CVE-2016-7541 in FortiOS
Summary
by MITRE
Long lived sessions in Fortinet FortiGate devices with FortiOS 5.x before 5.4.0 could violate a security policy during IPS signature updates when the FortiGate's IPSengine is configured in flow mode. All FortiGate versions with IPS configured in proxy mode (the default mode) are not affected.
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Analysis
by VulDB Data Team • 01/11/2020
The vulnerability described in CVE-2016-7541 represents a critical security flaw in Fortinet FortiGate devices running FortiOS version 5.x before 5.4.0. This issue specifically impacts devices configured with IPS engine operating in flow mode, creating a window of opportunity for security policy violations during the dynamic update process of IPS signatures. The flaw stems from the improper handling of long-lived sessions when the IPS engine processes signature updates, potentially allowing malicious traffic to bypass security controls that should be actively enforced.
The technical root cause of this vulnerability lies in the interaction between the IPS engine's flow mode configuration and the dynamic signature update mechanism. When FortiGate devices operate in flow mode, they maintain stateful connections and process traffic based on flow characteristics rather than individual packet inspection. During IPS signature updates, the system's handling of existing long-lived sessions becomes problematic, as the security policy enforcement may be temporarily disrupted or bypassed. This creates a scenario where traffic that should be subject to active IPS inspection and policy enforcement can pass through the device without proper security controls being applied. The vulnerability is particularly concerning because it operates silently during the update process, making detection difficult and potentially allowing persistent threats to exploit the window of vulnerability.
The operational impact of CVE-2016-7541 extends beyond simple policy bypass scenarios, creating potential attack vectors that align with several ATT&CK tactics including privilege escalation and defense evasion. Organizations running affected FortiGate devices face significant risk during IPS signature update windows, as malicious actors could potentially exploit this vulnerability to establish persistent access or conduct data exfiltration activities. The vulnerability affects the core security functionality of the device, potentially allowing attackers to bypass network intrusion prevention systems that are meant to protect against known attack patterns. This represents a fundamental failure in the security policy enforcement mechanism during critical maintenance operations, undermining the trust placed in the device's security controls. The risk is compounded by the fact that these long-lived sessions are typically established for legitimate business purposes, making the vulnerability particularly insidious as it operates within normal network traffic patterns.
Mitigation strategies for CVE-2016-7541 should prioritize immediate patching to FortiOS version 5.4.0 or later, which addresses the core issue in the IPS engine's handling of long-lived sessions during signature updates. Organizations should also consider temporarily switching affected devices to proxy mode if flow mode is not strictly required for their operational needs, as proxy mode is not affected by this vulnerability according to the advisory. Network administrators should implement monitoring solutions to detect unusual traffic patterns during IPS update windows and establish procedures for verifying the integrity of security policies during maintenance operations. Additionally, organizations should review their IPS signature update schedules to minimize the exposure window and ensure that updates are performed during planned maintenance periods when network traffic is predictable. The vulnerability highlights the importance of maintaining current security firmware versions and demonstrates the critical need for thorough testing of security policy enforcement mechanisms during system maintenance operations, aligning with cybersecurity best practices outlined in standards such as nist 800-53 and iso 27001.