CVE-2026-86251

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.

• https://github.com/h3js/h3/security/advisories/GHSA-72gr-qfp7-vwhw
• https://www.vulncheck.com/advisories/h3-before-1.15.9-path-traversal-via-double-decoding