CVE-2019-18198 in Linux
Summary
by MITRE • 01/25/2023
In the Linux kernel before 5.3.4, a reference count usage error in the fib6_rule_suppress() function in the fib6 suppression feature of net/ipv6/fib6_rules.c, when handling the FIB_LOOKUP_NOREF flag, can be exploited by a local attacker to corrupt memory, aka CID-ca7a03c41753.
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Analysis
by VulDB Data Team • 01/17/2024
The vulnerability identified as CVE-2019-18198 represents a critical reference count management flaw within the Linux kernel's IPv6 forwarding table implementation. This issue affects kernel versions prior to 5.3.4 and specifically targets the fib6_rule_suppress() function located in net/ipv6/fib6_rules.c. The flaw manifests when the kernel processes the FIB_LOOKUP_NOREF flag, which is designed to control reference counting behavior during IPv6 routing table lookups. The improper handling of reference counts creates a scenario where memory corruption can occur through local privilege escalation attacks. This vulnerability exemplifies a classic use-after-free condition that arises from incorrect reference count manipulation, making it particularly dangerous in multi-user environments where local attackers could leverage this weakness to gain elevated privileges.
The technical root cause of this vulnerability stems from a fundamental flaw in how the kernel manages object references during IPv6 routing table operations. When the fib6_rule_suppress() function processes routes with the FIB_LOOKUP_NOREF flag, it fails to properly account for reference counts, leading to situations where objects may be freed while still being referenced elsewhere in the system. This reference count error creates a window where memory can be overwritten or corrupted, potentially allowing an attacker to manipulate kernel memory structures. The vulnerability operates at the kernel level, meaning any local user with access to the system can exploit this flaw to execute arbitrary code with kernel privileges. The issue aligns with CWE-415: Double Free and CWE-416: Use After Free, as it involves improper reference counting that can lead to memory corruption through freed object reuse.
The operational impact of CVE-2019-18198 extends beyond simple privilege escalation, as it can potentially compromise the entire system integrity and confidentiality. Local attackers who successfully exploit this vulnerability can gain root-level access to affected systems, enabling them to modify system files, install persistent backdoors, or exfiltrate sensitive data. The attack vector is particularly concerning because it requires only local system access, making it accessible to users who may have legitimate login credentials but lack administrative privileges. This makes the vulnerability especially dangerous in multi-tenant environments, cloud deployments, or systems where users have legitimate access but should not possess elevated privileges. The vulnerability also aligns with ATT&CK technique T1068: Exploitation for Privilege Escalation, as it provides a direct path for local users to escalate their privileges to kernel level.
Mitigation strategies for this vulnerability primarily focus on kernel version updates and system hardening measures. The most effective immediate solution is upgrading to Linux kernel version 5.3.4 or later, where the reference count handling has been corrected. System administrators should also implement additional security controls such as disabling unnecessary IPv6 functionality when not required, implementing proper access controls, and monitoring for suspicious network activity that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper reference counting mechanisms in kernel space programming and highlights the need for thorough code review processes, particularly for functions that handle memory management and object lifecycle control. Organizations should also consider implementing kernel module signing, secure boot configurations, and regular security audits to detect and prevent similar vulnerabilities from being exploited in their environments.