CVE-2016-2572 in Squid
Summary
by MITRE
http.cc in Squid 4.x before 4.0.7 relies on the HTTP status code after a response-parsing failure, which allows remote HTTP servers to cause a denial of service (assertion failure and daemon exit) via a malformed response.
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Analysis
by VulDB Data Team • 07/08/2022
The vulnerability identified as CVE-2016-2572 represents a critical denial of service flaw within the Squid proxy server software version 4.x prior to 4.0.7. This issue resides in the http.cc component which handles HTTP response parsing operations. The vulnerability stems from the software's improper handling of malformed HTTP responses received from upstream servers, creating a scenario where malicious or compromised servers can trigger catastrophic failures in the proxy daemon. The flaw specifically manifests when Squid encounters an HTTP response parsing error and subsequently relies on the status code returned in the malformed response to determine its next course of action. This design decision creates a dangerous dependency on potentially unreliable data, allowing attackers to craft malicious responses that exploit this weak validation mechanism.
The technical exploitation of this vulnerability occurs through the manipulation of HTTP response data sent by remote servers to the vulnerable Squid proxy. When Squid attempts to parse a malformed HTTP response, it performs an assertion check that fails due to the improper handling of the response status code. This assertion failure triggers an immediate daemon exit, effectively causing a denial of service condition that impacts all users relying on the proxy service. The vulnerability operates at the application layer and can be triggered by any HTTP server that sends a malformed response to a Squid proxy configured to forward requests to that server. This flaw demonstrates poor defensive programming practices and highlights the importance of robust input validation and error handling in network services. The issue falls under CWE-248, which addresses "Uncaught Exception" conditions, and represents a classic example of how improper error handling can lead to system instability and service disruption.
The operational impact of this vulnerability extends beyond simple service interruption, as it can affect organizations relying on Squid for content filtering, caching, and proxy services. When exploited, the vulnerability can cause cascading failures in network infrastructure that depends on the proxy server, potentially affecting hundreds or thousands of users depending on the scale of the deployment. Organizations using vulnerable versions of Squid may experience complete service outages until the daemon is manually restarted or the system is rebooted. The vulnerability is particularly concerning because it requires no authentication or specialized privileges to exploit, making it accessible to any remote attacker who can influence HTTP responses to the proxy. This makes it a high-risk vulnerability for organizations with open proxy configurations or those that do not properly filter or validate upstream responses.
Mitigation strategies for CVE-2016-2572 primarily involve immediate patching of affected Squid installations to version 4.0.7 or later, where the vulnerability has been resolved through improved response parsing and error handling mechanisms. Organizations should also implement network-level controls to monitor and filter potentially malicious responses, though this approach provides only partial protection. The fix addresses the core issue by ensuring that Squid does not rely on potentially malformed status codes during error conditions and instead implements proper fallback mechanisms that prevent assertion failures from terminating the daemon. Security teams should also consider implementing intrusion detection systems that can identify unusual patterns of HTTP response handling that may indicate exploitation attempts. From an ATT&CK framework perspective, this vulnerability maps to the T1499.004 technique related to "Endpoint Denial of Service" and demonstrates the importance of maintaining software patches and implementing proper error handling practices. Additionally, organizations should conduct regular vulnerability assessments to identify similar issues in other network services and ensure that their incident response procedures include specific protocols for handling proxy service failures.