Listen to this Post
CVE-2025-43859 represents a critical security vulnerability impacting the h11 Python HTTP/1.1 library, which serves as an underlying dependency for packages such as greenbone-scap-api.
The core flaw originates from an excessive leniency in how the h11 library parses line terminators within chunked-coding message bodies.
Instead of strictly requiring the standard carriage return and line feed sequence, the parser accepts alternative non-standard byte sequences as valid chunk delimiters.
This permissive parsing behavior creates dangerous discrepancies when the downstream h11 server interacts with an upstream reverse proxy or load balancer.
If a front-end proxy parses chunked transfer encoding strictly while the backend h11 instance accepts lenient terminators, message boundaries become desynchronized.
An attacker can exploit this parsing gap by crafting an HTTP request with specially modified chunk terminators and extra payload data.
The front-end proxy interprets the chunked message stream differently than the backend server, cutting off or extending the request boundaries incorrectly.
Consequently, the trailing bytes of the malformed request are left unconsumed in the network buffer as a dangling data stream.
The backend server treats these leftover bytes as the start of an entirely separate, subsequent HTTP request sent by a different user.
This technique, known as HTTP request smuggling, allows an attacker to prepend unauthorized commands or hidden requests into a shared communication pipeline.
The server processes the smuggled request under the assumption that it originated from a legitimate, authenticated user session.
Successful exploitation can lead to severe security bypasses, cross-user request hijacking, and unauthorized data exposure.
Mitigation requires updating the vulnerable h11 parser library to version 0.16.0 or higher, where strict chunk terminator validation is enforced.
Dependent software packages like greenbone-scap-api must bump their dependency versions to eliminate exposure to this desynchronization vector.
DailyCVE Form:
Platform: Greenbone Scap API
Version: Prior To 0.16.0
Vulnerability: HTTP Request Smuggling
Severity: Critical Severity
Date: April 23, 2025
Prediction: April 23, 2025
What Undercode Say:
Bash Commands And Codes
pip install --upgrade greenbone-scap-api pip install h11==0.16.0 python -c "import h11; print(h11.<strong>version</strong>)" git log -S "h11" --oneline
Exploit: (Educational Purposes!)
Conceptual demonstration of malformed chunked transfer encoding request payload = ( b"POST / HTTP/1.1\r\n" b"Host: target\r\n" b"Transfer-Encoding: chunked\r\n" b"Content-Length: 6\r\n\r\n" b"0\r\n" b"X\r\n\r\n" Non-standard line terminator leniency exploit pattern b"GET /admin HTTP/1.1\r\n" b"Host: target\r\n\r\n" )
Protection: From This CVE
Upgrade the `h11` package to version `0.16.0` or later.
Update dependent repositories like `greenbone-scap-api` to their latest patched releases.
Configure reverse proxies to strictly validate and reject non-standard chunked transfer encodings and malformed line terminators.
Implement robust web application firewall rules to detect and drop request smuggling patterns.
Impact:
Allows malicious actors to perform HTTP request smuggling attacks via proxy desynchronization.
Enables unauthorized access to sensitive endpoints and confidential application data.
Facilitates session hijacking by injecting requests into valid user data streams.
Compromises data integrity through unauthorized command execution or cache poisoning.
🎯Let’s Practice Exploiting & Learn Patching For Free:
🎓 Live Courses & Certifications:
Join Undercode Academy for Verified Certifications
🚀 Request a Custom Project:
Secure, high-velocity infrastructure and disruptive technological engineering. Contact our engineering team for high-tier development and proprietary systems:
[email protected]
💎 Smart Architecture | 🛡️ Secure by Design | ⭐ Trusted by Thousands
Sources:
Reported By: github.com
Extra Source Hub:
Undercode
