CVE-2015-6997 in iOS
Summary
by MITRE
The X.509 certificate-trust implementation in Apple iOS before 9.1 does not recognize that the kSecRevocationRequirePositiveResponse flag implies a revocation-checking requirement, which makes it easier for man-in-the-middle attackers to spoof endpoints by leveraging access to a revoked certificate.
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Analysis
by VulDB Data Team • 06/24/2022
The vulnerability described in CVE-2015-6997 represents a critical flaw in Apple iOS certificate validation mechanisms that fundamentally undermines the trust model of secure communications. This issue affects iOS versions prior to 9.1 and stems from an improper interpretation of the kSecRevocationRequirePositiveResponse flag within the X.509 certificate trust implementation. The technical root cause lies in the operating system's failure to properly enforce certificate revocation checking when this specific flag is present, creating a significant security gap that attackers can exploit to bypass essential validation steps.
The operational impact of this vulnerability extends beyond simple certificate validation failures, as it enables sophisticated man-in-the-middle attacks that can successfully spoof legitimate endpoints. Attackers with access to revoked certificates can leverage this implementation flaw to establish fraudulent secure connections that appear legitimate to vulnerable iOS devices. This weakness specifically targets the certificate revocation checking process, which is designed to prevent the use of certificates that have been compromised, expired, or otherwise invalidated by certificate authorities. The vulnerability essentially allows attackers to circumvent the intended security controls that should prevent the acceptance of revoked certificates, even when the system is explicitly configured to require positive revocation responses.
From a cybersecurity perspective, this vulnerability aligns with CWE-295 which addresses improper certificate validation and represents a classic example of trust model bypass. The attack vector operates through the manipulation of certificate trust decisions, allowing adversaries to exploit the gap between the intended security behavior and the actual implementation. This flaw demonstrates the critical importance of proper flag interpretation in security-sensitive code and highlights how seemingly minor implementation details can have profound consequences for overall system security. The vulnerability's impact is particularly concerning given that it affects mobile device operating systems where users frequently engage in sensitive transactions and communications.
The mitigation strategy for this vulnerability requires immediate system updates to iOS 9.1 or later versions where Apple has corrected the certificate trust implementation. Organizations should also implement additional monitoring for suspicious certificate usage patterns and ensure that their security policies account for proper revocation checking procedures. Network administrators should consider implementing additional layers of security verification beyond certificate validation, particularly for high-value transactions and communications. The fix addresses the core issue by properly implementing the kSecRevocationRequirePositiveResponse flag behavior, ensuring that certificate validation correctly enforces revocation checking requirements when explicitly requested by the system configuration. This vulnerability serves as a reminder of the critical importance of thorough security testing and validation of cryptographic implementations, particularly those involving trust decisions that directly impact user security and privacy.