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fast-uri vulnerable to host confusion via failed IDN canonicalization

High severity GitHub Reviewed Published Jun 29, 2026 in fastify/fast-uri • Updated Sep 11, 2026

Package

npm fast-uri (npm)

Affected versions

>= 4.0.0, < 4.0.1
>= 3.0.0, < 3.1.3
>= 2.3.1, < 2.4.2

Patched versions

4.0.1
3.1.3
2.4.2

Description

Impact

fast-uri versions >= 2.3.1, <= 4.0.0 fail to canonicalize Unicode/IDN hostnames for HTTP-family URLs. The IDN conversion path calls URL.domainToASCII(...) on the global WHATWG URL constructor, where that helper does not exist. The resulting TypeError is silently routed into parsed.error, but parse(), normalize(), and equal() all return with the host left in its original Unicode form.

For example, http://127。0。0。1/ is treated by fast-uri as host 127。0。0。1, while Node's WHATWG URL parser and fetch() canonicalize the same input to 127.0.0.1.

Applications that use fast-uri to enforce host-based policy (denylists, loopback filtering, redirect validation, outbound proxy routing) before passing the same URL into Node's URL or fetch() consumers see a policy/use desync and can be steered to an unintended destination.

Patches

Upgrade to fast-uri v4.0.1, v3.1.3, or v2.4.2

Workarounds

None. Upgrade to the patched version.

References

@mcollina mcollina published to fastify/fast-uri Jun 29, 2026
Published by the National Vulnerability Database Jun 29, 2026
Published to the GitHub Advisory Database Jul 21, 2026
Reviewed Jul 21, 2026
Last updated Sep 11, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(39th percentile)

Weaknesses

Interpretation Conflict

Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state. Learn more on MITRE.

Incorrect Behavior Order: Authorization Before Parsing and Canonicalization

If a web server does not fully parse requested URLs before it examines them for authorization, it may be possible for an attacker to bypass authorization protection. Learn more on MITRE.

CVE ID

CVE-2026-13676

GHSA ID

GHSA-4c8g-83qw-93j6

Source code

Credits

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