CVE-1999-1549 in Lynx
Summary
by MITRE
Lynx 2.x does not properly distinguish between internal and external HTML, which may allow a local attacker to read a "secure" hidden form value from a temporary file and craft a LYNXOPTIONS: URL that causes Lynx to modify the user's configuration file and execute commands.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability identified as CVE-1999-1549 affects Lynx version 2.x web browsers and represents a critical security flaw in how the application handles HTML content processing. This issue stems from the browser's inadequate distinction between internal and external HTML elements, creating a dangerous condition where local attackers can exploit the system's handling of temporary files and configuration directives. The vulnerability specifically targets the Lynx browser's approach to processing URL schemes and configuration file modifications, allowing for unauthorized command execution through crafted input.
The technical implementation of this vulnerability exploits the browser's failure to properly validate and sanitize HTML content when processing LYNXOPTIONS URLs. When Lynx encounters a LYNXOPTIONS scheme URL, it attempts to modify the user's configuration file without adequate security checks. The flaw occurs because the browser does not properly differentiate between internal browser operations and external HTML content that should be treated with different security contexts. This allows attackers to craft malicious URLs that reference temporary files containing sensitive form values, which the browser then processes as configuration directives rather than temporary data.
The operational impact of this vulnerability extends beyond simple privilege escalation to include full system compromise through command execution. Local attackers can leverage this flaw to read secure hidden form values stored in temporary files, which typically contain sensitive information such as authentication tokens or system configuration parameters. Once these values are extracted, attackers can construct malicious LYNXOPTIONS URLs that instruct Lynx to modify the user's configuration file with malicious commands or settings. This creates a persistent backdoor mechanism that can execute arbitrary code with the privileges of the user running Lynx, effectively allowing complete system compromise.
The vulnerability aligns with several cybersecurity frameworks and threat models, particularly those addressing privilege escalation and command injection attacks. From a CWE perspective, this issue relates to CWE-20: Improper Input Validation, CWE-78: Improper Neutralization of Special Elements used in OS Command, and CWE-264: Permissions, Privileges, and Access Controls. The attack pattern follows ATT&CK framework techniques including T1068: Exploitation for Privilege Escalation and T1059: Command and Scripting Interpreter, demonstrating how local exploitation can lead to broader system compromise. The vulnerability also highlights the importance of proper input sanitization and the principle of least privilege in web browser security implementations.
Mitigation strategies for this vulnerability require immediate implementation of several security measures. System administrators should upgrade to Lynx versions that properly distinguish between internal and external HTML content, as the original 2.x versions contain fundamental design flaws that cannot be adequately patched. Users should be educated about the dangers of visiting untrusted websites and the importance of not clicking on suspicious URLs, particularly those that might contain LYNXOPTIONS schemes. Additional protective measures include implementing proper file permissions on temporary files and configuration directories, ensuring that temporary files are not world-readable and that configuration files are properly secured. Network administrators should monitor for suspicious URL patterns and consider implementing web filtering solutions that can block access to potentially malicious Lynx-specific URLs. The vulnerability also underscores the necessity of proper input validation and output encoding in browser implementations, particularly when handling custom URL schemes and configuration file modifications.