CVE-2026-71554Public exploitNetwork attack vector

Request smuggling via duplicate Host header forwarding

Published Aug 6, 2026 · Updated Aug 6, 2026

HTTP request smuggling in python-hyper h2 4.4.0 and earlier allows remote attackers to create ambiguous HTTP/1.1 requests during downgrade. Before 4.4.1, _validate_host_authority_header retains every duplicate Host field instead of rejecting the second, so the consumer receives both values. Exploitation requires an embedding consumer that converts HTTP/2 to HTTP/1.1; two emitted Host lines can cause divergent downstream parsing.

CVSS severity5.3
Medium
EPSS probability0.44%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in python-hyper h2 4.4.0 and earlier allows remote attackers to create ambiguous HTTP/1.1 requests during downgrade. Before 4.4.1, _validate_host_authority_header retains every duplicate Host field instead of rejecting the second, so the consumer receives both values. Exploitation requires an embedding consumer that converts HTTP/2 to HTTP/1.1; two emitted Host lines can cause divergent downstream parsing.

The record

CVE
CVE-2026-71554
Published
Aug 6, 2026
Updated
Aug 6, 2026
Vendor
python-hyper
Product
h2
Classifications
CWE-444
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 6, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 6, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 4.4.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

python-hyper · h2version=< 4.4.1

Published CVSS scores

5.3GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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