CVE-2021-20213 in Privoxy
Summary
by MITRE • 03/25/2021
A flaw was found in Privoxy in versions before 3.0.29. Dereference of a NULL-pointer that could result in a crash if accept-intercepted-requests was enabled, Privoxy failed to get the request destination from the Host header and a memory allocation failed.
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Analysis
by VulDB Data Team • 04/05/2021
The vulnerability identified as CVE-2021-20213 represents a critical NULL pointer dereference flaw within the Privoxy proxy software ecosystem. This issue affects versions prior to 3.0.29 and demonstrates a fundamental memory management weakness that can be exploited to cause service disruption. The flaw specifically manifests when the accept-intercepted-requests configuration option is enabled, creating a dangerous condition where the software fails to properly extract the destination request information from the Host header field. This configuration setting is typically used to intercept and modify HTTP requests passing through the proxy, making it a commonly enabled feature in many deployments. The vulnerability stems from inadequate input validation and memory allocation handling within the proxy's request processing pipeline, where the software assumes certain data structures will be properly initialized while failing to account for scenarios where memory allocation operations might fail.
The technical exploitation of this vulnerability occurs through a specific sequence of events that begins with the activation of accept-intercepted-requests functionality. When this option is enabled, Privoxy attempts to parse incoming HTTP requests and extract destination information from the Host header to determine where the request should be forwarded. However, the software's failure to properly validate memory allocation results combined with its reliance on potentially uninitialized pointers creates a condition where a NULL pointer dereference can occur. This type of vulnerability falls under CWE-476 which specifically addresses NULL pointer dereference conditions in software implementations. The crash occurs because the application attempts to access memory at address zero or an uninitialized pointer location when processing intercepted requests, resulting in an immediate termination of the proxy service and denial of service for all users relying on that proxy infrastructure.
The operational impact of CVE-2021-20213 extends beyond simple service disruption to encompass broader security implications within network infrastructure deployments. Organizations utilizing Privoxy as part of their network security posture face significant risk when running vulnerable versions, as the crash condition can be triggered through simple HTTP request manipulation without requiring authentication or advanced exploitation techniques. This makes the vulnerability particularly dangerous in environments where Privoxy serves as a critical network component for content filtering, privacy protection, or web traffic management. The vulnerability also aligns with ATT&CK technique T1499.004 which covers network disruption through service availability attacks, as the denial of service condition can be easily achieved by sending specifically crafted requests to the vulnerable proxy. Network administrators may experience unexpected service outages, particularly in environments where automated monitoring systems fail to detect the intermittent nature of the crash condition, leading to potential security gaps in network protection.
Mitigation strategies for CVE-2021-20213 primarily focus on immediate version upgrades to Privoxy 3.0.29 or later, which contains the necessary patches to address the NULL pointer dereference condition. Organizations should prioritize this update across all systems running vulnerable Privoxy versions, particularly those with accept-intercepted-requests enabled. Additionally, implementing network-level monitoring to detect unusual traffic patterns or service disruptions can help identify exploitation attempts before they cause significant impact. Security teams should also consider temporarily disabling the accept-intercepted-requests configuration option if immediate patching is not feasible, though this may reduce the proxy's functionality for legitimate interception requirements. The vulnerability highlights the importance of proper input validation and memory management practices in proxy software implementations, with recommendations to follow secure coding guidelines that emphasize defensive programming techniques to prevent similar conditions in future deployments. Organizations should also implement regular vulnerability scanning procedures to identify and remediate similar issues before they can be exploited in operational environments.