CVE-2013-6891 in CUPS
Summary
by MITRE
lppasswd in CUPS before 1.7.1, when running with setuid privileges, allows local users to read portions of arbitrary files via a modified HOME environment variable and a symlink attack involving .cups/client.conf.
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Analysis
by VulDB Data Team • 12/23/2024
The vulnerability identified as CVE-2013-6891 affects the lppasswd utility within the Common Unix Printing System CUPS version prior to 1.7.1. This flaw represents a significant security weakness that exploits the setuid execution context of the lppasswd program, creating an avenue for local privilege escalation and information disclosure. The vulnerability specifically targets the handling of environment variables and file system interactions within the printing system's authentication framework.
The technical implementation of this vulnerability relies on a combination of environment variable manipulation and symbolic link exploitation. When lppasswd executes with setuid privileges, it processes the HOME environment variable to locate configuration files. An attacker can manipulate this variable to point to a crafted directory structure containing malicious symbolic links. The vulnerability specifically involves the .cups/client.conf file which is accessed through the modified HOME environment variable, allowing unauthorized file access through the symlink attack mechanism.
This vulnerability operates under the principle of insecure file handling and environment variable manipulation that falls under CWE-276, which addresses improper privileges. The attack vector demonstrates how setuid programs can be exploited when they do not properly validate or sanitize environment variables before using them to access system resources. The flaw essentially allows attackers to bypass normal file access controls by leveraging the elevated privileges of the setuid program to read files that would normally be restricted.
The operational impact of this vulnerability is substantial as it enables local users to potentially access sensitive information that should remain protected. The ability to read arbitrary files through this mechanism could expose authentication credentials, system configuration data, or other confidential information stored in files accessible to the user. This represents a critical information disclosure vulnerability that undermines the security model of the printing system and could provide attackers with additional attack surface for further exploitation.
The attack requires local system access and involves a specific sequence of steps including environment variable modification and symbolic link creation. According to ATT&CK framework, this vulnerability aligns with T1068, which covers 'Exploitation for Privilege Escalation', and T1005, which addresses 'Data from Local System'. The exploitation technique demonstrates how attackers can leverage existing system utilities with elevated privileges to access restricted resources.
Mitigation strategies for this vulnerability include upgrading to CUPS version 1.7.1 or later where the issue has been resolved through proper environment variable validation and file access controls. System administrators should ensure that all CUPS components are updated to patched versions and that proper access controls are maintained. Additionally, monitoring for unauthorized changes to environment variables and file system permissions can help detect potential exploitation attempts. The fix typically involves implementing proper validation of the HOME environment variable and ensuring that symbolic links are properly handled when accessing configuration files in setuid contexts.