Security Digest · Vulnerability record
CVE-2025-43859: h11 accepts some malformed Chunked-Encoding bodies
h11 is a Python implementation of HTTP/1.1.
CVSS 9.1 Critical EPSS 0.6% · p47
Should you act?
Critical and remotely reachable
No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- No confirmed exploitation. It is not in CISA’s KEV catalog and CISA’s SSVC assessment records no known exploitation. That can change quickly after a public write-up; re-check if you are affected.
- EPSS 0.6%, percentile 47: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 47% of all scored CVEs (low). EPSS is a probability, not a verdict.
- Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
- Only applies if your dependency tree includes
h11(PyPI) < 0.16.0, fixed in 0.16.0. Check your lockfile, including transitive dependencies.
What it is
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.
From the GitHub advisory
Impact
A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions.
Details
HTTP/1.1 Chunked-Encoding bodies are formatted as a sequence of "chunks", each of which consists of:
- chunk length - \r\n - length bytes of content - \r\n
Weakness type: CWE-444.
Product: h11 (PyPI) · Assigned by: GitHub_M · Published: 2025-04-24
Affected and fixed versions
| Ecosystem | Package | Vulnerable | First fixed |
|---|---|---|---|
| PyPI | h11 | < 0.16.0 | 0.16.0 |
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| python-hyper | h11 | < 0.16.0 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| GitHub_M | 3.1 | 9.1 Critical | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
| GitHub | 3.1 | 9.1 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N |
- Attack vector: network (remotely reachable)
- Attack complexity: low
- Privileges required: none (no login)
- User interaction: none
- Scope: unchanged
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
- CISA SSVC assessment: exploitation none, automatable yes, technical impact total.
- FIRST EPSS (model of 2026-10-21): 0.58% probability of exploitation activity in the next 30 days; percentile 47.
Timeline
- — CVE record last updated
- — CVE record published by GitHub_M
- — GitHub-reviewed advisory GHSA-vqfr-h8mv-ghfj published (critical)
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vulnerability records
- GitHub Advisory GHSA-vqfr-h8mv-ghfj github.com
- CVE.org record CVE-2025-43859 cve.org
- NVD entry CVE-2025-43859 nvd.nist.gov
Other references from the CVE record and advisory
- github.com github.com
- github.com github.com