CVE-2004-0189 in Squid
Summary
by MITRE
The "%xx" URL decoding function in Squid 2.5STABLE4 and earlier allows remote attackers to bypass url_regex ACLs via a URL with a NULL ("%00") characterm, which causes Squid to use only a portion of the requested URL when comparing it against the access control lists.
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Analysis
by VulDB Data Team • 07/13/2025
The vulnerability described in CVE-2004-0189 represents a critical security flaw in Squid proxy server versions 2.5STABLE4 and earlier, specifically within the URL decoding mechanism that processes percent-encoded characters. This issue enables remote attackers to circumvent access control policies by exploiting how the software handles NULL character encoding in URLs. The flaw occurs when Squid processes URLs containing "%00" sequences, which are valid percent-encoded representations of NULL bytes in URL encoding standards. When the proxy server encounters such URLs, its internal URL decoding function fails to properly handle the NULL character, leading to incomplete URL processing during access control list comparisons.
The technical implementation of this vulnerability stems from the improper handling of percent-encoded sequences within Squid's URL parsing logic. The URL decoding function processes "%xx" encoded characters but does not adequately validate or sanitize the resulting decoded strings before performing access control checks. When a NULL character is present in the URL, it effectively truncates the URL string at that point, causing the proxy to compare only a portion of the original URL against configured access control lists. This truncation creates a bypass condition where maliciously crafted URLs can evade detection by access control mechanisms that would normally block access to certain resources or domains.
The operational impact of this vulnerability extends beyond simple access control bypass, as it fundamentally undermines the security posture of proxy servers relying on url_regex ACLs for content filtering and access management. Attackers can craft URLs that contain legitimate content followed by a NULL character, allowing them to access restricted resources that would normally be blocked by the proxy's access control policies. This vulnerability affects the integrity of the proxy's security model and can potentially enable unauthorized access to sensitive web resources, data exfiltration, or bypass of content filtering policies. The flaw particularly impacts organizations that depend on Squid for web filtering, corporate security policies, or network access control enforcement.
Organizations should prioritize immediate mitigation of this vulnerability by upgrading to Squid versions that address this specific URL decoding issue, as the flaw has been resolved in subsequent releases. The recommended approach involves implementing the latest stable Squid versions that contain proper URL decoding validation and NULL character handling. Additionally, administrators should review and audit existing url_regex ACL configurations to identify potential bypass opportunities, while considering implementing additional monitoring for unusual URL patterns that may indicate exploitation attempts. This vulnerability aligns with CWE-129, which addresses improper handling of length validation in input processing, and relates to ATT&CK technique T1071.004 for application layer protocol manipulation, highlighting the need for robust input validation and proper string handling in proxy server implementations to maintain network security integrity.