CVE-2025-43859: h11 HTTP/1.1 Request Smuggling Vulnerability

CVE-2025-43859 Overview

h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

Critical Impact
This vulnerability allows for HTTP request smuggling, which can lead to unauthorized information disclosure and potential session hijacking.

Affected Products

Python h11 < 0.16.0
Not Available
Not Available

Discovery Timeline

Not Available - Vulnerability discovered by Not Available
Not Available - Responsible disclosure to Not Available
Not Available - CVE CVE-2025-43859 assigned
Not Available - Not Available releases security patch
2025-04-24T19:15:47.060 - CVE CVE-2025-43859 published to NVD
2025-04-29T13:52:28.490 - Last updated in NVD database

Technical Details for CVE-2025-43859

Vulnerability Analysis

This vulnerability arises due to h11’s permissive parsing of HTTP line terminators. This behavior can be exploited for HTTP request smuggling, where an attacker constructs malicious HTTP requests to manipulate backend servers via a vulnerable proxy configuration.

Root Cause

The vulnerability stems from improper parsing of newline characters in chunked transfer encoding, where an incorrect split allows inconsistencies in message length, leading to potential request smuggling.

Attack Vector

The attack vector is network-based, exploiting proxies and web servers using manipulated HTTP requests.

python

# Example exploitation code (sanitized)
request = """
GET / HTTP/1.1
Host: victim.com
Content-Length: 40
Transfer-Encoding: chunked

0

GET /malicious HTTP/1.1
Host: victim.com
"""

Detection Methods for CVE-2025-43859

Indicators of Compromise

Unexpected HTTP requests in server logs
Inconsistent HTTP responses
Abnormal session or cookie behaviors

Detection Strategies

Network traffic should be monitored for malformed HTTP requests that exhibit unusual line terminators or double encoding conditions. Deploy web application firewalls with signatures tailored to detect anomalous HTTP payloads characteristic of request smuggling.

Monitoring Recommendations

Regularly review and analyze server and proxy logs for patterns indicative of request smuggling threats. Utilizing deep packet inspection (DPI) can help identify malformed requests in transit.

How to Mitigate CVE-2025-43859

Immediate Actions Required

Upgrade h11 to version 0.16.0 or later
Configure reverse proxy servers to enforce strict HTTP parsing
Regularly audit web application firewall rules

Patch Information

Ensure systems are updated with patches made available in h11 version 0.16.0 addressing this request smuggling vulnerability.

Workarounds

Consider configuring proxy servers to reject non-standard HTTP line terminators and malformed requests to mitigate the vulnerability impact temporarily.

bash

# Configuration example
http {
    server {
        location / {
            if ($http_transfer_encoding ~* "chunked") {
                return 400;
            }
        }
    }
}