CVE-1999-1273 is a high-severity vulnerability affecting Squid Internet Object Cache version 1.1.20, a widely used web proxy cache server. The vulnerability allows an attacker to bypass configured access control lists (ACLs) by encoding URLs with hexadecimal escape sequences. Squid uses ACLs to restrict or allow access to web resources based on various criteria such as source IP, destination URL, or user credentials. However, in this version, the URL parsing mechanism does not properly normalize or decode hexadecimal escape sequences before applying ACL rules. As a result, an attacker can craft a URL containing hexadecimal encoded characters that represent restricted URLs, which Squid fails to recognize as restricted due to the encoding. This bypass enables unauthorized users to access web resources that should be blocked by the proxy server. The vulnerability has a CVSS score of 7.5 (high), with an attack vector of network (remote exploitation), low attack complexity, no authentication required, and impacts on confidentiality, integrity, and availability. Although no patch is available for this legacy version and no known exploits in the wild have been reported, the vulnerability poses a significant risk if the affected version is still in use. Given the age of the vulnerability (published in 1998) and the specific affected version (1.1.20), modern deployments are unlikely to be affected, but legacy systems or unpatched environments remain at risk. The lack of patch availability means mitigation must rely on compensating controls or upgrading to a newer, fixed version of Squid.

For European organizations, this vulnerability could lead to unauthorized access to restricted web resources, potentially exposing sensitive internal or external data. By bypassing ACLs, attackers might access confidential information, introduce malicious content, or disrupt normal proxy operations, impacting confidentiality, integrity, and availability. Organizations relying on Squid 1.1.20 as a web proxy or caching server in their network infrastructure could face data leakage or unauthorized internet access. This risk is particularly relevant for sectors with strict data protection requirements such as finance, healthcare, and government agencies within Europe. Additionally, compromised proxy servers could be leveraged as pivot points for further network intrusion or data exfiltration. Although the vulnerability is old and no known exploits are reported, the presence of legacy systems in some European organizations means the threat remains relevant. The impact is heightened by the absence of patches, requiring organizations to consider alternative mitigation strategies.

1. Upgrade: The most effective mitigation is to upgrade Squid to a supported and patched version where this vulnerability is fixed. Versions later than 1.1.20 have addressed ACL bypass issues and improved URL normalization. 2. Network Segmentation: Isolate legacy proxy servers from critical internal networks to limit potential damage from exploitation. 3. Access Controls: Implement strict network-level access controls (firewalls, IP whitelisting) to restrict who can reach the proxy server, reducing exposure. 4. Monitoring and Logging: Enable detailed logging on the proxy server and monitor for unusual URL encoding patterns or unexpected access attempts that may indicate exploitation attempts. 5. Web Filtering: Use additional web filtering or security gateways in front of or behind the proxy to enforce access policies independently of Squid ACLs. 6. Incident Response Preparedness: Develop and test incident response plans specific to proxy server compromise scenarios. 7. Decommission Legacy Systems: Where possible, phase out legacy Squid versions and replace them with modern, supported proxy solutions to eliminate this and other vulnerabilities.