CVE-2026-86251: h3js h3

Medium severity, CVSS 5.9. EPSS: 0.4% chance of exploitation in the next 30 days.

h3 versions before 1.15.9 contain a path traversal vulnerability in the serveStatic utility. A double-decoding flaw allows a request path containing double-encoded dot sequences (e.g. %252e%252e) to be decoded to %2e%2e, which survives resolveDotSegments() because that function only checks for literal '.' characters. When the resulting asset ID is resolved by URL-based backends (CDN, S3, object storage), %2e%2e is interpreted as '..' per RFC 3986, enabling path traversal to read arbitrary files from the backend.

Affected products

• h3js h3: before 1.15.9 (fixed in 1.15.9)

Published 2026-09-06. Last modified 2026-09-09.