Request Smuggling Vulnerability in h2 by Python Hyper
CVE-2026-71554

5.3MEDIUM

Key Information:

Vendor

Python-hyper

Status
H2
Vendor
CVE Published:
6 August 2026

Badges

👾 Exploit Exists🟡 Public PoC

What is CVE-2026-71554?

The h2 library, which is a pure-Python implementation of the HTTP/2 protocol stack, has a vulnerability that allows for potential request smuggling. This occurs in versions up to and including 4.4.0, where the library incorrectly handles request header blocks containing multiple Host headers. Subsequently, when these requests are processed, and HTTP/2 is downgraded to HTTP/1.1, they carry multiple Host headers, creating a situation that could enable attackers to smuggle requests. This issue has been resolved in version 4.4.1, which ensures proper handling of Host headers.

Affected Version(s)

h2 < 4.4.1

Exploit Proof of Concept (PoC)

PoC code is written by security researchers to demonstrate the vulnerability can be exploited. PoC code is also a key component for weaponization which could lead to ransomware.

poc-h2-CVE-2026-71554
Created by SunandM

poc-h2-duplicate-host
Created by SunandM

References

https://github.com/python-hyper/h2/security/advisories/GH...
x_refsource_CONFIRM
https://github.com/python-hyper/h2/commit/292a40829feefda...
x_refsource_MISC

CVSS V3.1

Score:
5.3
Severity:
MEDIUM
Confidentiality:
None
Integrity:
None
Availability:
None
Attack Vector:
Network
Attack Complexity:
Low
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • 🟡

    Public PoC available

  • 👾

    Exploit known to exist

  • Vulnerability published

  • Vulnerability Reserved

.