C4 Bomb Attack Explained | How Hackers Cracked Chrome’s AppBound Cookie Encryption in 2025

Table of Contents

• What is the C4 Bomb Attack?
• Why Was AppBound Encryption Introduced?
• How Does the C4 Bomb Attack Work?
• Alternate Exploit Path: COM Hijacking
• Google and Microsoft’s Response
• The Bigger Picture: Is It Time to Retire AES-CBC?
• Key Takeaways for Users and Organizations
• What Should You Do?
• Conclusion
•  Frequently Asked Questions (FAQs)

What is the C4 Bomb Attack?

The C4 Bomb Attack—short for Chrome Cookie Cipher Cracker—is a newly discovered vulnerability that bypasses Google Chrome’s AppBound Cookie Encryption, a security mechanism introduced in July 2024. Developed to protect user cookies from infostealer malware, AppBound Encryption was thought to be robust—until researchers uncovered a way to decrypt cookies using a padding oracle attack on the underlying Windows DPAPI system.

The C4 attack effectively turns a trusted security layer into a potential weakness by exploiting error messages and cryptographic design flaws, requiring no admin privileges.

Why Was AppBound Encryption Introduced?

To combat growing threats from infostealer malware and session hijacking tools, Google introduced AppBound Cookie Encryption, which:

• Applies dual-layer encryption using Windows DPAPI:

  • User-DPAPI (bound to user-level access)

  • SYSTEM-DPAPI (accessible only by SYSTEM-level services)

• Restricts cookie decryption to the original executable (i.e., Chrome)

• Limits session theft even if malware compromises the user profile

  
  

However, as the C4 attack demonstrates, layering security doesn’t guarantee protection if there’s a cryptographic loophole underneath.

How Does the C4 Bomb Attack Work?

1. Vulnerability in DPAPI’s Padding Oracle

C4 targets a padding oracle vulnerability in Microsoft’s DPAPI encryption when used with AES-CBC and PKCS7 padding.

The Chrome elevation service, a SYSTEM-level COM server, is responsible for decrypting cookies. When fed malformed ciphertext, it leaks information through differentiated error messages in Windows Event Logs:

• “MAC check failed” → valid padding, invalid signature

• “Unknown error” → invalid padding

This forms a padding oracle, which can be exploited to decrypt data block-by-block.

2. Step-by-Step Breakdown of the Attack

• Automated Process: 16-hour process using continuous IPC and Windows Event Log monitoring

• Decryption Technique: Attacker tweaks ciphertext and monitors padding error responses

• Layer Extraction: Once SYSTEM-DPAPI is cracked, the remaining User-DPAPI layer is decrypted using CryptUnprotectData

• Low Privileges Needed: No administrative access or elevated privileges are required

Alternate Exploit Path: COM Hijacking

Beyond the padding oracle, researchers also demonstrated a COM hijacking technique where:

• Chrome’s elevation service is redirected to non-existent COM objects

• This forces Chrome to revert to legacy cookie encryption

• Legacy encryption lacks SYSTEM binding, making cookie theft easier

• Downside: previously encrypted cookies become unreadable (data loss)

Google and Microsoft’s Response

Google’s Action:

• Vulnerability disclosed in December 2024

• Moved to “Accepted” status by February 2025

• Partial fix released in June 2025 (disabled by default)

• A comprehensive patch is planned for future versions of Chrome

Microsoft’s Response:

• Microsoft declined to patch DPAPI, citing:

  “Low practical exploitability under real-world conditions.”

Despite that, experts argue the attack’s reliability undermines that position, as repeated oracle-based attacks across systems continue to expose AES-CBC’s flaws.

The Bigger Picture: Is It Time to Retire AES-CBC?

The C4 attack isn't just a Chrome flaw—it’s a wider indictment of CBC encryption, a mode already criticized for:

• Bit-flipping vulnerabilities

• Oracle-based decryption attacks

• Dependency on secure padding handling

Security experts now advocate for modern authenticated encryption modes like AES-GCM or ChaCha20-Poly1305, which offer built-in integrity checks and resist padding oracles.

Key Takeaways for Users and Organizations

What Should You Do?

