CVE-2017-13686 in Linux
Summary
by MITRE
net/ipv4/route.c in the Linux kernel 4.13-rc1 through 4.13-rc6 is too late to check for a NULL fi field when RTM_F_FIB_MATCH is set, which allows local users to cause a denial of service (NULL pointer dereference) or possibly have unspecified other impact via crafted system calls. NOTE: this does not affect any stable release.
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Analysis
by VulDB Data Team • 12/16/2022
The vulnerability described in CVE-2017-13686 represents a critical NULL pointer dereference flaw within the Linux kernel's routing subsystem, specifically in the net/ipv4/route.c file. This issue affects kernel versions 4.13-rc1 through 4.13-rc6, making it a regression that was introduced during the development cycle of what would become kernel version 4.13. The flaw manifests when the RTM_F_FIB_MATCH flag is set during routing table operations, creating a window where the fi field remains unvalidated before being dereferenced. This timing issue creates a race condition in the kernel's routing table processing logic, where the system fails to properly validate the existence of a routing information field before attempting to access it. The vulnerability is particularly concerning because it occurs in kernel space, where improper handling of memory references can lead to system crashes or potentially more severe consequences.
The technical implementation of this vulnerability stems from improper input validation within the kernel's routing table processing code. When the RTM_F_FIB_MATCH flag is present, the system should validate that the fi (flow information) field is properly initialized before proceeding with operations. However, the kernel's implementation delays this validation until after certain processing steps have already occurred, creating a scenario where a NULL pointer dereference can occur. This flaw falls under CWE-476 which specifically addresses NULL pointer dereference conditions, and it represents a classic example of improper validation of input data in kernel space. The vulnerability operates at the network routing layer, specifically affecting the IPv4 routing table management functionality where the kernel processes routing messages and maintains forwarding information.
The operational impact of this vulnerability extends beyond simple denial of service, though that is the primary immediate consequence. Local users with access to the system can exploit this flaw to trigger a kernel NULL pointer dereference, causing the system to crash and potentially leading to a complete system hang or reboot. This creates a denial of service condition that affects the availability of network services and system functionality. The vulnerability's potential for unspecified other impacts suggests that in certain configurations or under specific conditions, attackers might be able to escalate privileges or access other system resources. According to ATT&CK framework, this vulnerability aligns with T1499.004 which covers network denial of service attacks, and T1068 which addresses local privilege escalation through kernel exploits. The fact that this vulnerability exists only in development releases and not in stable versions indicates that it was likely caught during the testing phase but may have been overlooked due to its local nature and the complexity of kernel debugging.
The mitigation approach for CVE-2017-13686 involves immediate patching of affected kernel versions, as this vulnerability was addressed in subsequent kernel releases. System administrators should upgrade to kernel versions 4.13 or later where the fix has been implemented, as the patch corrects the timing issue by ensuring proper validation of the fi field before any dereference operations occur. Additionally, monitoring for unusual system behavior or kernel crashes that might indicate exploitation attempts is recommended. The fix typically involves reordering the validation steps in the routing table processing code to ensure that all required fields are properly initialized before any operations are performed on them. Organizations running kernel versions in the affected range should implement immediate patch management procedures and consider the potential for other similar timing-based validation issues in kernel networking code. This vulnerability serves as a reminder of the importance of rigorous testing for race conditions and proper input validation in kernel space operations, particularly in critical subsystems like routing table management that handle high volumes of network traffic and system calls.