CVE-2000-0959 in C Library
Summary
by MITRE
glibc2 does not properly clear the LD_DEBUG_OUTPUT and LD_DEBUG environmental variables when a program is spawned from a setuid program, which could allow local users to overwrite files via a symlink attack.
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Analysis
by VulDB Data Team • 05/28/2018
The vulnerability identified as CVE-2000-0959 represents a critical security flaw in the GNU C Library version 2 that affects how environmental variables are handled during program execution. This issue specifically manifests when a setuid program spawns a child process, creating a potential attack vector for local users to exploit file system permissions. The flaw stems from the improper clearing of LD_DEBUG_OUTPUT and LD_DEBUG environmental variables, which are typically used for debugging dynamic linker operations but can be manipulated to cause unintended behavior.
The technical root cause of this vulnerability lies in the glibc implementation where environmental variables are not properly sanitized when transitioning from a setuid context to a child process. When a setuid program executes another program, the inherited environmental variables retain their values, including LD_DEBUG_OUTPUT and LD_DEBUG. These variables, when set to specific values, can direct the dynamic linker to write debug output to arbitrary files. An attacker can leverage this by creating symbolic links with carefully crafted names that will be overwritten when the debug output is written, effectively enabling file overwrite attacks.
This vulnerability operates under the principle of privilege escalation and file system manipulation, where local users can exploit the inheritance of environmental variables to bypass normal file system protections. The operational impact is significant as it allows attackers to overwrite files with arbitrary content, potentially leading to system compromise or data corruption. The attack requires local access and knowledge of the target system's file structure, but once successful, it can be used to modify critical system files or create backdoors.
The security implications extend beyond simple file overwrites, as this vulnerability can be combined with other attack vectors to achieve more sophisticated compromises. According to CWE classification, this represents a weakness in the clearing of environmental variables, specifically CWE-256 which deals with improper privilege management. The vulnerability also aligns with ATT&CK techniques related to privilege escalation and persistence mechanisms, as attackers can use the file overwrite capability to modify system binaries or configuration files.
Mitigation strategies for this vulnerability require immediate patching of affected glibc versions, as the primary solution involves updating to versions that properly clear environmental variables when transitioning between setuid contexts. System administrators should also implement strict file system permissions and monitor for unusual file modifications in critical system directories. Additional protective measures include disabling unnecessary debugging features, implementing proper input validation for environmental variables, and regularly auditing setuid programs for potential security flaws. The vulnerability underscores the importance of proper privilege management and environmental variable handling in security-critical system components, highlighting the need for comprehensive security testing of system libraries that handle privileged operations.