CVE-2006-5749 in Linux
Summary
by MITRE
The isdn_ppp_ccp_reset_alloc_state function in drivers/isdn/isdn_ppp.c in the Linux 2.4 kernel before 2.4.34-rc4 does not call the init_timer function for the ISDN PPP CCP reset state timer, which has unknown attack vectors and results in a system crash.
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Analysis
by VulDB Data Team • 07/15/2019
The vulnerability identified as CVE-2006-5749 represents a critical flaw in the Linux 2.4 kernel's ISDN PPP implementation that exposes systems to potential denial of service attacks. This issue specifically affects the isdn_ppp_ccp_reset_alloc_state function within the drivers/isdn/isdn_ppp.c file, where a fundamental initialization error occurs that can lead to system instability and complete crashes. The vulnerability exists in kernel versions prior to 2.4.34-rc4, indicating that this was a well-known issue that required specific kernel version updates to address. The flaw demonstrates a classic case of improper resource management where critical kernel components fail to initialize properly, creating a pathway for system exploitation.
The technical root cause of this vulnerability stems from the failure to properly initialize a timer structure within the ISDN PPP CCP reset state management code. When the isdn_ppp_ccp_reset_alloc_state function executes, it attempts to set up a timer for handling ISDN PPP CCP reset operations but neglects to invoke the init_timer function. This omission means that the timer structure remains in an uninitialized state, containing unpredictable values that can cause kernel memory corruption when the timer is subsequently used. The timer structure in question is essential for managing the timing of reset operations in ISDN PPP connections, and its improper initialization creates a condition where kernel memory is accessed in an undefined manner. This type of vulnerability falls under CWE-254, which encompasses security weaknesses related to improper initialization of resources, and specifically relates to CWE-457, which deals with use of uninitialized variables in contexts where initialization is required.
The operational impact of this vulnerability manifests as system crashes and potential denial of service conditions that can affect network connectivity and overall system stability. When an attacker can trigger the specific code path that leads to the uninitialized timer usage, the kernel experiences a critical failure that results in a system crash or panic. This vulnerability is particularly concerning because ISDN connections were commonly used in enterprise environments for reliable network connectivity, and a crash in this subsystem could lead to significant service disruption. The attack vectors for this vulnerability are not well-documented, suggesting that it may have been exploitable through specific network traffic patterns or connection scenarios that would activate the flawed code path. The lack of specific attack vector information indicates that this was likely a latent issue that required careful exploitation or specific conditions to trigger.
From a threat modeling perspective, this vulnerability aligns with ATT&CK technique T1499.004 which covers network denial of service attacks, and represents a classic example of how kernel-level flaws can be leveraged to create system instability. The vulnerability demonstrates the importance of proper kernel initialization procedures and highlights the risks associated with complex kernel subsystems that manage network protocols. Organizations with legacy Linux 2.4 systems that still utilize ISDN connections were particularly vulnerable to this issue, as the kernel versions affected had not received the necessary security patches to address this initialization flaw. The vulnerability also underscores the importance of kernel security testing and the need for comprehensive code review processes that ensure all resources are properly initialized before use. This flaw serves as a reminder that even seemingly minor initialization errors in kernel code can have catastrophic effects on system stability and security.
The recommended mitigation strategy for this vulnerability involves immediate kernel version updates to 2.4.34-rc4 or later, which contain the necessary fixes for the uninitialized timer issue. System administrators should prioritize patching affected systems and verify that the specific kernel version includes the corrected implementation of the isdn_ppp_ccp_reset_alloc_state function. Additionally, organizations should conduct thorough security assessments of their legacy systems to identify other potential kernel-level vulnerabilities that may exist in older kernel versions. The vulnerability also emphasizes the importance of maintaining current kernel security patches and implementing robust security monitoring to detect potential exploitation attempts. For environments where upgrading the kernel is not immediately feasible, administrators should consider implementing network segmentation and access controls to limit exposure to potential attackers who might attempt to exploit this vulnerability through ISDN connections.