CVE-2025-43859Network attack vector

Request smuggling via unchecked chunk terminators

Published Apr 24, 2025 · Updated Apr 24, 2025

HTTP request smuggling in python-hyper h11 before 0.16.0 allows remote attackers to bypass proxy controls via malformed chunked bodies. ChunkedReader discards any two bytes after chunk data instead of verifying the required CRLF terminator, so h11 and a lenient proxy split the same byte stream at different boundaries. Exploitation requires an h11-based server behind a proxy that also mishandles chunk terminators, enabling protected-request injection or cross-user credential disclosure.

CVSS severity9.1
Critical
EPSS probability0.58%
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 h11 before 0.16.0 allows remote attackers to bypass proxy controls via malformed chunked bodies. ChunkedReader discards any two bytes after chunk data instead of verifying the required CRLF terminator, so h11 and a lenient proxy split the same byte stream at different boundaries. Exploitation requires an h11-based server behind a proxy that also mishandles chunk terminators, enabling protected-request injection or cross-user credential disclosure.

The record

CVE
CVE-2025-43859
Published
Apr 24, 2025
Updated
Apr 24, 2025
Vendor
python-hyper
Product
h11
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 24, 2025CVE publishedPublication date reported by the CVE source.
  2. Apr 24, 2025Record 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=< 0.16.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

python-hyper · h11version=< 0.16.0

Published CVSS scores

9.1GitHubCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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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 exploitUnconfirmed

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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