CVE-2018-6557 in Linux
Summary
by MITRE • 01/25/2023
The MOTD update script in the base-files package in Ubuntu 18.04 LTS before 10.1ubuntu2.2, and Ubuntu 18.10 before 10.1ubuntu6 incorrectly handled temporary files. A local attacker could use this issue to cause a denial of service, or possibly escalate privileges if kernel symlink restrictions were disabled.
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Analysis
by VulDB Data Team • 05/04/2023
The vulnerability described in CVE-2018-6557 resides within the motd update script functionality of the base-files package in Ubuntu 18.04 LTS and 18.10 releases. This issue represents a classic temporary file handling flaw that can be exploited by local attackers to compromise system integrity. The problem specifically manifests in how the script manages temporary files during the motd update process, creating opportunities for privilege escalation or denial of service conditions.
The technical flaw stems from improper handling of temporary files within the MOTD (Message of the Day) update mechanism. When the system processes updates to the motd configuration, it creates temporary files that are susceptible to manipulation by local users. This vulnerability falls under CWE-377, which addresses insecure temporary file handling, and can be classified as a privilege escalation vector when combined with kernel symlink restrictions being disabled. The flaw allows attackers to potentially create symbolic links in strategic locations or overwrite critical system files through the compromised temporary file handling process.
The operational impact of this vulnerability extends beyond simple denial of service conditions to include potential privilege escalation capabilities. A local attacker with basic system access can exploit this weakness to execute arbitrary code with elevated privileges, particularly when kernel protections against symlink attacks are disabled. This creates a significant security risk for systems where local users might have access to the command line or other interactive interfaces. The vulnerability affects the core system components that manage user login messages, potentially allowing attackers to manipulate these messages to hide malicious activities or gain unauthorized access to system resources.
Mitigation strategies for CVE-2018-6557 should focus on applying the vendor-provided security patches that address the temporary file handling issues in the base-files package. System administrators should ensure that Ubuntu 18.04 LTS systems are updated to version 10.1ubuntu2.2 or later, and Ubuntu 18.10 systems to version 10.1ubuntu6 or later. Additionally, implementing proper kernel security measures such as enabling symlink restrictions and maintaining strict file permissions on system directories can significantly reduce the attack surface. The vulnerability aligns with ATT&CK technique T1068, which covers local privilege escalation, and T1499, covering endpoint denial of service, making it a critical concern for system administrators monitoring for potential compromise indicators.