CVE-2016-4028 in OX Guard
Summary
by MITRE
An issue was discovered in Open-Xchange OX Guard before 2.4.0-rev8. OX Guard uses an authentication token to identify and transfer guest users credentials. The OX Guard API acts as a padding oracle by responding with different error codes depending on whether the provided token matches the encryption padding. In combination with AES-CBC, this allows attackers to guess the correct padding. Attackers may run brute-forcing attacks on the content of the guest authentication token and discover user credentials. For a practical attack vector, the guest users needs to have logged in, the content of the guest user's "OxReaderID" cookie and the value of the "auth" parameter needs to be known to the attacker.
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Analysis
by VulDB Data Team • 10/08/2022
The vulnerability described in CVE-2016-4028 represents a critical padding oracle attack against the Open-Xchange OX Guard authentication system. This flaw exists in versions prior to 2.4.0-rev8 and specifically targets the way the system handles authentication tokens for guest users. The vulnerability stems from the API's inconsistent error responses when processing authentication tokens, creating a cryptographic oracle that can be exploited by attackers to determine valid padding in encrypted data. The issue manifests when the system returns different error codes based on whether the provided token matches the encryption padding, which directly violates fundamental security principles of cryptographic implementations where error responses should be constant time and indistinguishable to prevent such attacks.
The technical implementation of this vulnerability leverages the AES-CBC encryption mode, which is particularly susceptible to padding oracle attacks when the encryption implementation does not properly mask timing differences or error responses. The authentication token structure in OX Guard contains sensitive guest user credentials that are encrypted using AES-CBC, and the padding oracle vulnerability allows attackers to systematically guess the correct padding bytes through iterative attacks. This process enables the extraction of the complete authentication token content, ultimately leading to credential compromise. The attack requires specific preconditions including prior guest user authentication, knowledge of the OxReaderID cookie value, and access to the auth parameter value, which aligns with the attack pattern described in the attack technique matrix under the MITRE ATT&CK framework for credential access and exploitation of cryptographic weaknesses.
The operational impact of this vulnerability extends beyond simple credential theft, as it represents a significant weakening of the authentication security model for guest users within the Open-Xchange environment. Successful exploitation allows attackers to impersonate legitimate guest users and potentially gain access to restricted resources or data that these users can access. The vulnerability affects the confidentiality and integrity of user credentials stored within the authentication tokens, making it particularly dangerous for environments where guest access is enabled. Organizations using OX Guard versions prior to 2.4.0-rev8 face substantial risk of unauthorized access and potential data breaches, especially in scenarios where guest users have access to sensitive information or administrative functions. The vulnerability directly relates to CWE-129, which describes improper handling of padding in cryptographic operations, and represents a classic example of how seemingly minor implementation flaws in cryptographic protocols can lead to severe security consequences.
The recommended mitigations for this vulnerability involve immediate upgrading to OX Guard version 2.4.0-rev8 or later, which implements proper padding validation techniques that do not leak information through error responses. Organizations should also consider implementing additional security controls such as rate limiting on authentication attempts, monitoring for unusual authentication patterns, and ensuring that all cryptographic operations use constant-time comparison functions to prevent timing-based side channel attacks. The fix addresses the root cause by ensuring that the API responses are consistent regardless of whether the padding is correct, which aligns with the security principle that cryptographic implementations should not leak information through error responses. Additionally, organizations should conduct thorough security assessments of their authentication systems to identify similar padding oracle vulnerabilities in other cryptographic implementations and ensure that all security patches are applied promptly to maintain a robust security posture against known attack vectors.