CVE-2021-20217 in Privoxy
Summary
by MITRE • 03/25/2021
A flaw was found in Privoxy in versions before 3.0.31. An assertion failure triggered by a crafted CGI request may lead to denial of service. The highest threat from this vulnerability is to system availability.
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Analysis
by VulDB Data Team • 04/05/2021
The vulnerability identified as CVE-2021-20217 affects Privoxy, a popular non-cache web proxy designed to enhance privacy and block advertisements. This issue resides in versions prior to 3.0.31 and represents a critical assertion failure that can be exploited through carefully crafted CGI requests. The flaw manifests when the proxy processes malformed input in its Common Gateway Interface handling mechanism, specifically targeting the assertion validation logic within the software's request processing pipeline. This vulnerability falls under the category of denial of service attacks as defined by CWE-400, where an attacker can disrupt service availability by triggering unexpected program termination.
The technical implementation of this vulnerability exploits the assertion mechanism within Privoxy's CGI processing module. When a maliciously crafted request reaches the proxy server, it triggers an assertion failure that causes the application to terminate unexpectedly. This occurs because the software fails to properly validate input parameters before performing assertion checks, creating a path where controlled input can force the program into an unrecoverable state. The assertion failure essentially represents a programming error where the software assumes certain conditions will always hold true, but malicious input violates these assumptions. This behavior aligns with CWE-617, which addresses reachable assertions that can be triggered through input manipulation.
The operational impact of CVE-2021-20217 extends beyond simple service disruption to potentially compromise the entire proxy infrastructure. When exploited successfully, the vulnerability can cause Privoxy to crash repeatedly, leading to complete service unavailability for all users relying on the proxy for web browsing and privacy protection. This denial of service condition affects not only individual users but also organizations that depend on Privoxy for network security and content filtering. The vulnerability's threat level is particularly concerning because it can be triggered remotely without requiring authentication or specialized privileges, making it accessible to any attacker with network access to the affected system. From an attack framework perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers network denial of service attacks that target availability.
Mitigation strategies for CVE-2021-20217 primarily involve immediate software updates to version 3.0.31 or later, which contains the necessary patches to address the assertion failure. Organizations should also implement network monitoring to detect unusual traffic patterns that might indicate exploitation attempts, particularly focusing on unexpected HTTP request sequences targeting CGI endpoints. Additional defensive measures include configuring firewalls to limit access to CGI interfaces and implementing intrusion detection systems that can identify malformed requests targeting known vulnerable components. The vulnerability demonstrates the importance of proper input validation and assertion handling in security-critical applications, emphasizing the need for robust error handling mechanisms that prevent exploitation through controlled input manipulation. System administrators should also consider implementing redundant proxy services to maintain availability during patch deployment and monitoring for similar issues in other network security tools.