CVE-2026-71554: CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') in python-hyper h2
python-hyper h2 versions up to and including 4.4.0 have a vulnerability where HTTP/2 request header blocks containing multiple Host headers are accepted and forwarded to the application. When the consumer downgrades HTTP/2 to HTTP/1.1, this results in two Host header lines 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 HTTP/2 request header blocks that contain more than one Host header. Versions up to 4.4.0 forward all Host headers to the consuming application. If the consumer downgrades the HTTP/2 request to HTTP/1.1, the resulting request contains duplicate Host headers, creating a request smuggling primitive classified under CWE-444. This vulnerability allows an attacker to exploit inconsistent interpretation of HTTP requests between HTTP/2 and HTTP/1.1. The issue is fixed in version 4.4.1.
Potential Impact
The vulnerability enables HTTP request smuggling by allowing multiple Host headers to be forwarded during HTTP/2 to HTTP/1.1 downgrade. This can potentially be used to bypass security controls or interfere with request routing. The CVSS score is 5.3 (medium severity), with no confidentiality or integrity impact, but with a low impact on availability.
Mitigation Recommendations
Upgrade python-hyper h2 to version 4.4.1 or later, where this issue is fixed. No other mitigation is indicated by the vendor advisory or data.
CVE-2026-71554: CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling') in python-hyper h2
Description
python-hyper h2 versions up to and including 4.4.0 have a vulnerability where HTTP/2 request header blocks containing multiple Host headers are accepted and forwarded to the application. When the consumer downgrades HTTP/2 to HTTP/1.1, this results in two Host header lines 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
python-hyper
h2
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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 HTTP/2 request header blocks that contain more than one Host header. Versions up to 4.4.0 forward all Host headers to the consuming application. If the consumer downgrades the HTTP/2 request to HTTP/1.1, the resulting request contains duplicate Host headers, creating a request smuggling primitive classified under CWE-444. This vulnerability allows an attacker to exploit inconsistent interpretation of HTTP requests between HTTP/2 and HTTP/1.1. The issue is fixed in version 4.4.1.
Potential Impact
The vulnerability enables HTTP request smuggling by allowing multiple Host headers to be forwarded during HTTP/2 to HTTP/1.1 downgrade. This can potentially be used to bypass security controls or interfere with request routing. The CVSS score is 5.3 (medium severity), with no confidentiality or integrity impact, but with a low impact on availability.
Mitigation Recommendations
Upgrade python-hyper h2 to version 4.4.1 or later, where this issue is fixed. No other mitigation is indicated by the vendor advisory or data.
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
Threat ID: 6a750711bf8831d5395f5e24
Added to database: 08/06/2026, 22:13:37 UTC
Last enriched: 08/14/2026, 15:52:26 UTC
Last updated: 09/21/2026, 22:46:08 UTC
Views: 60
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