A newly discovered security exploit, CVE-2024-6827, affects Gunicorn 21.2., a widely used Web Server Gateway Interface (WSGI) server for running Python web applications. This vulnerability stems from the server's improper validation of the 'Transfer-Encoding' header, causing the server to default to the 'Content-Length' header. Consequently, threat actors can potentially use the TE.CL request smuggling technique to exploit this vulnerability for various nefarious purposes, such as cache poisoning, data exposure, session manipulation, SSRF, XSS, DoS, data integrity compromise, security bypass, information leakage, and business logic abuse.
To better understand this vulnerability, let's first learn about TE.CL request smuggling and what it entails.
TE.CL Request Smuggling
In a nutshell, request smuggling is a technique attackers employ to manipulate how servers handle and process incoming requests. By exploiting inconsistencies and discrepancies in server configurations, an attacker can smuggle malicious requests to potentially bypass security measures or exploit other vulnerabilities.
Transfer-Encoding/Content-Length Request Smuggling (TE.CL) specifically leverages the misuse of 'Transfer-Encoding' and 'Content-Length' headers in HTTP requests. These are typically used to declare the size and encoding of the request payload. By manipulating these headers, an attacker can insert additional content or separate malicious request components, thus 'smuggling' them through the server.
Now, let's explore how Gunicorn 21.2. is susceptible to this threat.
Gunicorn 21.2. Vulnerability Details
Gunicorn version 21.2. fails to validate the 'Transfer-Encoding' header correctly as required by RFC standards. Instead, it defaults to the 'Content-Length' header to process the incoming request. An attacker can exploit this vulnerability by sending specifically crafted requests to the server, allowing them to smuggle TE.CL requests and expose the application to various attack vectors.
Here's a code snippet illustrating the exploit
POST /example HTTP/1.1
Host: vulnerable_server.com
Content-Type: application/x-www-form-urlencoded
Content-Length: 97
Transfer-Encoding: chunked
GET /potentially_sensitive_data HTTP/1.1
X-Header: smuggled_request
In this example, a malicious actor sends a seemingly harmless POST request to the server. However, the request contains both 'Content-Length' and 'Transfer-Encoding' headers, which Gunicorn 21.2. mishandles, allowing the smuggled GET request to bypass security measures and access potentially sensitive data.
Here are some links to understand further about this vulnerability
1. Gunicorn: Official Release Notes
2. TE.CL Request Smuggling
3. RFC 723: Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
Mitigating the Vulnerability
To mitigate and safeguard your Gunicorn-based applications against this vulnerability, consider the following steps:
Configure your server to deny requests with both 'Transfer-Encoding' and 'Content-Length' headers.
3. Implement a Web Application Firewall (WAF) with effective request filtering rules to detect and block potential smuggling attempts.
4. Regularly audit your applications, server configurations, and networks to identify and address potential vulnerabilities and inconsistencies promptly.
By taking a proactive approach to securing your Gunicorn-powered applications, you can defend against the CVE-2024-6827 vulnerability and reduce the threat of TE.CL request smuggling.
Timeline
Published on: 03/20/2025 10:15:33 UTC