Cybersecurity researchers have unveiled a new attack dubbed the “C4 Bomb” that bypasses Google Chrome’s AppBound Cookie Encryption. This significant breakthrough exposes millions of users to renewed risks of cookie theft, credential compromise, and other data breaches. This comes despite Google’s recent efforts to harden Chrome against the growing threat of sophisticated infostealer malware. Google originally rolled out this new protection with Chrome version 127 in July 2024 to thwart these malware attacks.
The AppBound encryption added a dual-layer mechanism using both the user’s and the SYSTEM account’s Windows Data Protection API. Chrome delegated the decryption task to a privileged COM server to block access from low-privileged malware. However, the C4 Bomb attack leverages a classic cryptographic weakness known as a padding oracle attack. It exploits subtle flaws in how DPAPI handles padding, using Windows Event Logs as an oracle to guess correctly. This allows attackers with only low-privileged access to systematically decrypt the protected cookie blobs without needing administrator rights.
The public disclosure of the C4 Bomb comes amid growing concern over the rapid pace of infostealer malware adaptations.
Recent months have seen malware families like Lumma, Meduza, and Vidar implement their own unique bypasses. These other malware families have been using techniques that range 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 for many users. This makes advanced cookie theft accessible to less sophisticated threat actors and increases the urgency for Google to respond.
This C4 Bomb attack highlights the ongoing cat-and-mouse game between browser developers and today’s very creative cybercriminals.
While AppBound Encryption certainly raised the bar for attackers, the latest research demonstrates even sophisticated protections can be undermined. Users and especially enterprises are now being urged to remain vigilant, update security tools, and avoid storing credentials in browsers. This is necessary until more robust and permanent defensive security measures can finally be put in place.
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