CVE-2011-4968 in nginx
Summary
by MITRE
nginx http proxy module does not verify peer identity of https origin server which could facilitate man-in-the-middle attack (MITM)
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Analysis
by VulDB Data Team • 08/25/2025
The vulnerability identified as CVE-2011-4968 resides within the nginx http proxy module and represents a critical security flaw that undermines the integrity of secure communications. This issue specifically affects the proxy module's handling of HTTPS connections to origin servers, where the software fails to perform proper peer identity verification during the SSL/TLS handshake process. The absence of certificate validation creates a significant attack surface that malicious actors can exploit to conduct man-in-the-middle attacks, potentially intercepting and modifying sensitive data transmitted between the nginx proxy and upstream HTTPS servers.
The technical flaw manifests when nginx processes requests destined for HTTPS origin servers through its proxy functionality. During the SSL/TLS negotiation, the proxy module should validate the server certificate presented by the origin server against trusted certificate authorities and verify that the certificate matches the expected hostname. However, CVE-2011-4968 demonstrates that this validation process is either completely bypassed or inadequately implemented, allowing connections to proceed even when the peer identity cannot be verified. This weakness directly violates fundamental security principles of secure communication and certificate-based authentication mechanisms that are essential for maintaining trust in network communications.
The operational impact of this vulnerability extends beyond simple data interception, as it fundamentally compromises the security posture of any system relying on nginx as a proxy for HTTPS traffic. Attackers can exploit this weakness by positioning themselves between the nginx proxy and the origin server, presenting fake certificates that appear valid to the proxy while establishing separate encrypted connections with both parties. This creates a scenario where sensitive information can be captured, modified, or redirected without detection, potentially affecting user sessions, authentication credentials, financial transactions, and confidential business data. The vulnerability affects organizations that depend on nginx for secure proxying of HTTPS content, making it particularly concerning for web applications handling sensitive data.
Organizations can mitigate this vulnerability by implementing several defensive measures that align with industry best practices and security frameworks. The primary remediation involves ensuring that nginx is configured to properly verify peer certificates when establishing HTTPS connections to origin servers, typically through the use of the proxy_ssl_verify directive set to on and configuring appropriate certificate authorities. Additionally, implementing proper certificate management practices, including regular certificate rotation and maintaining up-to-date trust stores, strengthens the overall security posture. This vulnerability aligns with CWE-295, which addresses improper certificate validation, and maps to ATT&CK technique T1041, which covers data obfuscation through proxying. Organizations should also consider implementing network monitoring solutions to detect anomalous traffic patterns that might indicate MITM activity, while ensuring that all nginx installations are updated to versions that properly address this certificate verification weakness. The remediation approach should follow security standards such as NIST SP 800-53 and ISO/IEC 27001, which emphasize the importance of proper cryptographic key management and secure communication protocols in maintaining system integrity.