CVE-2021-36740 : What You Need to Know

Varnish Cache, with HTTP/2 enabled, is vulnerable to request smuggling and VCL authorization bypass via a large Content-Length header for a POST request. This impacts Varnish Enterprise versions before 6.0.8r3, Varnish Cache 5.x and 6.x before 6.5.2, 6.6.x before 6.6.1, and 6.0 LTS before 6.0.8.

Understanding CVE-2021-36740

This section will delve into the details of the CVE-2021-36740 vulnerability.

What is CVE-2021-36740?

CVE-2021-36740 pertains to Varnish Cache's vulnerability when HTTP/2 is enabled, leading to request smuggling and VCL authorization bypass via a specific header manipulation.

The Impact of CVE-2021-36740

The vulnerability allows attackers to manipulate a Content-Length header and bypass VCL authorization, potentially leading to unauthorized access or other malicious activities.

Technical Details of CVE-2021-36740

This section will outline the technical aspects of the CVE-2021-36740 vulnerability.

Vulnerability Description

The vulnerability in Varnish Cache occurs due to improper handling of HTTP/2 requests, specifically a Content-Length header, allowing for request smuggling and authorization bypass.

Affected Systems and Versions

Varnish Enterprise versions prior to 6.0.8r3 and Varnish Cache 5.x, 6.x, and 6.0 LTS versions before 6.5.2, 6.6.1, and 6.0.8, respectively, are affected by this vulnerability.

Exploitation Mechanism

Attackers can exploit this vulnerability by crafting a POST request with a large Content-Length header to manipulate the VCL authorization and smuggle malicious requests.

Mitigation and Prevention

Here we'll cover the mitigation strategies and preventive measures for CVE-2021-36740.

Immediate Steps to Take

Update Varnish Cache to the latest patched version that addresses the vulnerability.
Disable HTTP/2 if not strictly required to mitigate the risk of exploitation.

Long-Term Security Practices

Regularly monitor for security advisories and update Varnish Cache promptly.
Implement network monitoring and intrusion detection systems to detect potential exploitation attempts.

Patching and Updates

Apply security patches released by Varnish Software to address the vulnerability. Maintain a proactive approach to security updates.