CVE-2005-0509 in Mono
Summary
by MITRE
Multiple cross-site scripting (XSS) vulnerabilities in the Mono 1.0.5 implementation of ASP.NET (.Net) allow remote attackers to inject arbitrary HTML or web script via Unicode representations for ASCII fullwidth characters that are converted to normal ASCII characters, including ">" and "<".
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Analysis
by VulDB Data Team • 07/04/2025
The vulnerability described in CVE-2005-0509 represents a significant cross-site scripting weakness within the Mono 1.0.5 implementation of ASP.NET frameworks. This issue stems from the improper handling of Unicode fullwidth characters during input validation processes, creating a pathway for malicious actors to bypass security controls. The flaw specifically affects how the system processes Unicode representations that are converted to standard ASCII characters, particularly targeting the angle brackets used in HTML markup. Attackers can exploit this vulnerability by injecting Unicode fullwidth characters that are automatically converted to their ASCII equivalents, thereby circumventing traditional input sanitization mechanisms that only check for standard ASCII characters.
The technical exploitation of this vulnerability leverages the inherent conversion behavior of the Mono framework's character processing system. When the system encounters Unicode fullwidth characters such as the fullwidth greater than sign U+FF1E or fullwidth less than sign U+FF1C, it converts these to their standard ASCII counterparts during processing. This conversion occurs before input validation occurs, allowing attackers to inject malicious scripts that would otherwise be blocked by standard security measures. The vulnerability specifically targets the HTML angle brackets that are commonly used to define HTML tags and script elements, making it particularly dangerous for web applications that process user input without proper sanitization. This type of vulnerability falls under CWE-79 - Improper Neutralization of Input During Web Page Generation, which is a core component of the OWASP Top Ten web application security risks.
The operational impact of CVE-2005-0509 extends beyond simple script injection, as it can enable attackers to perform a wide range of malicious activities including session hijacking, data theft, and redirection to malicious sites. When exploited successfully, the vulnerability allows attackers to inject HTML content that executes in the context of other users' browsers, potentially leading to complete compromise of user sessions and sensitive data exposure. The vulnerability affects web applications running on the Mono 1.0.5 framework, which was widely used for cross-platform .NET development, making it particularly concerning for organizations maintaining legacy systems. From an attacker's perspective, this vulnerability maps to ATT&CK technique T1059.007 - Command and Scripting Interpreter: JavaScript, as it enables the execution of malicious JavaScript code through web-based interfaces.
Mitigation strategies for this vulnerability require multiple layers of defensive measures to address both the immediate exploitation vector and underlying architectural weaknesses. Organizations should implement comprehensive input validation that considers Unicode character sets rather than relying solely on ASCII character filtering, ensuring that all character representations are properly sanitized regardless of their encoding form. The recommended approach includes deploying proper HTML encoding mechanisms that convert potentially dangerous characters to their safe HTML entity equivalents before processing user input. Additionally, implementing Content Security Policy (CSP) headers can provide an additional layer of protection by restricting script execution and preventing unauthorized code injection. Security patches and updates to the Mono framework should be prioritized, as this vulnerability was addressed in subsequent versions through improved character handling and validation routines. Organizations should also consider implementing Web Application Firewalls (WAFs) with rules specifically designed to detect and block Unicode-based XSS attacks, and conduct regular security testing to identify similar character encoding vulnerabilities across their web applications.