CVE-2025-43859 | h11 accepts some malformed Chunked-Encoding bodies

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.

Published: 2025-04-24 Last update: 2026-04-15 Assigner: [email protected] Source: [email protected]
NVD Status：Deferred，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2025-43859 is rated Moderate Risk (50.1/100): CVSS Critical severity, with low exploitation likelihood (EPSS 0.18%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2025-43859

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-02 0.35% 0.18% -0.16%
2 2026-02-18 0.07% 0.35% +0.27%
3 2025-11-18 — 0.07% —

Full EPSS history (8 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2025-43859

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
9.1 3.1 CRITICAL
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 5.2 [email protected]

Weakness enumeration for CVE-2025-43859

GitHub Security Advisory for CVE-2025-43859

GHSA-vqfr-h8mv-ghfj · Severity: critical · Ecosystem: pip — h11 accepts some malformed Chunked-Encoding bodies

OS Trackers for CVE-2025-43859

vendor priority summary link
debian unimportant CVE-2025-43859 unimportant priority: Debian including 1 source packages (python-h11), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2025-43859
redhat high — https://access.redhat.com/security/cve/CVE-2025-43859
suse critical CVE-2025-43859 severity critical: SUSE including 246 source package names (0.0.17-1.1:python311-h11-0.14.0-150400.9.6.1, 0.1.6-1.2:python311-h11-0.14.0-150400.9.6.1, …), 299 product×package rows across 55 product lines (Container containers/lmcache-lmstack-router, Container containers/lmcache-vllm-openai, … (55 product lines)): Known Affected 231, Fixed 68. https://www.suse.com/security/cve/CVE-2025-43859/
ubuntu medium CVE-2025-43859 medium priority: Ubuntu including 1 source packages (python-h11), 6 status rows across 6 suites (focal, jammy, noble, oracular, plucky, upstream): released 4, not-affected 2. https://ubuntu.com/security/CVE-2025-43859

Affected software / configurations for CVE-2025-43859

Vendor Product Version Raw CPE
No affected products in dataset.

References for CVE-2025-43859

cvelogic Threat Intelligence