CVE-2017-18188 in opentmpfiles
Summary
by MITRE
OpenRC opentmpfiles through 0.1.3, when the fs.protected_hardlinks sysctl is turned off, allows local users to obtain ownership of arbitrary files by creating a hard link inside a directory on which "chown -R" will be run.
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Analysis
by VulDB Data Team • 01/04/2020
The vulnerability CVE-2017-18188 affects OpenRC's opentmpfiles utility version 0.1.3 and earlier, presenting a significant local privilege escalation risk within Linux systems. This flaw specifically manifests when the fs.protected_hardlinks sysctl parameter is disabled, creating a dangerous condition where unprivileged users can manipulate file ownership through carefully crafted hard link creation. The vulnerability resides in how opentmpfiles handles directory permissions and file ownership during its operation, particularly when processing directories that will subsequently undergo recursive ownership changes via chown -R commands. The security implications extend beyond simple file manipulation as this vulnerability can be exploited to gain persistent control over critical system files and directories.
The technical mechanism behind this vulnerability involves the exploitation of race conditions and permission handling within the opentmpfiles utility. When the fs.protected_hardlinks parameter is disabled, the system allows hard links to be created across different filesystems and directories, which opentmpfiles does not properly account for during its processing. An attacker can create a hard link to a target file within a directory that will later be processed by chown -R, effectively allowing them to manipulate the ownership of files they would normally not have access to. This occurs because the utility does not properly validate or sanitize the hard link creation process, particularly in scenarios where the target directory structure contains files that will undergo recursive ownership changes. The vulnerability represents a classic case of improper input validation and insufficient access control mechanisms, aligning with CWE-284 access control weaknesses and potentially mapping to ATT&CK technique T1068 for local privilege escalation.
The operational impact of this vulnerability extends across multiple system components and can result in severe security compromise. Systems running affected versions of OpenRC opentmpfiles become vulnerable to persistent file ownership manipulation, potentially allowing attackers to escalate privileges and gain control over system-critical files. The vulnerability is particularly dangerous in multi-user environments where users may not have direct access to sensitive system directories but can exploit this weakness to gain unauthorized access. Attackers can leverage this to modify system configuration files, create backdoors, or manipulate system binaries, ultimately leading to full system compromise. The exploitation requires local access but can be performed by any user with basic system privileges, making it a significant concern for system administrators who must consider the security implications of disabled hard link protection.
Mitigation strategies for CVE-2017-18188 focus on both immediate patching and system configuration adjustments. The primary recommendation involves updating to a patched version of OpenRC opentmpfiles that properly handles hard link scenarios and implements proper access controls during ownership changes. System administrators should also consider re-enabling the fs.protected_hardlinks sysctl parameter to prevent unauthorized hard link creation across filesystem boundaries. Additional protective measures include monitoring for unauthorized hard link creation in system directories and implementing proper file system permissions that limit user access to critical system areas. Security teams should also review existing access control policies and ensure that directory structures are properly protected against manipulation during ownership change operations. The vulnerability highlights the importance of proper privilege separation and access control validation in system utilities, particularly those that perform recursive ownership changes on system directories. Organizations should implement regular security audits to identify and remediate similar weaknesses in system utilities and ensure that all system components properly validate inputs and enforce appropriate access controls.