CVE-2021-39241: 
HAProxy vulnerability analysis and mitigation

CVE-2021-39241 is a security vulnerability discovered in HAProxy versions 2.0 before 2.0.24, 2.2 before 2.2.16, 2.3 before 2.3.13, and 2.4 before 2.4.3. The vulnerability allows an HTTP method name to contain a space followed by the name of a protected resource, which could lead to security bypass (HAProxy Announce).

The vulnerability exists in HAProxy's HTTP/2 implementation where the ':method' field can contain spaces. When a space is included in the method, it's possible to build an invalid HTTP/1 request on the backend side, which some lenient servers might interpret as valid. This results in a discrepancy between the request seen by HAProxy and the server, potentially allowing attackers to circumvent switching rules (HAProxy Announce).

The vulnerability could allow attackers to bypass implemented security restrictions and routing rules. For example, a request intended for one server farm could potentially be routed to a different server if the backend server fails to properly validate the input. This could lead to unauthorized access to protected resources (Red Hat CVE).

Several mitigation strategies are available: 1) Reject invalid characters in the method by adding a filtering rule: 'http-request reject if { method -m reg ^A-Z0-9 }'. 2) For version 2.0, disable HTX internal representation using 'no option http-use-htx'. 3) Disable HTTP/2 communication entirely by setting 'tune.h2.max-concurrent-streams 0' in the global section (HAProxy Announce).

The vulnerability was addressed promptly by major Linux distributions. Red Hat rated this update as having a security impact of Moderate and included fixes in their OpenShift Container Platform updates (Red Hat Advisory). Fedora also released security updates to address this vulnerability in their distributions (Fedora Update).