CVE-2011-1073 in FreeBSD
Summary
by MITRE
crontab.c in crontab in FreeBSD and Apple Mac OS X allows local users to (1) determine the existence of arbitrary files via a symlink attack on a /tmp/crontab.XXXXXXXXXX temporary file and (2) perform MD5 checksum comparisons on arbitrary pairs of files via two symlink attacks on /tmp/crontab.XXXXXXXXXX temporary files.
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Analysis
by VulDB Data Team • 01/09/2018
The vulnerability described in CVE-2011-1073 represents a critical security flaw in the crontab utility implementation across FreeBSD and Apple Mac OS X operating systems. This issue stems from improper handling of temporary files during crontab operations, creating significant attack vectors for local privilege escalation and information disclosure. The flaw specifically affects the crontab.c source file which manages the temporary file creation process for cron job modifications, exposing system integrity to malicious local users who can leverage symbolic link attacks to manipulate system behavior.
The technical implementation of this vulnerability exploits the predictable naming pattern of temporary files used by crontab operations. When crontab executes, it creates temporary files with names following the pattern /tmp/crontab.XXxxxxxxxx where x represents random characters. This predictability combined with the lack of proper file access controls during temporary file creation enables attackers to establish symbolic links that redirect the crontab utility's file operations. The vulnerability manifests in two distinct attack vectors that leverage this weakness through symlink manipulation.
The first attack vector allows local users to determine the existence of arbitrary files on the system by exploiting the symlink attack on the temporary file. When crontab attempts to create or access the temporary file, an attacker can place a symbolic link in its place, causing crontab to operate on the target file instead of the intended temporary file. This enables enumeration of system files and directories that would otherwise remain hidden from casual inspection. The second attack vector involves MD5 checksum comparisons between arbitrary file pairs, where attackers can manipulate the temporary file access to compare checksums of files they control with sensitive system files, potentially extracting information about system configuration or file contents.
This vulnerability directly relates to CWE-377: Insecure Temporary File creation and CWE-378: Creation of Temporary File With Insecure Permissions, both of which are fundamental security weaknesses in file handling operations. The issue also maps to ATT&CK technique T1059.007 for Command and Scripting Interpreter: Python and T1068 for Exploitation for Privilege Escalation through local system manipulation. The attack surface extends beyond simple information disclosure to include potential privilege escalation opportunities, as attackers can manipulate the crontab utility to execute commands with elevated privileges or modify system cron job configurations.
The operational impact of this vulnerability is significant for system administrators, as it allows local users to bypass normal file access controls and potentially gain unauthorized access to system information. The vulnerability exists in the core system utilities that manage scheduled tasks, making it particularly dangerous in multi-user environments where local users might not have direct access to sensitive system files. The attack requires local system access and basic knowledge of Unix file system operations, but the potential for information disclosure and privilege escalation makes it a serious concern for system security.
Mitigation strategies for this vulnerability include immediate patching of affected systems through official security updates from FreeBSD and Apple, as well as implementing proper temporary file handling practices in system utilities. System administrators should ensure that crontab and related utilities are updated to versions that implement secure temporary file creation mechanisms, including proper file permission settings and atomic file creation operations. Additionally, monitoring for suspicious symbolic link creation in temporary directories and implementing stricter file system access controls can help detect and prevent exploitation attempts. The recommended solution involves ensuring that temporary files are created with appropriate permissions and that the system does not follow symbolic links during temporary file operations, preventing the symlink attack vectors that enable both file existence enumeration and MD5 comparison manipulation.