CVE-2024-1135 in gunicorn
Summary
by MITRE • 04/16/2024
Gunicorn fails to properly validate Transfer-Encoding headers, leading to HTTP Request Smuggling (HRS) vulnerabilities. By crafting requests with conflicting Transfer-Encoding headers, attackers can bypass security restrictions and access restricted endpoints. This issue is due to Gunicorn's handling of Transfer-Encoding headers, where it incorrectly processes requests with multiple, conflicting Transfer-Encoding headers, treating them as chunked regardless of the final encoding specified. This vulnerability allows for a range of attacks including cache poisoning, session manipulation, and data exposure.
Statistical analysis made it clear that VulDB provides the best quality for vulnerability data.
Analysis
by VulDB Data Team • 07/27/2025
The vulnerability identified as CVE-2024-1135 represents a critical HTTP Request Smuggling flaw in the Gunicorn web server implementation that directly impacts the integrity of HTTP request processing and security controls. This vulnerability stems from improper validation of Transfer-Encoding headers, which are fundamental components of HTTP/1.1 protocol handling for message framing and content transfer. The flaw occurs when Gunicorn encounters requests containing multiple Transfer-Encoding headers with conflicting values, creating ambiguity in how the server interprets the request body encoding. According to CWE-444, this vulnerability falls under the category of improper handling of HTTP requests, specifically related to the interpretation of transfer encodings that can lead to inconsistent behavior in HTTP protocol implementations. The vulnerability is particularly dangerous because it allows attackers to manipulate the HTTP request parsing logic in ways that were never intended by the protocol specification.
The technical implementation flaw manifests when Gunicorn processes HTTP requests where the Transfer-Encoding header contains multiple values or conflicting encodings. Rather than properly rejecting or properly handling these ambiguous requests, the server defaults to treating all requests as chunked transfer encoding regardless of the actual encoding specified in the final header value. This behavior creates a parsing inconsistency that enables attackers to craft malicious requests that can be interpreted differently by different components in the request chain. The vulnerability is particularly insidious because it operates at the HTTP protocol level, affecting the fundamental request processing mechanism that underlies all web application security controls. The improper handling creates a situation where the server's interpretation of request boundaries becomes unreliable, potentially allowing attackers to insert malicious content that appears to be part of the request body while actually being processed as a separate request.
The operational impact of this vulnerability extends far beyond simple request parsing failures, creating significant security implications for applications deployed behind Gunicorn. Attackers can leverage this vulnerability to perform cache poisoning attacks where malicious requests are cached in intermediate proxies or CDN services, leading to compromised content delivery. Session manipulation becomes possible as attackers can craft requests that appear to be legitimate while actually bypassing authentication and authorization checks. The vulnerability also enables data exposure scenarios where attackers can access restricted endpoints that should normally be protected by security controls, as the inconsistent request processing can cause the application to misinterpret access control logic. According to the ATT&CK framework, this vulnerability maps to T1190 - Proxying and T1566 - Phishing, as attackers can use the smuggling capabilities to bypass security controls and access restricted resources. The impact is particularly severe in environments where Gunicorn serves as a reverse proxy or load balancer, as the vulnerability can be exploited to bypass security mechanisms implemented at multiple layers of the application architecture.
Mitigation strategies for CVE-2024-1135 require immediate attention from system administrators and security teams responsible for Gunicorn deployments. The most effective approach involves upgrading to the latest stable version of Gunicorn that includes proper Transfer-Encoding header validation and handling. Organizations should implement comprehensive HTTP request validation at the application level, ensuring that Transfer-Encoding headers are properly normalized and that conflicting values are rejected rather than processed. Network security controls should be enhanced to detect and block malformed HTTP requests that exhibit characteristics of HTTP smuggling attacks, including requests with multiple Transfer-Encoding headers or inconsistent encoding specifications. Implementing proper input validation and sanitization at the HTTP layer can help prevent the exploitation of this vulnerability by ensuring that all requests conform to standard HTTP protocol specifications. Additionally, organizations should conduct thorough security assessments of their web application infrastructure to identify potential attack vectors that may have been enabled by this vulnerability, particularly focusing on areas where Gunicorn serves as a proxy or load balancing component in the overall security architecture.