New C4 Bomb Attack Breaks Through Chrome’s AppBound Cookie Protections

Cybersecurity researchers have unveiled a new attack—dubbed the “C4 Bomb” (Chrome Cookie Cipher Cracker)—that successfully bypasses Google Chrome’s much-touted AppBound Cookie Encryption.

This breakthrough exposes millions of users to renewed risks of cookie theft, credential compromise, and potential data breaches, despite Google’s recent efforts to harden Chrome against infostealer malware.

AppBound Cookie Encryption

In July 2024, Google rolled out AppBound Cookie Encryption with Chrome version 127, aiming to thwart a wave of malware targeting browser cookies.

According to the CyberArk report, the new protection added a dual-layer encryption mechanism: cookies were encrypted first using the logged-in user’s Windows Data Protection API (DPAPI), and then again using the SYSTEM account’s DPAPI.

To decrypt cookies, Chrome delegated the task to a privileged COM server (the “elevation service”), which checked that requests originated from the legitimate Chrome process, theoretically blocking low-privileged malware from accessing sensitive data.

Despite these layered defenses, researchers discovered a critical vulnerability in the encryption flow. By leveraging a classic cryptographic weakness known as a padding oracle attack, the C4 Bomb allows attackers—even those with only low-privileged access—to systematically decrypt the protected cookie blobs.

The attack exploits subtle flaws in how DPAPI handles padding and error reporting, using Windows Event Logs as an “oracle” to guess the correct padding and gradually recover the encrypted data1.

The C4 technique involves repeatedly sending modified versions of the encrypted cookie key to the elevation service and observing the resulting error messages.

Over thousands of iterations, the attacker can reconstruct the SYSTEM-encrypted key, then use standard user-level decryption to obtain the final cookie key and access all stored cookies—without ever needing administrator rights.

The public disclosure of the C4 Bomb comes amid growing concern over the rapid pace at which infostealer malware has adapted to Chrome’s new protections.

Recent months have seen several malware families—such as Lumma, Meduza, Vidar, and WhiteSnake—implement their own bypasses, using techniques ranging from direct process injection to exploiting privilege escalation flaws.

The release of open-source tools that automate the C4 attack has further heightened the risk, making advanced cookie theft accessible to less sophisticated threat actors and increasing the urgency for Google and the wider security community to develop new countermeasures.

A Chrome security engineer acknowledged the challenge: “As the malware landscape continually evolves, we are keen to continue engaging with others in the security community on improving detections and strengthening operating system protections for any bypasses”.

The C4 Bomb attack highlights the ongoing cat-and-mouse game between browser developers and cybercriminals.

While AppBound Encryption raised the bar for attackers, the latest research demonstrates that even sophisticated protections can be undermined by creative cryptographic attacks.

Users—especially enterprises—are urged to remain vigilant, update security tools, and avoid storing sensitive credentials in browsers until more robust defenses are in place

Find this News Interesting! Follow us on Google News, LinkedIn, and X to Get Instant Updates

Keep your files stored safely and securely with the SanDisk 2TB Extreme Portable SSD. With over 69,505 ratings and an impressive 4.6 out of 5 stars, this product has been purchased over 8K+ times in the past month. At only $129.99, this Amazon’s Choice product is a must-have for secure file storage.

Help keep private content private with the included password protection featuring 256-bit AES hardware encryption. Order now for just $129.99 on Amazon!

Help Power Techcratic’s Future – Scan To Support

If Techcratic’s content and insights have helped you, consider giving back by supporting the platform with crypto. Every contribution makes a difference, whether it’s for high-quality content, server maintenance, or future updates. Techcratic is constantly evolving, and your support helps drive that progress.

As a solo operator who wears all the hats, creating content, managing the tech, and running the site, your support allows me to stay focused on delivering valuable resources. Your support keeps everything running smoothly and enables me to continue creating the content you love. I’m deeply grateful for your support, it truly means the world to me! Thank you!