In July 2023, Mozilla disclosed a serious vulnerability tracked as CVE-2023-4048, which could lead to exploitable crashes in Firefox and Firefox ESR. The bug was found in the DOMParser component, which is used to convert HTML and XML text into DOM documents. In simple terms, if you use Firefox (before version 116) to open or parse some HTML data while your computer is running low on memory, an attacker could cause the browser to read data it shouldn't, leading to a crash or—worst case—a way to further compromise your device.

This post will break down what CVE-2023-4048 is, how it works, and what the risks are. We'll include code snippets to help illustrate the problem and reference the official sources. This vulnerability is exclusive to older versions of Firefox, but understanding it can help everyone recognize the importance of keeping browsers updated. Let's get started.

Before we get into the bug itself, here's a quick example of how DOMParser works in JavaScript

let parser = new DOMParser();
let doc = parser.parseFromString('<html><body>Hello, World!</body></html>', 'text/html');
console.log(doc.body.textContent); // Output: Hello, World!

DOMParser turns strings of HTML or XML into real DOM document objects that websites can manipulate, which is standard for a lot of web pages.

Where does the bug happen?

The vulnerability is triggered during memory-constrained situations. If the system is low on memory, Firefox's DOMParser component could perform an out-of-bounds read—an error that happens when a program accesses memory outside of what it's supposed to. In non-technical terms, it's like reading the next page of a book when you should only have permission to read the current one; the extra page might contain sensitive information not meant for your eyes.

Example Scenario

Here's a simplified JavaScript snippet that simulates what an attacker could try to do in order to trigger the bug (for *illustration* purposes only):

function stressMemoryAndParse() {
  // Allocate a lot of arrays to consume available memory
  let leakers = [];
  try {
    while (true) {
      leakers.push(new Array(1e6).fill('A'));
    }
  } catch (e) {
    // Now memory is tight, force DOMParser to work
    let parser = new DOMParser();
    let weirdHTML = "<html>" + "X".repeat(100000) + "</html>";
    let doc = parser.parseFromString(weirdHTML, "text/html");
    console.log(doc);
  }
}
stressMemoryAndParse();

*In real attacks, this would be more carefully crafted, and might involve further steps to leak or manipulate memory.*

What could go wrong?

If the out-of-bounds read lands on sensitive memory, the browser might return data it shouldn't or just crash in a way that attackers can predict and use (such as to bypass security protections with additional exploits). This issue is *especially* concerning when combined with other vulnerabilities, since it could be a stepping stone to full browser compromise.

Firefox ESR < 115.1

If you use these versions, you need to update right now.

Mozilla’s security advisory is the best authoritative source

- Mozilla Foundation Security Advisory 2023-26
- Bugzilla tracking bug for CVE-2023-4048 (requires account for full details)

Vulnerable Code Example

While the actual bug lives deep inside C++ code in the Firefox source, it *generally* looks like misuse of pointers/offsets when handling memory reallocation failures:

// This is a simplified C++ pseudo-code
char* buffer = (char*)malloc(size);
// Low memory, malloc fails, buffer=null
readDataInto(buffer, size); // Danger: reading into a null pointer!

In practice, when DOMParser tries to parse a large or deeply nested HTML in a low-memory state, it might try to read more data than its buffer has, going out-of-bounds.

Trigger out-of-bounds reads, possibly extracting information or causing predictable crashes.

While it's hard (but not impossible) to jump straight from this vulnerability to remote code execution, it *definitely* provides a way to crash the browser, which is a key step for more sophisticated attacks.

Mitigation

Update your browser!
If you’re running any version of Firefox or ESR listed above, go to Menu > Help > About Firefox and make sure auto-update installs version 116 or above.

For enterprises using ESR (Extended Support Release), update to at least 102.14 or 115.1.

Disable JavaScript
As a temporary measure, disabling JavaScript blocks most ways this bug can be exploited, but this breaks many modern websites.

Conclusion

CVE-2023-4048 is a solid reminder that memory management bugs are still a huge problem for browsers. Out-of-bounds reads can have unpredictable and dangerous effects, especially when attackers can trigger them under specific conditions like low RAM.

To stay safe, always keep your browser updated and watch for security notices from Mozilla and other browser vendors.

References

- Mozilla Foundation Security Advisory 2023-26
- Bugzilla entry for CVE-2023-4048
- DOMParser documentation (MDN)

Timeline

Published on: 08/01/2023 15:15:00 UTC
Last modified on: 08/11/2023 20:03:00 UTC