CVE-2015-3455 in Squid
Summary
by MITRE
Squid 3.2.x before 3.2.14, 3.3.x before 3.3.14, 3.4.x before 3.4.13, and 3.5.x before 3.5.4, when configured with client-first SSL-bump, do not properly validate the domain or hostname fields of X.509 certificates, which allows man-in-the-middle attackers to spoof SSL servers via a valid certificate.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 05/10/2022
The vulnerability described in CVE-2015-3455 affects Squid proxy servers running specific versions across multiple release branches including 3.2.x before 3.2.14, 3.3.x before 3.3.14, 3.4.x before 3.4.13, and 3.5.x before 3.5.4. This security flaw manifests when Squid is configured with client-first SSL-bump functionality, a feature designed to intercept and inspect encrypted SSL traffic passing through the proxy. The vulnerability stems from inadequate validation of X.509 certificate domain and hostname fields during the SSL interception process, creating a critical security gap that can be exploited by malicious actors.
The technical flaw resides in the certificate validation logic within Squid's SSL-bump implementation where the proxy fails to properly verify that the domain name or hostname presented in the X.509 certificate matches the actual server being accessed. This weakness specifically impacts the client-first SSL-bump configuration where the proxy establishes an initial SSL connection with the client before connecting to the target server. The improper certificate validation allows attackers to present a valid certificate that appears to be from the legitimate server but actually originates from a different entity, effectively enabling man-in-the-middle attacks.
The operational impact of this vulnerability is severe as it fundamentally undermines the security assurances provided by SSL/TLS encryption within the proxy environment. Attackers can exploit this weakness to intercept, modify, or redirect encrypted traffic without detection, potentially accessing sensitive information, credentials, or proprietary data. The vulnerability affects organizations that rely on Squid proxies for content filtering, caching, or security inspection, particularly those using SSL-bump functionality to monitor encrypted communications. This creates a significant risk for enterprises where SSL traffic inspection is critical for security policy enforcement and compliance monitoring.
Organizations should immediately upgrade to patched versions of Squid as specified in the CVE references, ensuring that all affected versions are updated to prevent exploitation. The mitigation strategy should include comprehensive network monitoring to detect potential exploitation attempts and implementation of additional security controls such as certificate pinning where applicable. Security teams must also review their SSL-bump configurations and consider alternative approaches to traffic inspection that do not rely on certificate validation bypasses. This vulnerability aligns with CWE-295 which addresses improper certificate validation, and represents a critical threat in the ATT&CK framework under the T1041 technique for data encryption for exfiltration, as it enables unauthorized access to encrypted communications.
The broader implications extend to organizations using proxy-based security solutions where SSL inspection is a core requirement for compliance and threat detection. This vulnerability demonstrates the critical importance of proper certificate validation in security appliances and the potential for catastrophic security failures when cryptographic validation is bypassed. Organizations should conduct thorough security assessments of their proxy infrastructure and implement robust monitoring solutions to detect anomalous certificate behavior or unauthorized certificate modifications that might indicate exploitation attempts.