CVE-2025-43859: Critical Vulnerability in h11 Python HTTP/1.1 Implementation

Overview

The cybersecurity community has recently identified a serious vulnerability in the h11 Python HTTP/1.1 implementation. This flaw, recognized as CVE-2025-43859, has the potential to compromise systems and leak sensitive data under certain conditions. It’s a critical issue that mainly affects developers and organizations that use versions of h11 prior to 0.16.0 in conjunction with certain buggy proxy servers. The severity of this vulnerability is underscored by its CVSS Severity Score of 9.1, indicating a high level of risk.

Vulnerability Summary

CVE ID: CVE-2025-43859
Severity: Critical – CVSS 9.1
Attack Vector: Network
Privileges Required: None
User Interaction: None
Impact: System compromise, potential data leakage

Affected Products

Product | Affected Versions

h11 Python HTTP/1.1 implementation | Prior to 0.16.0

How the Exploit Works

The vulnerability results from a leniency in h11’s parsing of line terminators in chunked-coding message bodies. This flaw can lead to request smuggling vulnerabilities when used with a buggy proxy server. An attacker can exploit this vulnerability to inject malicious payloads into the communication, leading to potential system compromise and data leakage.

Conceptual Example Code

Here’s a conceptual example of how an attacker might exploit this vulnerability. Note that this is a simplified example and actual exploitation would require specific knowledge of the target system and its configuration.

POST / HTTP/1.1
Host: target.example.com
Transfer-Encoding: chunked
Content-Length: 5
G
malicious_payload
0

In this example, the attacker is using a chunked transfer encoding to smuggle a malicious payload (‘malicious_payload’) into the server communication. This payload could be code designed to exploit other vulnerabilities in the system or to leak sensitive data.

Mitigation

This vulnerability has been patched in h11 version 0.16.0. Affected users should update to this version or later as soon as possible. If immediate patching is not feasible, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can provide temporary mitigation. These security measures may help detect and block attempts to exploit this vulnerability. However, they are not a substitute for applying the vendor patch and should only be considered as a temporary measure while the patching process is carried out.