained

@eslint/plugin-kit had a regex pattern that didn't properly handle malicious or unexpected input. Attackers could pass specially crafted strings that would cause the underlying regular expression engine to process the input inefficiently.

While the precise line of source code changed in version .2.3, here’s an example of the kind of vulnerable pattern (not the exact internal code):

// Hypothetical vulnerable regex usage
function sanitizeInput(input) {
  // Bad: This regex can be abused for ReDoS
  return input.replace(/(a+)+$/, '');
}

An attacker could send a string like 'aaaaaaaaaaaaaaaaaaaaaaaaaaaa!' (many 'a's followed by '!') to this function. The regex engine would backtrack heavily, consuming the CPU.

Demo: Exploiting the ReDoS Flaw

Let’s look at a real snippet that shows how this type of bug can be abused.

// Simulating the vulnerable function
const vulnerableSanitize = (input) => input.replace(/(a+)+$/, '');

const safe = 'a!';
const exploit = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa!';
const longTimeStart = Date.now();
vulnerableSanitize(exploit);
console.log('Exploit time (ms):', Date.now() - longTimeStart);

const shortTimeStart = Date.now();
vulnerableSanitize(safe);
console.log('Safe time (ms):', Date.now() - shortTimeStart);

When you run this, you’ll see the "exploit" input takes far longer, which would multiply if used on a server or in a build system. Large enough strings could tie up a Node.js process until it times out or is killed.

How Attackers Can Crash Your System

If the vulnerable function sits in a webserver route or a processing pipeline for user inputs, an attacker could POST the bad pattern repeatedly, consuming all your server resources. In a build system, a malicious developer (or even an accident!) could inject a bad config or source file, stalling builds and CI pipelines.

In short: with enough repetitions, ReDoS becomes a denial of service.

The Fix: Upgrading & Input Validation

The maintainers addressed this in @eslint/plugin-kit v.2.3 by fixing the problematic regex and improving how input is sanitized. You should:

`bash

npm install @eslint/plugin-kit@^.2.3

`

2. Validate inputs before processing: Always make sure that user input is sanitized both for format and length.
3. Avoid problematic regexes: When writing custom regex patterns, test them with online tools (like regex101.com) and use safe-regex to detect unsafe patterns.

References and Original Advisory

- GitHub Security Advisory GHSA-7jcq-4m38-q7gh
- NPM Package page
- CVE details on NVD

Conclusion

CVE-2024-21539 is yet another reminder that even tiny mistakes in how regular expressions are written can have big consequences. Upgrading your dependencies regularly and scanning code for logic issues is the best way to keep your applications safe.

If you’re using @eslint/plugin-kit below v.2.3, upgrade now—don’t leave your development environment or CI pipelines open to CPU exhaustion attacks!

Want more security write-ups? [Follow us for exclusive breakdowns of the latest CVEs!]

Timeline

Published on: 11/19/2024 05:15:16 UTC
Last modified on: 11/19/2024 21:57:32 UTC