CVE-2026-71554: CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') in python-hyper h2
The python-hyper h2 library versions up to and including 4.4.0 have a vulnerability where multiple Host headers in HTTP/2 requests are accepted and forwarded to the application. When the consumer downgrades HTTP/2 to HTTP/1.1, this results in two Host headers in the HTTP/1.1 request, enabling HTTP request smuggling. This issue is resolved in version 4.4.1.
AI Analysis
Technical Summary
The h2 library, a pure-Python HTTP/2 protocol stack, improperly handles request header blocks containing multiple Host headers in versions up to 4.4.0. It forwards all Host headers to the consuming application. If the application downgrades HTTP/2 requests to HTTP/1.1, the presence of two Host headers in the resulting HTTP/1.1 request creates a request smuggling primitive (CWE-444). This vulnerability was fixed in version 4.4.1.
Potential Impact
This vulnerability allows an attacker to craft HTTP/2 requests with multiple Host headers that, when downgraded to HTTP/1.1, result in ambiguous or inconsistent interpretation of the request by downstream components. This can facilitate HTTP request smuggling attacks, potentially leading to request routing issues or bypassing security controls. The CVSS score is 5.3 (medium severity), indicating limited impact with no confidentiality or integrity loss but some availability impact.
Mitigation Recommendations
Upgrade to python-hyper h2 version 4.4.1 or later, where this issue is fixed. Patch status is confirmed by the version fix. No other mitigation is indicated.
CVE-2026-71554: CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') in python-hyper h2
Description
The python-hyper h2 library versions up to and including 4.4.0 have a vulnerability where multiple Host headers in HTTP/2 requests are accepted and forwarded to the application. When the consumer downgrades HTTP/2 to HTTP/1.1, this results in two Host headers in the HTTP/1.1 request, enabling HTTP request smuggling. This issue is resolved in version 4.4.1.
CVSS v3.1
Score 5.3medium
Affected software
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Weaknesses
AI-Powered Analysis
Machine-generated threat intelligence
Technical Analysis
The h2 library, a pure-Python HTTP/2 protocol stack, improperly handles request header blocks containing multiple Host headers in versions up to 4.4.0. It forwards all Host headers to the consuming application. If the application downgrades HTTP/2 requests to HTTP/1.1, the presence of two Host headers in the resulting HTTP/1.1 request creates a request smuggling primitive (CWE-444). This vulnerability was fixed in version 4.4.1.
Potential Impact
This vulnerability allows an attacker to craft HTTP/2 requests with multiple Host headers that, when downgraded to HTTP/1.1, result in ambiguous or inconsistent interpretation of the request by downstream components. This can facilitate HTTP request smuggling attacks, potentially leading to request routing issues or bypassing security controls. The CVSS score is 5.3 (medium severity), indicating limited impact with no confidentiality or integrity loss but some availability impact.
Mitigation Recommendations
Upgrade to python-hyper h2 version 4.4.1 or later, where this issue is fixed. Patch status is confirmed by the version fix. No other mitigation is indicated.
Technical Details
- Data Version
- 5.2
- Assigner Short Name
- GitHub_M
- Date Reserved
- 2026-08-06T21:24:15.376Z
- Cvss Version
- 3.1
- State
- PUBLISHED
- Remediation Level
- null
Threat ID: 6a750711bf8831d5395f5e24
Added to database: 08/06/2026, 22:13:37 UTC
Last enriched: 08/06/2026, 22:27:22 UTC
Last updated: 08/07/2026, 04:14:08 UTC
Views: 9
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