CVE-2018-7191 in Linux
Summary
by MITRE
In the tun subsystem in the Linux kernel before 4.13.14, dev_get_valid_name is not called before register_netdevice. This allows local users to cause a denial of service (NULL pointer dereference and panic) via an ioctl(TUNSETIFF) call with a dev name containing a / character. This is similar to CVE-2013-4343.
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Analysis
by VulDB Data Team • 09/21/2023
The vulnerability identified as CVE-2018-7191 represents a critical flaw in the Linux kernel's tun subsystem that affects versions prior to 4.13.14. This issue stems from a fundamental lack of input validation within the network device registration process, creating a pathway for local attackers to execute a denial of service attack through carefully crafted ioctl system calls. The vulnerability specifically targets the dev_get_valid_name function call sequence, which is crucial for validating network device names before device registration occurs. When an attacker submits a device name containing a forward slash character through the TUNSETIFF ioctl operation, the kernel fails to properly validate this input, leading to a cascade of failures in the device registration mechanism.
The technical exploitation of this vulnerability occurs through the improper handling of device names in the tun subsystem where the register_netdevice function is called without first validating the device name through dev_get_valid_name. This validation function serves as a critical gatekeeper that ensures device names meet specific criteria and do not contain characters that could lead to path traversal issues or other malformed inputs. When a device name containing a forward slash character is processed, the kernel's internal data structures become corrupted, resulting in a NULL pointer dereference that ultimately triggers a kernel panic. This panic effectively crashes the entire system, rendering it unavailable to legitimate users and applications. The vulnerability demonstrates a classic pattern of insufficient input validation that has been previously observed in similar issues such as CVE-2013-4343, indicating a recurring problem in kernel network subsystem development practices.
The operational impact of CVE-2018-7191 extends beyond simple service disruption to potentially compromise system availability in environments where local user access is possible. Attackers with local privileges can leverage this vulnerability to cause system-wide outages, making it particularly dangerous in multi-tenant environments or systems where local access cannot be strictly controlled. The vulnerability affects any system running Linux kernel versions earlier than 4.13.14, which includes numerous enterprise and embedded systems that may not have been updated to the patched versions. This creates a substantial attack surface across various deployment scenarios including servers, virtual machines, and containerized environments where the tun subsystem is utilized for network virtualization purposes. The vulnerability's classification aligns with CWE-20, which describes "Improper Input Validation" as a fundamental weakness that can lead to various security issues including denial of service conditions.
Mitigation strategies for CVE-2018-7191 primarily focus on kernel version updates, which provide the most effective solution by incorporating the proper input validation mechanisms that were missing in vulnerable versions. System administrators should prioritize updating their Linux kernels to version 4.13.14 or later to eliminate the vulnerability. Additionally, implementing network access controls to limit local user privileges can reduce the attack surface, though this approach provides only partial protection since the vulnerability is exploitable through local user accounts. The fix implemented in the patched kernel versions involves ensuring that dev_get_valid_name is called before register_netdevice, thereby preventing invalid device names from being processed and causing the subsequent NULL pointer dereference. This remediation aligns with ATT&CK technique T1499.004, which covers "Endpoint Denial of Service" and demonstrates the importance of proper input validation in preventing kernel-level exploits that can lead to system-wide disruptions. Organizations should also consider implementing monitoring solutions to detect unusual ioctl activity patterns that might indicate exploitation attempts, while maintaining comprehensive backup and recovery procedures to minimize downtime during potential attacks.