CVE-2013-2503 in Privoxy
Summary
by MITRE
Privoxy before 3.0.21 does not properly handle Proxy-Authenticate and Proxy-Authorization headers in the client-server data stream, which makes it easier for remote HTTP servers to spoof the intended proxy service via a 407 (aka Proxy Authentication Required) HTTP status code.
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Analysis
by VulDB Data Team • 02/03/2025
The vulnerability identified as CVE-2013-2503 affects Privoxy versions prior to 3.0.21 and represents a significant security flaw in the handling of HTTP proxy authentication headers. This issue stems from the software's inadequate processing of Proxy-Authenticate and Proxy-Authorization headers within the client-server communication stream, creating a potential avenue for man-in-the-middle attacks and authentication spoofing. The vulnerability specifically impacts the proxy server's ability to properly validate and authenticate proxy requests, allowing malicious HTTP servers to exploit the authentication mechanism through crafted 407 status codes.
The technical flaw manifests when Privoxy receives a 407 Proxy Authentication Required response from a remote HTTP server. Normally, this status code should be handled with proper validation to ensure it originates from the legitimate proxy server rather than an attacker. However, the vulnerable version fails to adequately inspect the authentication headers and their associated metadata, allowing an attacker to craft responses that appear to come from the intended proxy service. This improper handling creates a trust relationship vulnerability where the proxy server may inadvertently accept spoofed authentication challenges, effectively bypassing the intended security controls.
From an operational perspective, this vulnerability significantly weakens the security posture of any system relying on Privoxy for web filtering and proxy services. An attacker could exploit this flaw to intercept or manipulate proxy authentication flows, potentially gaining unauthorized access to protected resources or redirecting traffic through malicious endpoints. The impact extends beyond simple authentication bypass as it undermines the fundamental trust model that proxy services rely upon for secure communication between clients and servers. Network administrators and security professionals face increased risk of unauthorized access to filtered content, potential data exfiltration, and compromised network visibility.
The vulnerability aligns with CWE-284 Access Control Issues and specifically relates to improper handling of authentication headers within proxy services. It also maps to ATT&CK technique T1071.004 Application Layer Protocol: DNS where attackers may leverage proxy authentication flaws to establish persistent access or redirect traffic. Organizations using Privoxy should immediately implement the patch available in version 3.0.21 and conduct thorough network monitoring to detect any anomalous proxy authentication behavior. Additional mitigations include implementing strict header validation rules, deploying network segmentation to limit proxy server exposure, and establishing robust logging and alerting mechanisms for unusual authentication patterns. Security teams should also review their proxy configurations to ensure proper header sanitization and consider implementing additional authentication layers to compensate for the vulnerability's impact on the primary authentication mechanism.