CVE-2015-5736 in FortiClient
Summary
by MITRE
The Fortishield.sys driver in Fortinet FortiClient before 5.2.4 allows local users to execute arbitrary code with kernel privileges by setting the callback function in a (1) 0x220024 or (2) 0x220028 ioctl call.
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Analysis
by VulDB Data Team • 07/19/2025
The vulnerability identified as CVE-2015-5736 represents a critical privilege escalation flaw within the Fortishield.sys kernel driver component of Fortinet FortiClient security software. This issue affects versions prior to 5.2.4 and demonstrates a fundamental failure in input validation and privilege management within the kernel-mode driver interface. The vulnerability exists in the driver's handling of specific ioctl (input/output control) commands, specifically targeting the 0x220024 and 0x220028 control codes that are used for communication between user-mode applications and the kernel driver. The flaw allows local attackers who have already gained user-level access to manipulate the driver's callback function pointers, effectively enabling them to execute arbitrary code with the highest system privileges.
The technical exploitation of this vulnerability stems from improper validation of user-supplied data within the ioctl handler functions of the Fortishield.sys driver. When legitimate user-mode applications make ioctl calls to these specific control codes, the driver fails to properly sanitize or validate the parameters passed in the input buffer. This lack of input validation creates an opportunity for attackers to overwrite critical function pointers or callback mechanisms within the driver's memory space. The vulnerability is particularly dangerous because it operates at kernel level, meaning that successful exploitation grants full system compromise including the ability to bypass all operating system security mechanisms, access encrypted data, modify system files, and establish persistent backdoors. The attack vector requires only local user access since the vulnerability is not exposed over the network but rather through legitimate driver interfaces that are accessible to local processes.
From an operational perspective, this vulnerability presents a severe threat to organizations relying on FortiClient for endpoint protection, as it allows attackers who have already compromised a user account to escalate their privileges to SYSTEM level without requiring additional attack vectors. The impact extends beyond simple privilege escalation since kernel-level access enables complete system compromise, data exfiltration, and potential lateral movement within the network. The vulnerability aligns with CWE-121, which describes "Stack-based Buffer Overflow" and the broader category of improper input validation in kernel drivers, and maps to ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation." Organizations using vulnerable versions of FortiClient are particularly at risk because the attack can be executed without requiring specialized tools or advanced knowledge of system internals, making it a popular target for both automated exploitation tools and sophisticated attackers.
The recommended mitigations for this vulnerability include immediate deployment of Fortinet FortiClient version 5.2.4 or later, which contains patches addressing the input validation flaws in the ioctl handler functions. System administrators should also implement additional security controls such as disabling unnecessary kernel driver interfaces, monitoring for suspicious ioctl activity, and ensuring that only authorized applications can communicate with the vulnerable driver. Network segmentation and privilege separation practices should be reinforced to limit the potential impact of successful exploitation. The vulnerability also highlights the importance of kernel driver security auditing and proper input validation practices, particularly in security software that operates with elevated privileges. Organizations should conduct thorough vulnerability assessments of other security software components and consider implementing runtime application control measures to prevent unauthorized code execution within kernel space. The remediation process must include comprehensive testing to ensure that the patch does not introduce compatibility issues with existing security policies or legitimate system functionality while maintaining the integrity of the endpoint protection framework.