CVE-2000-0824 in C Library
Summary
by MITRE
The unsetenv function in glibc 2.1.1 does not properly unset an environmental variable if the variable is provided twice to a program, which could allow local users to execute arbitrary commands in setuid programs by specifying their own duplicate environmental variables such as LD_PRELOAD or LD_LIBRARY_PATH.
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Analysis
by VulDB Data Team • 12/17/2024
The vulnerability described in CVE-2000-0824 represents a critical flaw in the GNU C Library implementation that affects how environmental variables are handled during program execution. This issue specifically targets the unsetenv function in glibc version 2.1.1, where the library fails to properly remove environmental variables when duplicates are present in the program's environment. The flaw arises from the improper handling of duplicate variable entries, creating a scenario where attackers can manipulate the program's environment in unexpected ways. The vulnerability is particularly dangerous because it can be exploited in setuid programs, which execute with elevated privileges, making the potential impact significantly more severe than typical environment variable manipulation attacks.
The technical root cause of this vulnerability stems from the improper implementation of the unsetenv function within the glibc library. When a program receives environmental variables that contain duplicate entries for the same variable name, the unsetenv function fails to properly remove all instances of that variable from the environment. This creates a situation where duplicate variables can persist in memory even after attempts to unset them, allowing attackers to inject malicious values into critical environment variables such as LD_PRELOAD or LD_LIBRARY_PATH. The flaw occurs because the function does not iterate through all instances of a variable when performing unsetting operations, leaving behind duplicate entries that can be exploited. This behavior violates the expected semantics of environment variable handling and creates a persistent security risk that can be leveraged by local attackers to gain unauthorized privileges.
The operational impact of this vulnerability is severe and multifaceted, particularly when exploited in setuid programs where privilege escalation becomes possible. Attackers can craft malicious environment variable sequences that include duplicate entries, allowing them to bypass normal security checks and execute arbitrary code with elevated privileges. The vulnerability specifically affects the LD_PRELOAD and LD_LIBRARY_PATH variables, which are commonly used to load shared libraries and can be manipulated to inject malicious code into running processes. When exploited successfully, this vulnerability enables local users to execute arbitrary commands with the privileges of the setuid program, potentially leading to complete system compromise. The attack vector is particularly insidious because it requires no network connectivity and can be executed locally, making it difficult to detect and prevent through traditional network-based security measures.
This vulnerability aligns with several cybersecurity frameworks and threat modeling concepts, including CWE-15 for improper neutralization of data within an environment variable and ATT&CK technique T1546.008 for hijacking execution flow through environment variable manipulation. The flaw demonstrates the importance of proper input validation and environment handling in security-critical libraries, as it shows how seemingly minor implementation details can create significant security vulnerabilities. Organizations should prioritize patching affected systems and implementing strict environment variable validation practices to prevent exploitation. Mitigation strategies include updating to patched versions of glibc, implementing environment variable sanitization in applications, and monitoring for suspicious environment variable patterns in setuid programs. Additionally, the vulnerability underscores the importance of thorough testing of security-critical library functions and the need for comprehensive security reviews of system libraries that handle sensitive operations like environment variable manipulation. The issue also highlights the risks associated with privilege escalation through environment variable manipulation and the necessity of implementing defense-in-depth strategies to protect against such attacks.