CVE-2016-10003 in Squid
Summary
by MITRE
Incorrect HTTP Request header comparison in Squid HTTP Proxy 3.5.0.1 through 3.5.22, and 4.0.1 through 4.0.16 results in Collapsed Forwarding feature mistakenly identifying some private responses as being suitable for delivery to multiple clients.
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Analysis
by VulDB Data Team • 05/15/2026
The vulnerability identified as CVE-2016-10003 affects Squid HTTP proxy versions 3.5.0.1 through 3.5.22 and 4.0.1 through 4.0.16, specifically within the Collapsed Forwarding feature implementation. This flaw represents a critical security issue that undermines the proxy's ability to properly handle HTTP request header comparisons, creating potential for unauthorized information disclosure and cross-client request interference. The vulnerability stems from improper handling of HTTP header field comparisons where the proxy fails to correctly distinguish between different client requests, leading to incorrect assumptions about response reusability across multiple clients. This issue directly violates the fundamental security principle of isolation between client sessions, which is essential for maintaining the confidentiality and integrity of private network communications.
The technical root cause of this vulnerability lies in the flawed HTTP header comparison logic within Squid's Collapsed Forwarding mechanism. When processing HTTP requests, the proxy incorrectly evaluates header field values and their variations, particularly focusing on headers such as User-Agent, Accept, and other client-specific fields that should prevent response reuse between different clients. This improper comparison leads to situations where private responses intended for a single client are mistakenly marked as cacheable and forwarded to multiple clients. The vulnerability manifests when the proxy's internal logic fails to properly account for header field variations that indicate different client contexts, essentially allowing a malicious actor to potentially access responses that should remain isolated to specific client sessions. This behavior aligns with CWE-200, which addresses improper information exposure, and represents a significant deviation from proper HTTP protocol implementation standards.
The operational impact of CVE-2016-10003 extends beyond simple information disclosure to encompass potential session hijacking and cross-client data leakage scenarios. When private responses are incorrectly forwarded to multiple clients, it creates opportunities for attackers to intercept sensitive information that should remain confidential to specific users or sessions. This vulnerability particularly affects environments where Squid proxies handle authenticated content, personal data, or corporate confidential information, as the improper header comparison can result in responses intended for one user being served to another user within the same proxy cluster. The attack surface is further expanded in environments where the proxy is used for content filtering, web caching, or transparent proxying, as these configurations increase the likelihood of encountering scenarios where the flawed header comparison logic can be exploited. The vulnerability creates a persistent risk for organizations relying on Squid proxies for network security and access control, as it undermines the fundamental security assumptions of proxy-based filtering and content delivery systems.
Organizations should immediately implement mitigations including upgrading to patched versions of Squid that address the HTTP header comparison logic, typically those beyond the affected version ranges mentioned in the vulnerability description. System administrators should also consider implementing additional network segmentation measures and monitoring for unusual proxy behavior patterns that might indicate exploitation attempts. The vulnerability demonstrates the importance of proper HTTP protocol implementation and the critical need for thorough testing of caching and forwarding mechanisms within proxy servers. Security teams should also review their existing proxy configurations to ensure that any custom header handling logic does not compound the vulnerability risk. From an operational security perspective, this vulnerability highlights the necessity of maintaining current proxy software versions and implementing robust security monitoring that can detect anomalous proxy behavior related to response forwarding and caching decisions. Organizations should also consider implementing additional layers of security controls such as web application firewalls or network access controls to provide defense-in-depth against potential exploitation of this vulnerability. The ATT&CK framework categorizes this as a privilege escalation or information disclosure technique, emphasizing the need for comprehensive security monitoring and response procedures to detect and mitigate such proxy-based vulnerabilities effectively.