CVE-2016-4556 in Squid
Summary
by MITRE
Double free vulnerability in Esi.cc in Squid 3.x before 3.5.18 and 4.x before 4.0.10 allows remote servers to cause a denial of service (crash) via a crafted Edge Side Includes (ESI) response.
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Analysis
by VulDB Data Team • 08/17/2022
The vulnerability identified as CVE-2016-4556 represents a critical double free condition within the Edge Side Includes (ESI) processing module of Squid proxy software. This flaw exists in versions 3.x prior to 3.5.18 and 4.x prior to 4.0.10, affecting a widely deployed caching and forwarding proxy server that serves millions of organizations worldwide. The double free vulnerability occurs when the Esi.cc component processes maliciously crafted ESI responses from remote servers, leading to memory management corruption that can be exploited for remote denial of service attacks.
The technical implementation of this vulnerability stems from improper memory deallocation handling within the ESI processing code path. When Squid receives an HTTP response containing crafted ESI directives, the software's Esi.cc module fails to properly track memory allocations and deallocations, resulting in attempts to free the same memory block twice. This condition violates fundamental memory safety principles and creates exploitable conditions that can be triggered by remote attackers who control content served through the proxy. The flaw specifically manifests during the parsing and processing of ESI includes, where multiple code paths may attempt to release identical memory references, causing heap corruption and subsequent application crash.
The operational impact of this vulnerability extends beyond simple service disruption, as it can be leveraged to create persistent denial of service conditions that affect network availability and user access to web resources. Organizations relying on Squid as a core infrastructure component face significant risk since attackers can craft malicious ESI responses that will cause the proxy to crash and restart automatically, potentially leading to extended outages. The vulnerability's remote exploitability means that any user accessing content through an affected Squid proxy could trigger the condition, making it particularly dangerous in environments where the proxy serves as a gateway for multiple users or applications. This issue aligns with CWE-415, which describes improper handling of memory allocation and deallocation leading to double free conditions.
Mitigation strategies for CVE-2016-4556 require immediate patching of affected Squid installations to versions 3.5.18 or 4.0.10, which contain the necessary memory management fixes. Network administrators should also implement monitoring and alerting for unusual proxy behavior or frequent restarts that might indicate exploitation attempts. Additional protective measures include configuring Squid to limit ESI processing capabilities, disabling ESI support entirely if not required, and implementing network segmentation to reduce the attack surface. The vulnerability's classification under the ATT&CK framework would place it within the T1499 category of Network Denial of Service, with potential for lateral movement if attackers can establish persistent access through other means. Organizations should also consider implementing intrusion detection systems that can identify patterns consistent with ESI-based attack vectors and maintain comprehensive incident response procedures for handling proxy server compromise scenarios.