CVE-2026-16221: high-severity vulnerability in fast-uri
fast-uri vulnerable to host confusion via literal backslash authority delimiter
Published · Updated
E-mail alert when it lands in KEV, gets a public PoC or its score goes up.
21Vexday Risk Score
No sign of exploitation. No public exploitation artifact known so far.
ssvc Trackcvss 7.5epss 0.4%
Impact: fast-uri versions from 2.3.1 through 4.1.0 (including the 3.x line up to 3.1.3 and the 2.x line up to 2.4.2) do not treat a literal backslash character (U+005C) as an authority delimiter. Node's native WHATWG URL parser, used by fetch, undici, and Node's http and https clients, normalizes the backslash to a forward slash for special schemes such as http, https, ws, wss, ftp, and file. As a result, the two parsers extract different hosts from the same input string. Applications that use fast-uri to enforce host-based policy such as allowlists, denylists, loopback or SSRF filtering, redirect validation, or outbound proxy routing before passing the same URL into Node's URL or fetch consumers can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts.
Patches: upgrade to fast-uri 4.1.1, 3.1.4, or 2.4.3.
Workarounds: none.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
Affected products
fast-uri · fast-uriRelated CVEs — fast-uri
In the same product, most dangerous first.
CVE-2026-13676HIGHfast-uri vulnerable to host confusion via failed IDN canonicalizationEPSS 0.7%CVE-2026-6321HIGHfast-uri vulnerable to path traversal via percent-encoded dot segmentsEPSS 0.6%CVE-2026-6322HIGHfast-uri vulnerable to host confusion via percent-encoded authority delimitersEPSS 0.5%CVE-2026-75931HIGHfast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative referencesEPSS 0.4%CVE-2026-84394HIGHfast-uri vulnerable to host confusion via an unclosed bracket in the URI authorityEPSS 0.4%CVE-2026-18446HIGHfast-uri vulnerable to host confusion via backslash authority introducerEPSS 0.4%