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Fencer Vulns
medium 6.9

CVE-2025-57804 json

h2 allows HTTP Request Smuggling due to illegal characters in headers

published 2025-08-25 14 mo ago modified 2026-09-10 1 mo ago also GHSA-847f-9342-265h , PYSEC-2026-1435 , EUVD-2025-28631

cvss
6.9
version 4.0, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.9%
EPSS 1.9% chance of exploitation in 30 days, percentile 79%
public exploits
0
none known

Summary

h2 is a pure-Python implementation of a HTTP/2 protocol stack. Prior to version 4.3.0, an HTTP/2 request splitting vulnerability allows attackers to perform request smuggling attacks by injecting CRLF characters into headers. This occurs when servers downgrade HTTP/2 requests to HTTP/1.1 without properly validating header names/values, enabling attackers to manipulate request boundaries and bypass security controls. This issue has been patched in version 4.3.0.

Patch / fix

  • pypi h2 4.3.0
  • python-h2 Debian 14 4.3.0-1

debian advisories DEBIAN-CVE-2025-57804

  • python-h2 SLES for SAP applications 16.0, SLES 16.0 4.2.0-160000.3.1
  • python-h2 SLE Module for Server Applications 15 SP7, SLE Module for Server Applications 15 SP6 3.2.0-150200.3.5.1
  • python-h2 SLE Module for Python 3 15 SP7, SLE Module for Python 3 15 SP6, openSUSE Leap 15.6 4.1.0-150400.8.6.1

suse advisories SUSE-SU-2026:20187-1SUSE-SU-2025:03273-1SUSE-SU-2025:03199-1

  • python-h2 openSUSE Leap 16.0 4.2.0-160000.3.1
  • python-h2 openSUSE Tumbleweed 4.3.0-1.1

opensuse advisories openSUSE-SU-2026:20122-1openSUSE-SU-2025:15496-1

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2025-57804 updated cvss 5.3

h2: h2 allows HTTP Request Smuggling due to illegal characters in headers

  • Red Hat OpenShift Service Mesh 3 istio-cni-rhel9, istio-pilot-rhel9, istio-proxyv2-rhel9, istio-ztunnel-rhel9, istio-rhel9-operator, istio-must-gather-rhel9, +1 more Red Hat Ansible Automation Platform 2 ansible-automation-platform-24/ee-supported-rhel9, ansible-automation-platform-25/ee-cloud-services-rhel9
    affected: workaround, no fix available “Mitigation is either unavailable or does not meet Red Hat Product Security standards for usability, deployment, applicability, or stability.”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
partial
Assessed
14 mo ago

Exploit prediction

1.88% chance of exploitation in the next 30 days, higher than 79% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 4.0 6.9 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
CNA 4.0 6.9 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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Affected packages

ecosystem package affected fixed in
pypi h2
<4.3.0
4.3.0

Affected products

  • python-hyper h2

Classification

Weakness
  • CWE-93 Improper Neutralization of CRLF Sequences

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.