CVE-2025-43859

CRITICALNVD 9.19.1—
EchelonGraph scoreMEDIUM confidence

This critical-severity CVE scores 9.1 under NVD CVSS v3. EPSS exploit probability: 0.6%, top 57% of all CVEs by exploit prediction. GitHub Security Advisory data not yet ingested — confidence will rise once GHSA publishes (typical lag: hours to days for open-source ecosystem CVEs; never for infrastructure-only CVEs).

Triggered by: NVD CVSS baseline
Sources: epss, nvd
9.1EG
EchelonGraph verdictPlan a fixSerious severity, but no confirmed exploitation yet.
  • High severity, but no confirmed exploitation yet
CISA-KEV: Not listedEPSS PROB: 1%CVSS: 9.1Exploit: None knownExposed: 0

A fix is available — apply it.

h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.

CVSS v3
9.1
EG Score
9.1(medium)
EG Risk
56(Track)
EG Risk 56/100SSVC: Track

EG Risk is EchelonGraph's 0–100 priority score: it fuses intrinsic severity with real-world exploitation and automatability so you can rank equal-severity CVEs and fix the most dangerous first. Higher = act sooner. Distinct from the 0–10 EG Score (severity).

How it’s computed
Severity91% × 45%
Exploitation1% × 40%
Automatability100% × 15%
Action: Routine — remediate on your standard cadence.
EPSS PROB
1%
EPSS %ILE
46%
KEV
Not listed

Published

April 24, 2025

Last Modified

April 15, 2026

Advisory Details (2)

Auto-updated Aug 1, 2026
Patch available. Sources: github, github_commit.
github Patch Available

h11 accepts some malformed Chunked-Encoding bodies · Advisory · python-hyper/h11 · GitHub

https://github.com/python-hyper/h11/security/advisories/GHSA-vqfr-h8mv-ghfj
github_commit

commit 114803a29ce5 (python-hyper/h11)

Fix landed in python-hyper/h11 commit 114803a29ce5 — awaiting tagged release

https://github.com/python-hyper/h11/commit/114803a29ce50116dc47951c690ad4892b1a36ed

Vendor Advisories for CVE-2025-43859(4)

These vendors published their own advisory mentioning this CVE — often with vendor-specific remediation steps + affected product lists not in NVD.

Patch Availability(5)

Vendor / EcosystemFixed in / PatchReleasedSource
ubuntupython3-h11 (0.14.0-1ubuntu0.25.04.1) @ plucky2026-05-21ubuntu
redhatansible-automation-platform-24/lightspeed-rhel8:2.4.250225-122025-06-05redhat
redhatansible-automation-platform-25/ee-supported-rhel9:1.0.0-10152025-05-27redhat
redhatpython-h11-0:0.12.0-4.el9ost2025-05-14redhat
redhatpython-h11-0:0.12.0-2.1.el9ost2025-05-14redhat

Patches are aggregated from vendor advisories (Red Hat, Microsoft, Cisco, GitHub) and package ecosystems (OSV, GHSA). Multiple rows for the same upstream release have been deduplicated.

Affected Packages

(4 across 4 ecosystems)
Debian:12(1)
PackageVulnerable rangeFixed inDependents
python-h110.14.0-10.14.0-1.1~deb12u1—
Debian:13(1)
PackageVulnerable rangeFixed inDependents
python-h11—0.14.0-1.1—
Debian:14(1)
PackageVulnerable rangeFixed inDependents
python-h11—0.14.0-1.1—
PyPI(1)
PackageVulnerable rangeFixed inDependents
h110.0.1 ... 0.9.0 (13 versions)0.16.0—

Weakness Classification(1)

MITRE Common Weakness Enumeration — the root-cause categories this CVE belongs to.

Additional Vendor Advisories

(1)

Vendors that published advisories for this CVE beyond the curated set above. Broader coverage but minimal per-row detail — click through for the original advisory.

Data Freshness Timeline

(refreshed 6× in last 7d / 31× in last 30d)

Each row is a source pipeline that fetched or updated this CVE on that date, with what changed. For example, "NVD update" means NVD published or revised its analysis for this CVE; "MITRE cvelistV5" means we ingested or refreshed it from the CNA feed. Most recent first.

Showing the most recent 100 of 187 total refreshes for this CVE.

  1. 2026-09-18 19:27 UTCEPSS rescore
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Frequently asked(5)

What is CVE-2025-43859?
CVE-2025-43859 is a critical vulnerability published on April 24, 2025. h11 is a Python implementation of HTTP/1.1. Prior to version 0.16.0, a leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions. This issue has been patched in version 0.16.0. Since exploitation requires…
When was CVE-2025-43859 disclosed?
CVE-2025-43859 was first published in the National Vulnerability Database on April 24, 2025, with the most recent update on April 15, 2026. EchelonGraph re-ingests CVE updates from NVD on a 2-hour cycle, so this page reflects the latest published state.
Is CVE-2025-43859 actively exploited?
CVE-2025-43859 is not currently on CISA's Known Exploited Vulnerabilities catalog. FIRST EPSS estimates a 1% probability of exploitation in the next 30 days, which ranks it in the top 53.6% of all scored CVEs.
What is the CVSS score of CVE-2025-43859?
CVE-2025-43859 has a CVSS v3 base score of 9.1 (NVD).
How do I remediate CVE-2025-43859?
Patch to the fixed version published by the affected vendor. Where vendor advisories exist for CVE-2025-43859, EchelonGraph cross-links them in the Vendor Advisories panel below — those typically contain the canonical remediation steps, fixed version numbers, and any vendor-specific mitigations.

Dependency Blast Radius

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