CVE-2015-5400 in Squid
Summary
by MITRE
Squid before 3.5.6 does not properly handle CONNECT method peer responses when configured with cache_peer, which allows remote attackers to bypass intended restrictions and gain access to a backend proxy via a CONNECT request.
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Analysis
by VulDB Data Team • 06/19/2022
The vulnerability identified as CVE-2015-5400 affects Squid proxy servers version 3.5.5 and earlier, representing a critical security flaw in the handling of HTTP CONNECT method responses from peer proxies. This issue specifically manifests when Squid is configured with cache_peer directives, which establish connections to upstream proxy servers for content caching and filtering purposes. The flaw stems from insufficient validation of peer responses during CONNECT method processing, creating a pathway for malicious actors to circumvent configured access controls and gain unauthorized access to backend proxy infrastructure.
The technical implementation of this vulnerability resides in Squid's peer response handling mechanism where the proxy server fails to properly validate the response received from cache_peer connections during CONNECT requests. When a client makes a CONNECT request through the Squid proxy, the server typically forwards this request to configured peer proxies for authentication and access control decisions. However, due to inadequate response validation, attackers can manipulate the peer response to appear as if it originates from an authorized source, effectively bypassing the intended access restrictions and gaining direct access to backend proxy services that should remain protected.
This vulnerability directly impacts the security posture of organizations relying on Squid as a proxy server for content filtering and access control. The operational consequences extend beyond simple unauthorized access, as attackers can potentially establish persistent connections to backend services, exfiltrate sensitive data, or use the compromised proxy as a pivot point for further network exploration. The flaw particularly affects environments where Squid is configured with multiple cache_peer entries and where strict access controls are implemented, making it a significant concern for enterprise networks, data centers, and organizations with complex proxy infrastructures.
The vulnerability aligns with CWE-284, which addresses improper access control mechanisms, and can be categorized under ATT&CK technique T1071.002 for application layer protocol communication. Organizations should implement immediate mitigation strategies including upgrading to Squid version 3.5.6 or later, which contains the necessary patches to properly validate peer responses during CONNECT method processing. Additional defensive measures include implementing strict access controls on cache_peer configurations, monitoring proxy logs for anomalous CONNECT request patterns, and conducting regular security assessments of proxy server configurations to ensure proper access control enforcement.
The root cause analysis reveals that this vulnerability exploits a fundamental flaw in Squid's trust model with peer proxies, where the server assumes peer responses are legitimate without sufficient validation. This design oversight creates a trust boundary breach that allows attackers to craft malicious peer responses that appear authentic to the Squid server. Organizations should also consider implementing network segmentation, firewall rules to restrict direct access to backend proxy services, and comprehensive logging to detect unauthorized access attempts. Regular security updates and vulnerability assessments remain critical for maintaining proxy server security posture, particularly given the complexity of modern proxy configurations and the evolving threat landscape targeting network infrastructure components.