CVE-1999-1273 in Squid
Summary
by MITRE
squid internet object cache 1.1.20 allows users to bypass access control lists (acls) by encoding the url with hexadecimal escape sequences.
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Analysis
by VulDB Data Team • 04/17/2026
The vulnerability identified as CVE-1999-1273 affects squid internet object cache version 1.1.20 and represents a critical access control bypass flaw that undermines the security mechanisms designed to protect network resources. This vulnerability exploits a fundamental weakness in how the squid proxy server processes uniform resource locator requests, specifically when those urls contain hexadecimal escape sequences that are not properly validated or normalized during processing.
The technical flaw resides in the squid proxy server's URL parsing logic which fails to adequately decode and normalize hexadecimal escape sequences before applying access control list restrictions. When users submit urls containing encoded characters using the %xx hexadecimal notation, the system does not properly convert these sequences back to their original form before checking against configured access controls. This normalization failure creates a pathway for malicious users to circumvent restrictions that should prevent access to specific resources or domains, effectively allowing unauthorized access to protected content.
The operational impact of this vulnerability is significant as it directly compromises the integrity of network security policies implemented through squid's access control lists. Attackers can exploit this flaw to gain access to resources that should be restricted based on various criteria such as source ip addresses, destination domains, time-based restrictions, or user authentication requirements. The vulnerability affects organizations that rely on squid as their primary proxy server for content filtering, bandwidth management, and access control enforcement, potentially exposing sensitive corporate data, blocking legitimate security measures, and undermining network monitoring capabilities.
This vulnerability aligns with CWE-15 which describes improper neutralization of special elements used in data queries, and relates to ATT&CK technique T1071.004 for application layer protocol tunneling. The flaw demonstrates how insufficient input validation and normalization can create security gaps in network infrastructure components. Organizations using squid versions prior to 1.1.21 should immediately implement patches or upgrades to address this vulnerability. Mitigation strategies include configuring proper url normalization rules within the squid configuration, implementing additional monitoring for suspicious url patterns, and establishing network segmentation to limit the potential impact of successful exploitation attempts. The vulnerability highlights the importance of thorough input validation in proxy server implementations and underscores the critical need for proper encoding handling in security-critical network components to prevent bypass of access control mechanisms.