Interpretation Conflict in fast-uri - CVE-2026-16221
Published: July 22, 2026
Vulnerability identifier: #VU139081
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-16221
CWE-ID: CWE-436
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: fastify.io
Affected software:
fast-uri
fast-uri
Detailed vulnerability description
The vulnerability allows a remote attacker to bypass host-based policy checks and route requests to an unintended destination.
The vulnerability exists due to interpretation conflict in the fast-uri URL parser when parsing URLs containing a literal backslash in the authority section for special schemes. A remote attacker can supply a specially crafted URL to bypass host-based policy checks and route requests to an unintended destination.
This issue can occur when an application validates a URL with fast-uri and then passes the same input to Node's WHATWG URL parser or fetch-related consumers, which extract a different host from the same string.
How to mitigate CVE-2026-16221
Install security update from vendor's website.