What is CVE-2026-86205?

CVE-2026-86205 is a medium-severity vulnerability in H3js H3, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 5.4/10. Published 2026-09-06.

How severe is CVE-2026-86205?

Medium severity. CVSS v3 base score is 5.4 out of 10.

Open Redirect in H3js H3

CVE-2026-86205

h3 versions before 2.0.1-rc.18 contain an open redirect vulnerability in the redirectBack() utility that fails to sanitize protocol-relative paths in the Referer header pathname. Attackers can craft a same-origin URL with a double-slash path segment that passes origin validation but produces a Location header interpreted by browsers as a protocol-relative redirect to an external domain.

Vulnerability class: Open Redirect

Published 2026-09-06 · last modified 2026-09-08.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.003 (18.3th percentile), scored 2026-10-22.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 18th percentile — 18.3% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.002 · newest: 0.003 · change: +0.001

Affected products

H3js H3

Weakness classification (CWE)

CWE-601 — URL Redirection to Untrusted Site (Open Redirect)

References

github.com/h3js/h3/security/advisories/GHSA-fp4x-ggrf-wmc6 (vendor-advisory)
www.vulncheck.com/advisories/h3-before-2.0.1-rc.18-open-redirect-via-redirectba… (third-party-advisory)

Frequently asked questions

What is CVE-2026-86205?: CVE-2026-86205 is a medium-severity vulnerability in H3js H3, classified under URL Redirection to Untrusted Site (Open Redirect). CVSS score: 5.4/10. Published 2026-09-06.
How severe is CVE-2026-86205?: Medium severity. CVSS v3 base score is 5.4 out of 10.