CVE-2016-3948 in Squid
Summary
by MITRE
Squid 3.x before 3.5.16 and 4.x before 4.0.8 improperly perform bounds checking, which allows remote attackers to cause a denial of service via a crafted HTTP response, related to Vary headers.
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Analysis
by VulDB Data Team • 02/04/2019
The vulnerability identified as CVE-2016-3948 affects Squid proxy servers version 3.x prior to 3.5.16 and 4.x prior to 4.0.8, representing a critical bounds checking flaw that enables remote attackers to execute denial of service attacks through carefully crafted HTTP responses. This issue specifically targets the handling of Vary headers within the HTTP response processing pipeline, where improper bounds validation creates exploitable conditions that can crash the proxy server. The vulnerability resides in the application layer of the network stack, making it particularly dangerous as it can be exploited without requiring authentication or privileged access to the network infrastructure.
The technical flaw manifests when Squid processes HTTP responses containing malformed Vary headers that exceed expected buffer boundaries. During the parsing and validation of these headers, the software fails to properly enforce boundary limits on header field values, allowing attackers to craft malicious responses that trigger buffer overflows or memory corruption conditions. This improper bounds checking vulnerability directly maps to CWE-129, which describes implementations that do not properly validate input boundaries, and specifically relates to CWE-787, which addresses out-of-bounds write operations. The vulnerability allows attackers to cause the proxy server to crash or become unresponsive, effectively rendering the service unavailable to legitimate users and creating a denial of service condition that can persist until the server is manually restarted or the process is terminated.
From an operational perspective, this vulnerability presents a significant risk to organizations that rely on Squid as a critical component of their network infrastructure, including corporate proxies, content filtering systems, and web caching services. The remote exploit nature means that attackers can target vulnerable systems from anywhere on the internet without requiring physical access or network privileges, making it particularly attractive for automated attack campaigns. The impact extends beyond simple service disruption as the vulnerability can be leveraged as part of larger attack chains, potentially enabling further exploitation or serving as a stepping stone for more sophisticated attacks. Organizations using vulnerable versions of Squid are exposed to potential business disruption, especially in environments where proxy services are critical for network access control and content filtering operations. The vulnerability can be triggered through various attack vectors including web browsing, email attachments, or any HTTP traffic that passes through the affected proxy server.
The recommended mitigation strategy involves immediate deployment of patched versions of Squid software, specifically upgrading to versions 3.5.16 or 4.0.8 and later, which contain the necessary bounds checking fixes. System administrators should also implement network monitoring to detect potential exploitation attempts and consider temporary network segmentation to limit the attack surface. Organizations should conduct comprehensive vulnerability assessments to identify all instances of affected Squid versions within their infrastructure and establish incident response procedures for handling potential exploitation attempts. Additionally, implementing proper input validation and bounds checking mechanisms at the application level can help prevent similar vulnerabilities from occurring in other components of the network infrastructure. The ATT&CK framework categorizes this vulnerability under T1499.004 for network denial of service attacks, highlighting its potential for creating persistent service disruption that can impact business continuity and operational efficiency.