CVE-2018-6925 in FreeBSD
Summary
by MITRE
In FreeBSD before 11.2-STABLE(r338986), 11.2-RELEASE-p4, 11.1-RELEASE-p15, 10.4-STABLE(r338985), and 10.4-RELEASE-p13, due to improper maintenance of IPv6 protocol control block flags through various failure paths, an unprivileged authenticated local user may be able to cause a NULL pointer dereference causing the kernel to crash.
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Analysis
by VulDB Data Team • 03/28/2020
The vulnerability identified as CVE-2018-6925 represents a critical kernel-level issue affecting FreeBSD operating systems across multiple release versions. This flaw manifests as a NULL pointer dereference within the IPv6 protocol control block management system, creating a potential for system instability and denial of service conditions. The vulnerability specifically impacts systems running FreeBSD versions prior to 11.2-STABLE(r338986), 11.2-RELEASE-p4, 11.1-RELEASE-p15, 10.4-STABLE(r338985), and 10.4-RELEASE-p13, making it a widespread concern for organizations maintaining legacy FreeBSD installations.
The technical root cause of this vulnerability stems from inadequate handling of IPv6 protocol control block flags during error conditions and failure paths within the kernel's networking subsystem. When certain network operations fail or encounter exceptional conditions, the system fails to properly maintain the state of IPv6 control blocks, leading to situations where pointers that should reference valid memory locations become NULL. This improper state management creates a scenario where subsequent kernel operations attempt to dereference these NULL pointers, resulting in immediate system crashes and potential kernel panics. The flaw is particularly concerning because it occurs within the kernel space, meaning that exploitation can lead to complete system compromise.
The operational impact of CVE-2018-6925 extends beyond simple system crashes, as it represents a privilege escalation vector that can be leveraged by authenticated local users. While the vulnerability requires local authentication, the potential for system instability makes it particularly dangerous in environments where multiple users have access to systems or where automated services operate with elevated privileges. The vulnerability aligns with CWE-476 which describes NULL pointer dereference conditions, and can be categorized under the ATT&CK technique T1068 for local privilege escalation. Organizations running affected FreeBSD versions face significant risk of service disruption, potential data loss, and increased attack surface due to the inherent instability introduced by this flaw.
Mitigation strategies for CVE-2018-6925 primarily focus on immediate system updates and patches provided by FreeBSD developers. Organizations should prioritize upgrading to patched versions of FreeBSD, specifically targeting releases that include the fixes implemented in r338986 and subsequent patches. Additionally, system administrators should implement monitoring solutions to detect unusual kernel crash patterns that might indicate exploitation attempts. Network segmentation and privilege minimization practices can help reduce the potential impact of exploitation, though these measures do not address the underlying vulnerability. The fix implemented by FreeBSD developers addresses the core issue by ensuring proper maintenance of IPv6 protocol control block flags throughout all execution paths, including error handling routines, thereby preventing the NULL pointer dereference condition that previously occurred during network failure scenarios.