CVE-2000-0996 in OpenBSD
Summary
by MITRE
Format string vulnerability in OpenBSD su program (and possibly other BSD-based operating systems) allows local attackers to gain root privileges via a malformed shell.
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Analysis
by VulDB Data Team • 04/06/2019
The vulnerability identified as CVE-2000-0996 represents a critical format string flaw within the OpenBSD su program that affects not only OpenBSD systems but potentially other BSD-based operating systems. This vulnerability resides in the way the su command processes user input during authentication and privilege escalation operations. The flaw manifests when the program fails to properly validate and sanitize format specifiers in user-provided input, creating an opportunity for malicious exploitation.
Format string vulnerabilities occur when a program uses user-supplied data as format strings without proper validation, allowing attackers to manipulate memory layout and execute arbitrary code. In the context of the su program, this vulnerability specifically impacts the authentication process where the system prompts for a password and processes user input through functions like printf or similar formatting routines. The vulnerability is classified under CWE-134 as "Use of Externally-Controlled Format String" which directly maps to the core technical flaw present in the OpenBSD su implementation.
The operational impact of this vulnerability is severe as it allows local attackers to escalate privileges from regular user level to root access without requiring authentication. Attackers can craft malicious shell commands or input strings that contain format specifiers such as %x, %s, or %n, which when processed by the vulnerable su program can result in memory corruption, information disclosure, or code execution. This type of privilege escalation is particularly dangerous because it does not require network access or complex exploitation techniques, making it highly accessible to attackers with local system access.
The exploitation process typically involves an attacker creating a specially crafted shell environment or command that includes format specifiers in the input stream. When the su program processes this input, it can cause the program to read from or write to arbitrary memory locations, potentially overwriting critical system structures or executing malicious code. This vulnerability directly relates to ATT&CK technique T1068 which covers "Exploitation for Privilege Escalation" and T1548.001 which covers "Abuse Elevation Control Mechanism" through the manipulation of system privilege escalation tools.
Mitigation strategies for CVE-2000-0996 involve immediate patching of the su program and related system utilities to ensure proper input validation and sanitization. System administrators should implement strict input validation procedures that prevent format specifiers from being processed as part of user input, particularly in authentication contexts. Additionally, the use of stack canaries, address space layout randomization, and other exploit mitigation techniques should be employed. The vulnerability also highlights the importance of secure coding practices in system utilities, specifically ensuring that functions like printf are not called with user-controllable format strings without proper sanitization. Organizations should conduct regular security audits of system utilities and maintain up-to-date patches for all operating system components to prevent exploitation of similar vulnerabilities in the future.