CVE-2015-5965 in FortiOS
Summary
by MITRE
The SSL-VPN feature in Fortinet FortiOS before 4.3.13 only checks the first byte of the TLS MAC in finished messages, which makes it easier for remote attackers to spoof encrypted content via a crafted MAC field.
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Analysis
by VulDB Data Team • 06/07/2022
The vulnerability identified as CVE-2015-5965 affects Fortinet FortiOS versions prior to 4.3.13 and specifically targets the SSL-VPN functionality within the operating system. This issue resides in the implementation of Transport Layer Security protocols where the system fails to properly validate the Message Authentication Code (MAC) values in TLS finished messages. The flaw represents a critical weakness in the cryptographic verification process that underpins secure communications through the SSL-VPN tunnel.
The technical implementation flaw stems from the insufficient validation of MAC fields during the TLS handshake process. When TLS finished messages are exchanged between client and server, these messages contain MAC values that serve as cryptographic proof of message integrity and authenticity. In the affected FortiOS versions, the system only examines the first byte of the MAC field rather than validating the complete MAC value. This partial validation creates a significant security gap that allows attackers to manipulate the MAC field while maintaining the appearance of valid authentication.
This vulnerability falls under the category of cryptographic weakness and can be categorized as CWE-310, which deals with Cryptographic Issues. The operational impact of this flaw is substantial as it enables remote attackers to perform man-in-the-middle attacks or content spoofing operations against SSL-VPN connections. An attacker who can manipulate the MAC field could potentially inject malicious content into encrypted communications or alter existing data without detection, effectively undermining the confidentiality and integrity guarantees that SSL-VPN is designed to provide.
The attack vector for this vulnerability is particularly concerning as it enables remote exploitation without requiring authentication to the target system. Attackers can craft specially formatted MAC fields that pass the first-byte validation check while containing malicious content. This weakness directly relates to the MITRE ATT&CK framework's technique T1071.004, which covers Application Layer Protocol: DNS, and can be leveraged for broader network infiltration activities. The vulnerability essentially creates a backdoor for attackers to compromise the secure communication channels that organizations rely upon for remote access.
The remediation for this vulnerability requires upgrading to FortiOS version 4.3.13 or later, which implements proper full MAC validation. Organizations should also consider implementing additional monitoring mechanisms to detect anomalous MAC validation patterns and establish network segmentation to limit the potential impact of successful exploitation. Security teams should conduct comprehensive vulnerability assessments of their SSL-VPN infrastructure and review access controls to ensure that any potential compromise is contained. The fix addresses the root cause by implementing complete MAC field validation as required by industry standards for secure TLS implementation.