BlackSheep, White Noise: Breaking the HTTP Client with CRLF Injection

Amit Schendel

Senior Security Researcher

Jan 14, 2026·7 min read·6 visits

PoC Available

Executive Summary (TL;DR)

BlackSheep's `ClientSession` trusted user input a little too much. By failing to strip Carriage Return and Line Feed characters from headers and URLs, the framework allowed attackers to rewrite HTTP requests on the wire. This could lead to Header Injection, Request Splitting, and general protocol anarchy.

A deep dive into how the BlackSheep Python web framework's HTTP client failed to sanitize input, allowing attackers to inject headers and split requests via classic CRLF injection.

Attack Flow Diagram

Official Patches

NeoteroiOfficial fix commit on GitHub

Fix Analysis (1)

Technical Appendix

CVSS Score

6.3/ 10

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

EPSS Probability

0.04%

Top 100% most exploited

Affected Systems

BlackSheep Web Framework (Python)Applications using BlackSheep.ClientSessionMicroservices relying on BlackSheep for HTTP proxying

Affected Versions Detail

Product	Affected Versions	Fixed Version
BlackSheep Neoteroi	< 2.4.6	2.4.6

Attribute	Detail
CWE ID	CWE-113 (CRLF Injection)
CVSS v4.0	6.3 (Medium)
Attack Vector	Network
Vendor	Neoteroi
Fix Commit	bd4ecb9542b5d52442276b5a6907931b90f38d12
Exploit Status	PoC Available / Logic Flaw