CVE-2011-0017 in Server
Summary
by MITRE
The open_log function in log.c in Exim 4.72 and earlier does not check the return value from (1) setuid or (2) setgid system calls, which allows local users to append log data to arbitrary files via a symlink attack.
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Analysis
by VulDB Data Team • 03/18/2021
The vulnerability identified as CVE-2011-0017 represents a critical privilege escalation flaw within the Exim mail transfer agent version 4.72 and earlier. This issue stems from improper error handling in the open_log function located in the log.c source file, where the system fails to validate the return values of critical system calls during the logging process. The flaw specifically affects the setuid and setgid system calls that are essential for proper privilege management when Exim initializes its logging functionality. When these system calls do not return successfully, the application continues execution without proper privilege elevation, creating a dangerous condition that can be exploited by local attackers.
The technical exploitation of this vulnerability relies on a classic symlink attack vector where malicious users manipulate symbolic links to redirect log file operations to arbitrary locations on the filesystem. During normal operation, Exim creates log files in designated directories with appropriate permissions, but due to the missing return value checks, the application does not properly validate whether the setuid/setgid operations have completed successfully. This allows a local attacker to create carefully crafted symbolic links that point to sensitive system files or directories, enabling them to append log data to files they would normally not have write access to. The vulnerability is particularly dangerous because it operates at the privilege level where Exim runs, typically with elevated permissions, allowing attackers to potentially write to system-critical files or overwrite existing log data.
From an operational impact perspective, this vulnerability presents a significant risk to system integrity and security posture. The ability to append data to arbitrary files through log manipulation can be leveraged for various malicious activities including data corruption, privilege escalation, or even system compromise. Attackers could potentially target critical system files such as configuration files, authentication databases, or system logs to either corrupt essential data or create backdoors for persistent access. The vulnerability's local nature means it requires physical access or execution privileges on the target system, but the impact can be substantial when combined with other attack vectors or when the target system runs with elevated privileges. Organizations using Exim versions prior to 4.73 are particularly at risk as the flaw affects the core logging mechanism that is fundamental to system monitoring and security auditing processes.
The vulnerability maps directly to CWE-252, which describes an "Unchecked Return Value" condition where a program fails to check the results of a function call that might indicate failure. This weakness is classified as a privilege escalation vulnerability under the ATT&CK framework's privilege escalation tactics, specifically within the T1068 technique for "Local Privilege Escalation" and T1566 for "Phishing with Malicious Attachments" when considering the broader attack surface. The flaw also aligns with ATT&CK's T1078 for "Valid Accounts" as it allows attackers to potentially gain access to resources that would otherwise be protected by proper privilege controls. Organizations should prioritize immediate patching of affected Exim installations, as version 4.73 and later include proper return value checking for the setuid/setgid system calls. Additionally, implementing proper file permissions, monitoring for suspicious log file modifications, and maintaining regular security audits can help detect and prevent exploitation attempts. The vulnerability serves as a reminder of the critical importance of proper error handling in security-sensitive applications, particularly when dealing with privilege management operations that form the foundation of system security controls.