CVE-2019-12522 in Web Proxy
Summary
by MITRE
An issue was discovered in Squid through 4.7. When Squid is run as root, it spawns its child processes as a lesser user, by default the user nobody. This is done via the leave_suid call. leave_suid leaves the Saved UID as 0. This makes it trivial for an attacker who has compromised the child process to escalate their privileges back to root.
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Analysis
by VulDB Data Team • 05/26/2024
This vulnerability exists in Squid proxy server versions through 4.7 where the software operates with elevated privileges during startup but fails to properly drop these privileges in child processes. The issue stems from how Squid handles user privilege management when transitioning from root to a lesser user account. When Squid runs as root and spawns child processes, it utilizes the leave_suid mechanism to switch user contexts. However, this mechanism does not properly clear the saved set-user-ID bit, leaving the effective user ID set to zero, which represents root privileges. This design flaw creates a persistent security weakness that allows attackers who gain control of any child process to easily escalate their privileges back to the root user level without requiring additional exploitation techniques.
The technical implementation of this vulnerability involves the improper handling of Unix user privilege mechanisms. When Squid initializes and creates child processes, it calls leave_suid to switch from root to the nobody user account. The function fails to properly reset the saved set-user-ID bit which remains set to zero, maintaining the capability to regain root privileges. This represents a fundamental flaw in privilege management and process isolation. The vulnerability directly maps to CWE-276, which addresses improper privileges, and specifically relates to improper privilege management in Unix-like systems where the saved set-user-ID bit is not properly cleared during privilege transitions. Attackers can exploit this by compromising any child process and then leveraging the preserved root privileges to execute arbitrary code with full system access.
The operational impact of this vulnerability is severe as it provides attackers with a straightforward path to complete system compromise. Once an attacker gains control of any Squid child process through other means such as buffer overflows, injection attacks, or other vulnerabilities, they can immediately escalate privileges to root without additional exploitation steps. This makes Squid installations particularly attractive targets for attackers seeking persistent system access. The vulnerability affects systems where Squid is configured to run as root initially and spawn child processes, which is the default configuration for many deployments. The attack vector is particularly concerning because it does not require additional compromise of the main Squid process or other system components, making it a critical issue for network infrastructure security. This vulnerability aligns with ATT&CK technique T1068, which covers the use of privilege escalation techniques, and specifically demonstrates how improper privilege management can be exploited to gain unauthorized access.
The primary mitigation strategy involves upgrading to Squid versions beyond 4.7 where this privilege escalation issue has been resolved. Organizations should also implement proper network segmentation and access controls to limit exposure to potential attackers. Additionally, running Squid with reduced privileges from the start, rather than starting as root and then dropping privileges, would prevent this specific vulnerability from being exploitable. System administrators should also monitor for unauthorized access to Squid child processes and implement process monitoring to detect potential compromise. The fix typically involves ensuring that the saved set-user-ID bit is properly cleared during the privilege transition process, preventing the ability to regain root privileges from compromised child processes. Organizations should also consider implementing additional security controls such as mandatory access controls, process isolation, and regular security audits to protect against similar privilege escalation vulnerabilities in other system components.