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

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.7% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-06-29 · updated 2026-09-11
Weakness: CWE-436

Description

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 products

fast-uri: from 2.3.1 before 3.1.3; from 4.0.0 before 4.0.1 — fixed in 3.1.3, 4.0.1

How to fix

Upgrade to a fixed version: fast-uri 3.1.3, 4.0.1. Check the vendor advisory for mitigations.

References

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