CVE-2026-13676 — fast-uri vulnerable to host confusion via failed IDN canonicalization

Description (CNA)

fast-uri versions 2.3.1 through 3.1.2 and 4.0.0 fail to canonicalize Unicode (IDN) hostnames for HTTP-family URLs. The IDN conversion path calls a helper that does not exist on the global URL constructor, silently leaving the host in its original Unicode form while normalize() and equal() still return values that differ from a WHATWG-compatible URL parser. Applications that use fast-uri to enforce host-based policy (denylists, loopback filtering, redirect validation, outbound proxy routing) before passing the same URL to Node's URL or fetch can be bypassed when the two implementations resolve the same input to different hosts. Patches: upgrade to fast-uri 3.1.3 for the 3.x line or 4.0.1 for the 4.x line. Workarounds: enforce host policy using the same URL parser used for the actual request, or reject non-ASCII hosts before policy checks.

Affected (CNA)

Vendor / product	Versions
fast-uri / fast-uri	affected 4.0.0 before 4.0.1; unaffected 4.0.1; affected 2.3.1 before 3.1.3; unaffected 3.1.3

CWE

CWE-436

References

ADP enrichment

CISA-ADP

No English description supplied.

No references supplied.

redhat-SADP

A flaw was found in fast-uri. This vulnerability occurs because fast-uri fails to properly convert Unicode (Internationalized Domain Name - IDN) hostnames for HTTP-family URLs. This can lead to a situation where security policies, such as denylists or redirect validations, are bypassed when applications use fast-uri to enforce these policies before passing the URL to another parser. A remote attacker could exploit this to circumvent security controls and potentially access unauthorized resources or perform malicious redirects.