CVE-2021-25317 in Linux Enterprise Server
Summary
by MITRE • 05/05/2021
A Incorrect Default Permissions vulnerability in the packaging of cups of SUSE Linux Enterprise Server 11-SP4-LTSS, SUSE Manager Server 4.0, SUSE OpenStack Cloud Crowbar 9; openSUSE Leap 15.2, Factory allows local attackers with control of the lp users to create files as root with 0644 permissions without the ability to set the content. This issue affects: SUSE Linux Enterprise Server 11-SP4-LTSS cups versions prior to 1.3.9. SUSE Manager Server 4.0 cups versions prior to 2.2.7. SUSE OpenStack Cloud Crowbar 9 cups versions prior to 1.7.5. openSUSE Leap 15.2 cups versions prior to 2.2.7. openSUSE Factory cups version 2.3.3op2-2.1 and prior versions.
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Analysis
by VulDB Data Team • 05/07/2021
The vulnerability identified as CVE-2021-25317 represents a critical incorrect default permissions flaw within the Common Unix Printing System (CUPS) packaging across multiple SUSE and openSUSE distributions. This issue specifically impacts systems where local attackers with lp group membership can exploit the flawed permission model to create files with root ownership and 0644 permissions. The vulnerability stems from improper default file creation permissions that allow malicious users to establish files with elevated privileges while maintaining the ability to read and write content, though not execute it. This misconfiguration creates a significant privilege escalation vector that directly violates fundamental security principles of least privilege and proper access control enforcement.
The technical implementation of this vulnerability resides in the CUPS printing subsystem's file creation mechanisms where default permissions are not properly enforced during file generation operations. When users belonging to the lp group execute certain printing operations or file creation processes, the system creates files with root ownership but grants world-readable permissions, effectively allowing any local user to read the created files while maintaining root-level file ownership. This flaw specifically affects CUPS versions prior to 1.3.9 for SUSE Linux Enterprise Server 11-SP4-LTSS, 2.2.7 for SUSE Manager Server 4.0, 1.7.5 for SUSE OpenStack Cloud Crowbar 9, and 2.2.7 for openSUSE Leap 15.2, with the openSUSE Factory version being impacted up to 2.3.3op2-2.1. The underlying cause aligns with CWE-276, which addresses incorrect default permissions, and represents a classic case of insufficient access control enforcement in system-level services.
The operational impact of this vulnerability extends beyond simple privilege escalation as it enables attackers to potentially compromise system integrity and confidentiality. While the attacker cannot directly set file content, they can create files with root ownership and world-readable permissions, which may contain sensitive information or serve as staging points for further attacks. This vulnerability can be leveraged in combination with other techniques to establish persistent access or escalate privileges further. The attack surface is particularly concerning in enterprise environments where the lp group typically contains legitimate users who require printing capabilities but should not possess the ability to create files with root permissions. This vulnerability directly maps to ATT&CK technique T1068, which covers privilege escalation through local exploitation, and T1548.001, covering abuse of group permissions.
Mitigation strategies for CVE-2021-25317 should prioritize immediate patching of affected CUPS versions across all impacted distributions. Organizations must ensure that all systems running affected CUPS versions are updated to the latest stable releases that address the incorrect default permissions issue. System administrators should also conduct comprehensive audits of file permissions and ownership across all printing-related directories to identify any potentially compromised files. Additional defensive measures include implementing proper monitoring of file creation events in printing directories and establishing stricter access controls for the lp group membership. The remediation process should involve verifying that file creation operations now properly enforce default permissions and that files created by lp group members do not inherit root ownership with excessive permissions. Security teams should also consider implementing privilege separation mechanisms and ensuring that the principle of least privilege is properly enforced for all system services that handle user input or file operations.