CVE-1999-0706 in Linux
Summary
by MITRE
Linux xmonisdn package allows local users to gain root privileges by modifying the IFS or PATH environmental variables.
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Analysis
by VulDB Data Team • 04/21/2026
The vulnerability described in CVE-1999-0706 represents a classic privilege escalation flaw within the Linux xmonisdn package that was prevalent during the late 1990s era of Unix-like system development. This issue resides in the package's improper handling of environmental variables, specifically the IFS and PATH variables that are fundamental to shell execution behavior. The xmonisdn package was designed to manage ISDN (Integrated Services Digital Network) connections and provided monitoring capabilities for these digital communication systems. The flaw emerged from the package's failure to properly sanitize or validate environmental variables before executing shell commands, creating a pathway for local users to manipulate system behavior through variable modification. This vulnerability operates under the broader category of environment variable manipulation attacks that have been documented in various security frameworks including CWE-15, which specifically addresses external control of critical system parameters.
The technical implementation of this vulnerability exploits the inherent behavior of Unix shells where IFS (Internal Field Separator) and PATH variables control how command arguments are parsed and where executable files are searched respectively. When the xmonisdn package executed shell commands without proper variable sanitization, attackers could modify these variables to inject malicious code or manipulate command execution paths. By altering IFS, an attacker could control how shell arguments were separated, potentially allowing for command injection through crafted input. Similarly, PATH variable manipulation could redirect command execution to malicious binaries that were positioned earlier in the search path, effectively allowing privilege escalation. This particular flaw demonstrates the importance of proper input validation and environment variable handling, aligning with ATT&CK technique T1068 which covers local privilege escalation through environment variable manipulation. The vulnerability essentially allowed local users to elevate their privileges to root level by exploiting the package's trust in unvalidated environmental context.
The operational impact of CVE-1999-0706 was significant within the security landscape of the late 1990s when Linux systems were rapidly gaining adoption in enterprise environments. Organizations running xmonisdn packages were vulnerable to local privilege escalation attacks that could result in complete system compromise, as attackers could gain root access without requiring authentication credentials. The vulnerability was particularly dangerous because it required no special privileges to exploit, making it accessible to any user with access to the system. The attack vector was straightforward and reliable, as it leveraged fundamental shell behavior rather than complex exploitation techniques. This made the vulnerability particularly attractive to attackers and highlighted the critical need for proper security practices in system administration and software development during that era. The vulnerability also demonstrated the broader security implications of trusting environmental variables without proper validation, a principle that remains relevant in modern security practices.
Mitigation strategies for CVE-1999-0706 centered around immediate patching of the xmonisdn package to implement proper environmental variable validation and sanitization. System administrators were advised to update their installations to versions that properly handled shell environment variables without allowing user modification to affect privileged operations. The recommended approach included implementing strict variable validation mechanisms and ensuring that critical system processes did not inherit potentially compromised environmental contexts. Security practitioners should have enforced proper access controls and monitoring to detect unauthorized modification of system variables. Additionally, this vulnerability underscored the importance of following secure coding practices that prevent environment variable manipulation, as outlined in various security standards and frameworks. The incident highlighted the necessity of comprehensive security testing that includes environment variable handling and input validation, particularly for packages that execute shell commands with elevated privileges. Organizations were encouraged to implement regular security audits and vulnerability assessments to identify similar issues in other system components and prevent similar privilege escalation scenarios from occurring in their environments.