For Users:

• Update Chrome regularly and ensure security patches are applied

• Use full-disk encryption to mitigate credential theft

• Monitor suspicious browser behavior or unexpected logout sessions

For Developers:

• Refrain from using CBC mode in new implementations

• Ensure error messages are indistinguishable in cryptographic code

• Use authenticated encryption modes wherever possible

Conclusion

The C4 Bomb Attack is a reminder that cryptographic strength isn’t just about algorithms—it’s about implementation details and edge cases. While Chrome’s dual-layer encryption was a step forward, it was only as strong as the underlying platform allowed.

As security evolves in layers, transparency, patching, and cryptographic hygiene remain the strongest defense.

 FAQs 

What is the C4 Bomb Attack?

The C4 Bomb Attack is a method to decrypt Chrome’s AppBound cookie encryption using a padding oracle vulnerability in DPAPI.

Why is the C4 Bomb Attack significant?

It allows attackers to steal encrypted cookies without admin access, defeating Google’s AppBound cookie security.

What does C4 stand for in this context?

C4 stands for Chrome Cookie Cipher Cracker.

What vulnerability does the C4 attack exploit?

It exploits a padding oracle vulnerability in the Windows Data Protection API (DPAPI).

What encryption method is exploited in the C4 attack?

AES in CBC (Cipher Block Chaining) mode with PKCS7 padding.

How long does the C4 attack take to execute?

Approximately 16 hours of automated execution.

Does the C4 attack require administrative privileges?

No, the attack works with user-level access.

What part of Chrome’s security is bypassed?

The AppBound Cookie Encryption, which is designed to bind cookies to Chrome and block malware access.

What is a padding oracle attack?

It’s a cryptographic attack that uses error messages to decrypt encrypted data block-by-block.

How does Windows Event Viewer help the C4 attack?

It leaks different error messages for valid vs. invalid padding, allowing the oracle to work.

What does DPAPI stand for?

Data Protection API, a Windows feature for encrypting sensitive data.

What is AppBound Cookie Encryption?

A dual-layer encryption system in Chrome designed to prevent malware from accessing session cookies.

Can the attack be mitigated currently?

Google released a partial fix in June 2025, but it’s disabled by default.

What are the two layers in Chrome’s cookie encryption?

User-DPAPI and SYSTEM-DPAPI.

What is COM hijacking in this context?

A method to force Chrome to revert to legacy encryption by redirecting its elevation service.

What happens if COM hijacking is used?

Chrome loses access to previously encrypted cookies but becomes more vulnerable.

What is CryptUnprotectData?

A Windows API used to decrypt DPAPI-protected data.

Is Microsoft patching the DPAPI vulnerability?

No, Microsoft has declined to patch it, citing low real-world exploitability.

Why is CBC encryption mode criticized?

Because it is prone to padding oracle and bit-flipping attacks.

What could attackers do with decrypted cookies?

They could hijack browser sessions, access accounts, and bypass 2FA.

What is the purpose of the elevation service in Chrome?

It handles SYSTEM-level decryption requests for AppBound cookies.

What is the difference between SYSTEM-DPAPI and User-DPAPI?

SYSTEM-DPAPI is more secure and used by system processes, while User-DPAPI is accessible by user-level apps.

Can malware use the C4 attack?

Yes, malware could automate the attack to steal user sessions without triggering defenses.

What should users do to stay protected?

Keep Chrome updated and monitor for security patches and suspicious behavior.

What should enterprises do?

Implement cookie protection monitoring, restrict inter-process communication, and use endpoint detection.

Is this vulnerability actively exploited in the wild?

As of now, there are no known widespread real-world exploits, but the threat is real.

Who discovered the C4 vulnerability?

CyberArk researchers uncovered and reported the attack in December 2024.

When did Google acknowledge the vulnerability?

In February 2025, with a partial fix released in June 2025.

Why is this called a 'bomb' attack?

Due to the impact and destruction of cookie security, akin to dropping a digital bomb on Chrome’s encryption model.

What is the future of CBC mode in encryption?

Security experts suggest deprecating CBC in favor of more secure AEAD modes like AES-GCM.