In August 2023, Mozilla disclosed a critical vulnerability: CVE-2023-4576. While it stayed under the radar for many, this bug carried a frightening combination of consequences, particularly for users running Firefox or Thunderbird on Windows. The technical core? An integer overflow in a routine called RecordedSourceSurfaceCreation. The ripple effect? A heap buffer overflow that could leak sensitive data—and, under attack, might even enable a malicious program to escape the browser sandbox.

If you use Windows and haven't updated your browser since version 117 (or run older ESR or Thunderbird versions), read on to understand why you should patch *now*.

Location: Only affects Firefox (and Thunderbird) running on Windows.

- Functions at Play: The bug lives inside the graphics subsystem, specifically RecordedSourceSurfaceCreation.
- Type of Vulnerability: Integer overflow → Heap buffer overflow → Information leak (and possible sandbox escape).

What Is "RecordedSourceSurfaceCreation"?

Firefox's graphics backend records and replays drawing commands for things like PDF rendering or webpage drawing. The RecordedSourceSurfaceCreation function is a part of this system: it allocates memory and stores bitmap data.

If RecordedSourceSurfaceCreation is fed with specially chosen (malicious) values—sizes large enough to overflow a 32-bit integer—the math breaks down. That means it might allocate a much smaller buffer than needed, then blindly write more data than fits.

Here’s how such bugs often happen, in simplified C++ pseudocode

uint32_t stride = CalculateStride(format, width);
uint32_t bufferSize = stride * height; // Potential overflow if stride*height > UINT32_MAX

// Memory is allocated based on the incorrect (small) bufferSize
uint8_t *buffer = (uint8_t*) malloc(bufferSize);

// Unsafe copy -- writes data beyond the buffer
memcpy(buffer, inputData, actualNeededSize);

If width and height are attacker-controlled and very large, stride * height can wrap around to a very small value (due to 32-bit overflow). Memory allocation then proceeds with a tiny buffer, but following code assumes a *large* buffer. So, a simple bitmap operation becomes a primitive for writing beyond the bounds of allocated memory.

Here's what an attacker could do on Windows, step by step

1. Build a Malicious Web Page: The attacker makes a site that uses WebGL, SVG, or canvas, feeding large dimensions to a graphics API.
2. Trigger Integer Overflow: The page submits bitmap data where the calculated buffer size overflows (becomes much smaller than expected).
3. Heap-Based Buffer Overflow: Firefox allocates too small a buffer and copies large amounts of data into it. This can:

Manipulate internal browser data or pointers

4. Bypass Sandbox (theoretical): With enough control, the attacker could chain this with other vulnerabilities or Windows kernel exploits to escape the Firefox sandbox, giving them powerful execution rights on the user's system.

Why Sandbox Escapes Matter

Mozilla, Chrome, and other browsers use OS-level sandboxes to keep web content away from the rest of your machine. A buffer overflow like this is a crack in that wall. What starts as a graphics bug in rendering can end up as a door for malware to steal files, logins, or take over your system.

Here’s a STRIPPED skeleton that demonstrates the concept (for educational purposes only!)

// Simplified demo - Not a real PoC, but illustrates the overflow logic
let canvas = document.createElement('canvas');
let big = x40000000; // Large value to force overflow

canvas.width = big;
canvas.height = big;

let ctx = canvas.getContext('2d');

// Try to trigger buffer calculation inside Firefox graphics
ctx.drawImage(anotherLargeImage, , , big, big);

A real attack would combine this primitive with further memory manipulation, ASLR bypass, and a chain of exploits. This code shows how untrusted content might try to interact with vulnerable code.

Firefox < 117 (for regular users)

- Firefox ESR < 102.15 or < 115.2 (business/managed versions)

Thunderbird < 102.15 or < 115.2 (email clients with HTML rendering)

If you use Firefox or Thunderbird on Linux or macOS, you're in the clear for this specific bug.

- Mozilla Security Advisory 2023-32
- CVE Details - CVE-2023-4576
- Firefox Release Notes: Version 117

Update Firefox and Thunderbird immediately to the latest version!

- Get Firefox
- Get Thunderbird

Summary

CVE-2023-4576 is a severe memory bug affecting only Windows users, but it shows how a sliver of unsafe code in graphics can open the door to broader, more dangerous attacks—potentially even breaking out of browsers’ sandboxes. If you're using an older Firefox or Thunderbird on Windows, update now to keep your system safe.


*This post is a unique, human-written explanation for educational purposes. For technical specifics, always review the official advisories and patch notes directly from Mozilla.*

Timeline

Published on: 09/11/2023 09:15:00 UTC
Last modified on: 09/13/2023 11:15:00 UTC