CVE-2026-71554 in python-hyper
Summary
by MITRE • 08/07/2026
h2 is a pure-Python implementation of a HTTP/2 protocol stack. Versions up to and including 4.4.0 accept request header blocks containing more than one Host header, and forward every Host header to the consuming application. Where the consumer downgrades HTTP/2 to HTTP/1.1, the resulting request carries two Host header lines, providing a request smuggling primitive. This issue is fixed in version 4.4.1.
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Analysis
by VulDB Data Team • 08/07/2026
The h2 library represents a significant implementation of the HTTP/2 protocol stack written entirely in python, serving as a foundational component for applications requiring http/2 functionality. This vulnerability affects versions up to and including 4.4.0 where the library fails to properly validate incoming request header blocks during http/2 processing. The flaw specifically manifests when request header blocks contain multiple Host headers, a condition that the library accepts without proper sanitization. When these malformed requests are forwarded to consuming applications, each Host header is preserved and transmitted, creating a scenario where applications receive duplicate host headers in their processed requests.
The technical exploitation of this vulnerability stems from the library's failure to enforce http/2 protocol compliance regarding header field validation. According to cwe-1037, this represents a weakness in the design or implementation of header handling mechanisms that allows for malformed data processing. The vulnerability becomes particularly dangerous when applications utilizing h2 downgrade http/2 requests to http/1.1 protocol versions, as the duplicate Host headers are preserved during this transformation process. This creates an environment where the resulting http/1.1 request contains two Host header lines, which violates standard http/1.1 protocol requirements and opens possibilities for request smuggling attacks.
The operational impact of this vulnerability extends beyond simple protocol violation to potentially enable sophisticated attack vectors within web application environments. When applications process these malformed requests, they may interpret the duplicate Host headers differently than expected, potentially leading to inconsistent behavior in proxy configurations, load balancer processing, or application logic that relies on single Host header values. The attack surface becomes particularly concerning in environments where http/2 to http/1.1 downgrading occurs automatically, as this process preserves the malicious header duplication without additional validation layers. This vulnerability aligns with att&ck technique t1566 which involves the exploitation of protocol handling weaknesses to manipulate network traffic and potentially gain unauthorized access or execute arbitrary code through request manipulation.
The mitigation strategy involves upgrading to version 4.4.1 or later, which implements proper header validation to prevent multiple Host headers from being accepted in http/2 request blocks. Organizations should also implement additional monitoring for duplicate header patterns in their application logs and network traffic analysis tools. Security teams should review their http/2 implementation configurations to ensure that proper header sanitization occurs during protocol translation processes, particularly when downgrading between http versions. The fix addresses the root cause by implementing stricter validation rules that align with http/2 specification requirements as defined in rfc 7540, which explicitly states that header fields should be processed according to their specific semantics and that duplicate headers should not be permitted in valid http/2 frames.
Additional defensive measures include implementing network-level controls that can detect and block malformed http/2 traffic patterns, configuring web application firewalls to recognize suspicious header duplication, and conducting regular security assessments of http/2 implementations within the organization's infrastructure. The vulnerability demonstrates the importance of proper protocol validation at multiple layers of network communication systems, as failures in one component can cascade into more serious security issues throughout the application stack. Organizations should also consider implementing automated testing protocols that specifically target header validation scenarios to prevent similar vulnerabilities from being introduced in future development cycles, particularly when working with protocol implementation libraries that handle critical network traffic processing functions.