CVE intelligence and bounded remediation
CVE-2025-43859 — h11 is a Python implementation of HTTP/1.1
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.
- Severity
- Critical
- CVSS
- 9.1 (3.1)
- Published
- 2025-04-24
- CISA KEV
- Not currently listed
- Ecosystem
- python/pypi
- Weaknesses
- CWE-444
Affected products
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Matched remediation archetype
HTTP request smuggling and message-boundary ambiguity
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Check exposure
- Inventory every proxy, CDN, gateway, load balancer, service mesh, and application server hop on affected request paths.
- Compare documented parsing and normalization behavior for message length, transfer coding, duplicate headers, and protocol translation.
- Identify connection reuse and which downstream services trust headers added by intermediaries.
Remediate safely
- Update affected intermediaries and origin servers and align them on a single standards-compliant request framing policy.
- Reject ambiguous length, transfer-coding, duplicate, malformed, and unsupported framing before forwarding.
- Normalize or remove hop-by-hop and identity headers at one controlled boundary and add multi-hop regression tests.
Authoritative sources
Complete CVE record and remediation plan
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