CVE-2019-20372 in Xcode
Summary
by MITRE
NGINX before 1.17.7, with certain error_page configurations, allows HTTP request smuggling, as demonstrated by the ability of an attacker to read unauthorized web pages in environments where NGINX is being fronted by a load balancer.
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Analysis
by VulDB Data Team • 10/01/2021
The vulnerability identified as CVE-2019-20372 represents a critical HTTP request smuggling flaw in NGINX web server versions prior to 1.17.7. This vulnerability specifically manifests when NGINX is configured with certain error_page directives and operates in front of load balancers or other reverse proxy configurations. The flaw exploits the inconsistent handling of HTTP headers and request parsing between NGINX and downstream components, creating a pathway for attackers to manipulate HTTP requests and potentially access unauthorized content. The vulnerability is particularly dangerous in environments where NGINX serves as a front-end proxy for load balancers, as it can bypass security controls implemented at the load balancer level.
The technical root cause of this vulnerability stems from NGINX's improper handling of HTTP request parsing when error_page directives are configured in specific ways. When an HTTP request is processed through NGINX with certain error_page configurations, the server fails to properly normalize or validate HTTP headers, leading to inconsistent interpretation of request boundaries. This behavior creates a scenario where an attacker can craft malicious requests that appear legitimate to NGINX but are interpreted differently by the load balancer or backend servers. The vulnerability is classified under CWE-444 as improper handling of HTTP requests, specifically related to inconsistent interpretation of HTTP methods and headers across different components in the request processing chain.
The operational impact of CVE-2019-20372 extends beyond simple unauthorized content access, as it enables sophisticated attack patterns that can compromise entire application stacks. Attackers can leverage this vulnerability to perform cache poisoning attacks, bypass authentication mechanisms, and access sensitive data that should be protected by load balancer security policies. In environments where NGINX serves as a critical security boundary, this vulnerability essentially undermines the security model by allowing attackers to smuggle requests that can access resources they shouldn't be able to reach. The vulnerability is particularly concerning in cloud environments and microservices architectures where NGINX often serves as a front-end gateway for multiple backend services, creating a single point of failure that can compromise the entire system.
Organizations affected by this vulnerability should immediately implement mitigations including upgrading to NGINX version 1.17.7 or later, which contains the necessary patches to address the HTTP request smuggling issue. Additionally, administrators should review all error_page configurations to ensure they do not create conditions that could trigger this vulnerability. Network segmentation and additional security controls such as API gateways or web application firewalls can provide additional layers of protection while the upgrade is being implemented. The ATT&CK framework categorizes this vulnerability under T1071.004 for application layer protocol, and T1566 for phishing, as attackers can use this vulnerability to bypass security controls and gain unauthorized access to protected resources through manipulated HTTP requests. Organizations should also consider implementing comprehensive monitoring to detect anomalous HTTP request patterns that might indicate exploitation attempts, as the vulnerability can be used to perform stealthy attacks that may not immediately trigger traditional security alerts.