CVE-2025-57804Network attack vector

Request smuggling via incomplete header character validation

Published Aug 25, 2025 · Updated Nov 3, 2025

HTTP request splitting in python-hyper h2 before 4.3.0 allows remote attackers to smuggle requests across HTTP/2-to-HTTP/1.1 gateways. The validate_headers path in src/h2/utilities.py rejected only CR, LF, and NUL, leaving other RFC-illegal header-name bytes and embedded colons accepted. Exploitation requires an embedding server or proxy to downgrade and serialize the accepted HTTP/2 headers as HTTP/1.1, where ambiguous syntax can alter request boundaries and bypass downstream controls.

CVSS severity6.9
Medium
EPSS probability1.73%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request splitting in python-hyper h2 before 4.3.0 allows remote attackers to smuggle requests across HTTP/2-to-HTTP/1.1 gateways. The validate_headers path in src/h2/utilities.py rejected only CR, LF, and NUL, leaving other RFC-illegal header-name bytes and embedded colons accepted. Exploitation requires an embedding server or proxy to downgrade and serialize the accepted HTTP/2 headers as HTTP/1.1, where ambiguous syntax can alter request boundaries and bypass downstream controls.

The record

CVE
CVE-2025-57804
Published
Aug 25, 2025
Updated
Nov 3, 2025
Vendor
python-hyper
Product
h2
Classifications
CWE-93, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 25, 2025CVE publishedPublication date reported by the CVE source.
  2. Nov 3, 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=< 4.3.0

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

python-hyper · h2version=< 4.3.0

Published CVSS scores

6.9GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:N/SC:N/SI:L/SA: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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-93Improper Neutralization of CRLF Sequences ('CRLF Injection')
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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