CVE-2025-4609 - Exploiting Mojo Handle Confusion in Chrome for Sandbox Escape (with Code Example)

A serious vulnerability, now tracked as CVE-2025-4609, hit Google Chrome on Windows recently. Before version 136..7103.113, Chrome had a bug in the Mojo IPC (Inter-Process Communication) framework which could let an attacker break out of Chrome's sandbox with a cleverly crafted file. Google rated this issue High on their security scale.

This post explains what went wrong, how the exploit might work (with code), and explains the fix. All explanations use simple, straight-forward language.

What Is Mojo in Chrome?

Mojo is a system inside Chromium that lets different parts of the browser safely talk to each other. It uses "handles," which are like references to resources (such as files or communication channels). The security of Chrome relies on these handles being managed correctly. If not, dangerous things can happen.

Affected Versions: Chrome for Windows, before 136..7103.113.

- Risk: Allows a remote attacker to potentially escape the browser sandbox via a malicious file.

References

- Chromium release notes
- CVE entry at NIST
- Chromium bug tracker (restricted)

How The Attack Works

By tricking Chrome into accepting an inappropriate handle (maybe a file or pipe handle), a remote attacker could get the browser process to read or write outside the sandbox, possibly elevating their privileges or running code on the user's machine.

Representative Exploit Pseudocode

*Note: This is an educational example, not real exploit code. The actual bug is in the way Chrome passes handles via Mojo, but here’s a simplified illustration!*

# Malicious file structure (simplified)
# The attacker embeds a payload that, when parsed, tricks Mojo into passing a privileged system handle

malicious_payload = {
    "MojoHandle": "xDEADBEEF",  # Invalid or forged handle value
    "Action": "invoke_privileged_operation"
}

# The exploit might trigger something like this when Chrome opens the file:

def on_file_open(payload):
    handle = parse_handle(payload["MojoHandle"])
    # Chrome fails to validate 'handle', passes it to privileged process
    do_mojo_ipc(handle, payload["Action"])

# The privileged process now treats this attacker-controlled handle as legitimate,
# potentially exposing the system.

*The above code is a conceptual example for learning only.*

In real attacks, hackers could use this bug to

- Break out of the Chrome sandbox (the security zone meant to protect the rest of your device from web threats).

Steal data, install malware, or pivot to other attacks.

All that could happen just by opening a specially-crafted file—no other user action needed.

Patch and Fix

Google's quick response means that Chrome version 136..7103.113 and above are no longer affected. The patch ensures that only the correct handles are passed between processes, and extra validation is performed before acting on any handle in Mojo.

Conclusion

CVE-2025-4609 is proof that even small mistakes—like mishandling a simple "handle"—can have serious security consequences. Chrome’s rapid fix protected millions, but it’s a good reminder: always keep your browser up to date and be careful with files you open.

Stay safe online!

*For security professionals or researchers: Always test on isolated systems and with permission only. This post is for learning and awareness.*

Timeline

Published on: 08/22/2025 21:05:46 UTC
Last modified on: 08/25/2025 20:44:35 UTC