CVE-1999-0347 is a critical vulnerability affecting Internet Explorer version 4.01, discovered in early 1999. The flaw arises from improper handling of the "%01" character within an "about:" JavaScript URL. Specifically, when this character is included, Internet Explorer misinterprets the domain specified after "%01", allowing a remote attacker to bypass the browser's same-origin policy. This enables the attacker to read local files on the victim's machine and spoof web pages by making them appear as if they originate from a trusted domain. The vulnerability effectively allows unauthorized disclosure of local file contents and manipulation of web content, which can lead to further exploitation such as phishing or injection of malicious scripts. The CVSS v2 base score is 10.0, indicating critical severity, with network attack vector, no authentication required, and complete compromise of confidentiality, integrity, and availability. No patches are available for this vulnerability, and there are no known exploits in the wild documented. Given the age of the vulnerability and the affected software, this issue primarily concerns legacy systems still running Internet Explorer 4.01, which is an extremely outdated browser version.

For European organizations, the impact of this vulnerability is largely dependent on the presence of legacy systems running Internet Explorer 4.01. If such systems exist, attackers could remotely read sensitive local files, potentially exposing confidential corporate data, credentials, or configuration files. The spoofing capability could facilitate phishing attacks by making malicious web pages appear legitimate, increasing the risk of credential theft or malware infection. This could disrupt business operations, damage reputation, and lead to regulatory non-compliance, especially under GDPR where data breaches must be reported. However, given the obsolescence of IE 4.01, the practical risk is low for most modern European enterprises. Nonetheless, organizations in sectors with legacy infrastructure—such as industrial control systems, government agencies, or critical infrastructure operators—may face higher risks if these outdated browsers remain in use.

Since no official patches are available for this vulnerability, the primary mitigation is to discontinue the use of Internet Explorer 4.01 entirely. Organizations should upgrade to supported, modern browsers that receive regular security updates. For legacy applications that require IE, consider isolating these systems from the internet and sensitive networks using network segmentation and strict firewall rules. Employ endpoint protection solutions that can detect and block suspicious scripts or URL manipulations. Additionally, implement strict content security policies and disable or restrict JavaScript execution in legacy browsers where possible. User education to avoid interacting with suspicious links and URLs is also critical. Regularly audit systems to identify any remaining IE 4.01 installations and remove or upgrade them promptly.