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Introduction: Simple Words, Critical Issue

In February 2023, Google patched a major security issue in Chrome: CVE-2023-0941. If you’ve ever heard about hackers taking control of your browser just by visiting a web page, this is that kind of bug. CVE-2023-0941 is a “use-after-free” vulnerability affecting how Chrome handled “prompts,” and it was so risky, Google tagged it as Critical.

Let’s break down what happened, how hackers could use it, and what the code behind it looked like—all in plain American English, with references.

What is a Use-After-Free?

A use-after-free bug means the program tries to use memory after it’s already been freed (released). This causes unpredictable behavior. In Chrome, if this happens, an attacker might convince Chrome to run bad code instead of normal stuff.

Where Was the Bug? (prompts)

This bug was hiding in the “prompts” logic of Chrome. Prompts are those pop-up messages you sometimes get (for example, asking for permission to access your mic or camera, or showing an alert).

When Chrome deals with prompts, it creates some internal objects in memory. If these objects are freed (gone), but Chrome still tries to use them, problems happen.

When a website triggers a prompt (like alert() or confirm()), Chrome allocates some memory.

- If, during certain sequences (like rapidly opening/closing prompts or redirecting), the object for that prompt is freed but Chrome tries to use it again (read/write in that spot), a use-after-free occurs.

Let’s mimic a simplified, vulnerable C++ pattern

class Prompt {
public:
    void Show() {
        // Do stuff, maybe trigger callbacks, or user closes prompt
        delete this;  // Oops! Frees object
    }

    void HandleResponse() {
        // Code still tries to use 'this' pointer--danger!
    }
};

void TriggerPrompt() {
    Prompt* p = new Prompt();
    p->Show();
    // Some condition causes HandleResponse to run after 'p' is deleted!
    p->HandleResponse(); // <-- Use-after-free happens here
}

In the real Chrome engine, the problem was more complex, but it boiled down to objects being reused after being freed via prompt events.

3. Pushes Chrome to interact with the freed memory—possibly even filling the heap with attacker-controlled data.
4. If successful, the attacker could control what happens next—potentially taking over the browser tab (or beyond!).

On a high level

<script>
for (let i = ; i < 10000; i++) {
    window.setTimeout(function() {
        alert("Exploit here!");
        // Trigger allocation/frees in strange ways
    }, );
}
</script>

While the actual attack flow is more sophisticated, this is the gist: the attacker abuses prompt events to smash Chrome’s memory safety.


### Who Reported/Fixed It?

- Reported by: Anonymous researcher

Fixed in: Chrome 110..5481.177 and above

- Severity: Critical

Proof of Concept (PoC) Reference

Though the full exploit code isn’t public (as it might be used for bad purposes), the Chromium issue tracker has some valuable context:

Chromium Security Advisory

- Chromium Blog - Release Announcement
- Chromium Issue #141140 (may need special access)

Summary

CVE-2023-0941 is a reminder: even the safest software can have hidden dangers. Chrome’s prompt-handling code had a “use-after-free” bug—until Google patched it, attackers could've hijacked your browser just by loading a crafted web page. The fix? Update now. Stay safe, and check the provided references if you want to dive deeper.

References:

- Chrome Release Blog: Stable Channel Update for Desktop - 2023-02-14
- CVE-2023-0941 at NIST NVD
- Chromium Bug Tracker: 141140
- Understanding Use-After-Free (Google Security Blog)

Timeline

Published on: 02/22/2023 20:15:00 UTC
Last modified on: 02/28/2023 02:17:00 UTC