CVE-2023-25739 is a serious security vulnerability found in Mozilla Firefox (before version 110), Thunderbird (before 102.8), and Firefox ESR (before 102.8). At the core, it’s a use-after-free (UAF) bug in a browser component called ScriptLoadContext.

In plain English: When a website asks Firefox to load a script module (a chunk of JavaScript code), the browser keeps track of this job using a “context” object. If something interrupts or cancels this loading—say, you navigate away, or a network problem occurs—Firefox was supposed to clean up the context safely. But in this case, due to a logic oversight, Firefox sometimes tried to use a context object after it had already been deleted. That’s dangerous: hackers can exploit use-after-free bugs to crash browsers or, worse, run malicious code.

Module load requests: The browser asks for an external JavaScript file, often as a module.

2. Request cancellation: Sometimes, these requests are canceled—maybe the page is closed, or a script loader decides it doesn’t need the file anymore.
3. No check on cancelled requests: Firefox didn’t properly check if the load request had been canceled *before* touching the corresponding ScriptLoadContext.
4. Use-After-Free: If the context was already deleted, and Firefox tried to use it (for example, to report an error), memory corruption could occur.

Here’s a simplified snippet showing what can happen in code

// Pseudocode!
ScriptLoadContext* context = GetScriptLoadContext(request);
if (/* something went wrong loading the script */) {
    // Oops! If the context was already freed, this is unsafe.
    context->ReportError("Failed to load script");
}


If context is already destroyed, accessing it is like reading a letter after shredding it: the data is gone, but you’re pretending it’s still there.

Impact: Why Should You Care?

Use-after-free vulnerabilities are gold for attackers. They can sometimes trick the browser into running code of the attacker’s choice. That means, simply by visiting a malicious website, an attacker may:

Run unwanted programs on your computer

That’s why Mozilla gave this bug a high severity rating.

Exploit Details

As of now, there’s no public exploit known in the wild, but here’s how a theoretical attack might work:

The attack triggers the buggy situation: Firefox tries to clean up using the already-freed context.

4. If the attacker carefully primes the browser’s memory, they can cause code reuse (or a crash), taking over some browser control.

An example attack in JavaScript (hypothetical and simplified)

let script = document.createElement('script');
script.type = 'module';
script.src = 'evil-script.js';
document.head.appendChild(script);

// Malicious timing: quickly remove the script to cancel the load
setTimeout(() => {
    document.head.removeChild(script);
    // Try to exploit browser's delayed cleanup...
}, 10);

It’s technically tricky, but professional attackers with browser exploit experience might pull it off.

How Was It Fixed?

Mozilla patched this by adding proper checks before accessing the ScriptLoadContext. The browser now verifies that the context is still valid before touching it.

Here’s an improved pseudocode

ScriptLoadContext* context = GetScriptLoadContext(request);
if (context && !context->IsCancelled()) {
    context->ReportError("Failed to load script");
}


This ensures Firefox never uses a destroyed object.

Official References

- Mozilla Advisory: https://www.mozilla.org/en-US/security/advisories/mfsa2023-07/#CVE-2023-25739
- National Vulnerability Database: https://nvd.nist.gov/vuln/detail/CVE-2023-25739
- Firefox Bug #1812702: https://bugzilla.mozilla.org/show_bug.cgi?id=1812702

If you can’t update, be extra cautious visiting untrusted sites (but really, just update!).

3. If you’re a developer, understand why use-after-free bugs are dangerous and always check object validity before use, especially with asynchronous requests.

The Takeaway

CVE-2023-25739 is a classic example of how small coding mistakes can become big security risks. Modern browsers are incredibly complex, and even simple logic errors—like forgetting to check if a canceled request’s context is still there—can open the door to attackers. Thanks to responsible disclosure and fast patching by Mozilla, the damage here was limited.

Stay safe: Keep your browsers and email clients updated!

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Timeline

Published on: 06/02/2023 17:15:00 UTC
Last modified on: 06/08/2023 16:15:00 UTC