CVE-2025-34091: 
Google Chrome vulnerability analysis and mitigation

A padding oracle vulnerability exists in Google Chrome's AppBound cookie encryption mechanism due to observable decryption failure behavior in Windows Event Logs when handling malformed ciphertext in SYSTEM-DPAPI-encrypted blobs. The vulnerability was discovered in July 2025 and affects Google Chrome with AppBound Encryption enabled, as well as potentially other Chromium-based browsers that implement similar COM-based encryption mechanisms (VulnCheck Advisory, CyberArk Blog).

The vulnerability exploits a padding oracle attack against Chrome's AppBound cookie encryption implementation. A local attacker can repeatedly send malformed ciphertexts to the Chrome elevation service and distinguish between padding and MAC errors through Windows Event Log messages. This allows partial decryption of the SYSTEM-DPAPI layer and eventual recovery of the user-DPAPI encrypted cookie key. The attack, dubbed C4 (Chrome Cookie Cipher Cracker), takes approximately 16 hours to complete due to multiple file operations and IPC requests required for each guess. The vulnerability has been assigned a CVSS v4.0 base score of 8.8 HIGH with vector CVSS:4.0/AV:L/AC:H/AT:P/PR:L/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H (VulnCheck Advisory).

This vulnerability undermines the core purpose of AppBound Encryption by enabling low-privileged cookie theft through cryptographic misuse and verbose error feedback. Additionally, the attack technique can potentially be used to decrypt any SYSTEM-DPAPI encrypted data, expanding the impact beyond just Chrome cookies (CyberArk Blog).

As of June 23rd, 2025, Google has implemented a partial solution in Chrome but it is disabled by default. A full solution is planned for a future release. No specific workarounds have been published (CyberArk Blog).

Microsoft initially rejected the vulnerability report, citing low practical exploitability due to environmental constraints. However, the security community, through CyberArk's research, has demonstrated the significance of the vulnerability and its broader implications for DPAPI encryption in Windows (CyberArk Blog).