CVE-2026-110306 · GHSA-w74x-v6f9-j3xg

fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding

Risk score: 44/100
CVSS: 6.1 v3.1
Exploit chance (30 days): 0.2%
Published: 20 Oct 2026

Open in the live forecast Today's forecast

What to know

What it is: fast-uri is a dependency-free RFC 3986 URI parser and serializer for Node.js. In versions before 2.4.8, from 3.0.0 through 3.1.8, and from 4.0.0 through 4.2.2, fast-uri percent-decodes the scheme component with the legacy global unescape() function before validating it, so an input whose decoded scheme is a dangerous …
Affected: fast-uri (npm) < 2.4.8; fast-uri (npm) >= 3.0.0, < 3.1.9; fast-uri (npm) >= 4.0.0, < 4.2.3
Fix: Upgrade fast-uri to 2.4.8 or 3.1.9 or 4.2.3 (whichever matches your release line) or later.
Exploitation: No known exploitation

Story map

Story map: the flaws, the fix, how it unfolded and what it connects to. Boxes link to the record or article.

At a glance

fast-uri: Amber, patch this week. fast-uri accepts a host that contains an unbalanced or misplaced authority bracket ([ or ]) without reporting an error. A host that starts with [ but does not end with ], such as [@127.0.0.1, is neither validated as an IP literal nor canonicalized as a domain name, so parse() returns it as the host with error undefine…

The flaws

CVE-2026-110306: Fast-uri vulnerable to URI scheme spoofing via percent-encoded scheme decoding (CVSS 6.1 · EPSS 0%)
CVE-2026-84394: Fast-uri vulnerable to host confusion via an unclosed bracket in the URI authority (CVSS 7.5 · EPSS 0%)
CVE-2026-84292: Fast-uri vulnerable to authority injection via an unvalidated port in serialize (CVSS 7.5 · EPSS 0%)

The fix

Upgrade fast-uri: 2.4.8 or later on your release line

Who is exposed: fast-uri (npm) = 2.4.5; fast-uri (npm) = 3.1.6; fast-uri (npm) = 4.1.3

Where it stands

No known exploitation

Connects to

Same weakness: CWE-172 (CWE-172). 4 other warnings in 30 days

Why it is Yellow

CVSS 6.1 medium
On your watchlist: npm

Affected packages

Package	Ecosystem	Vulnerable	Fixed in
fast-uri	npm	< 2.4.8	2.4.8
fast-uri	npm	>= 3.0.0, < 3.1.9	3.1.9
fast-uri	npm	>= 4.0.0, < 4.2.3	4.2.3

References

Original description

fast-uri is a dependency-free RFC 3986 URI parser and serializer for Node.js. In versions before 2.4.8, from 3.0.0 through 3.1.8, and from 4.0.0 through 4.2.2, fast-uri percent-decodes the scheme component with the legacy global unescape() function before validating it, so an input whose decoded scheme is a dangerous token is emitted as a live scheme even though RFC 3986 does not allow percent-encoding in a scheme and no other parser accepts it. An application that screens dangerous schemes on the raw input and then stores or renders the normalized or resolved result can turn an inert string into an executable javascript, data, or vbscript URL, producing a stored or reflected cross-site scripting vector, and the equal() comparison treats an encoded and a plain scheme as the same, which can bypass equality-based allowlists. The issue is fixed in fast-uri 2.4.8, 3.1.9, and 4.2.3, and users should upgrade to one of these versions or later. As a workaround, reject input whose scheme part contains a percent sign, or check the parsed scheme against an allowlist after normalization and use that same parsed result rather than the raw string.

Last activity 23 Oct 2026. Patch Forecast updates every hour.