# CVE-2026-13676: fastify fast-uri host confusion via failed IDN canonicalization

Source: https://junglewise.ai/threats/cve-2026-13676-fastify-fast-uri-host-confusion-via-failed-idn-canonicalization  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-29. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-13676 · Severity: high · CVSS 7.5 · Published 2026-06-29

fast-uri is a library used to parse and validate URLs in Node.js applications. A flaw in how it handles international domain names (IDN) causes it to fail converting Unicode hostnames to their ASCII equivalents, leaving them in their original form. This creates a mismatch: when applications use fast-uri to enforce host-based security policies (like blocking loopback addresses or enforcing allow-lists), an attacker can bypass these checks by submitting URLs with visually similar Unicode characters that fast-uri accepts but that Node's URL parser converts to standard ASCII, leading to unintended destinations and policy bypass.

Technologies: [fast-uri (npm)](https://junglewise.ai/threats/technologies/npm-fast-uri). Vendors: [npm](https://junglewise.ai/threats/vendors/npm).

## Quick answers

**Is CVE-2026-13676 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-13676 being exploited in the wild.

**How severe is CVE-2026-13676?** High severity, with a CVSS score of 7.5, published 29 June 2026.

**Which products and versions are affected?** fast-uri (npm) SEMVER:introduced=4.0.0,fixed=4.0.1.

**Is there a fix for CVE-2026-13676?** Yes. fast-uri (npm): upgrade to 4.0.1 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-13676 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-13676?** Check whether you run fast-uri (npm) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- fast-uri (npm): upgrade to 4.0.1 or later

  ```sh
  npm install fast-uri@4.0.1
  ```


## Technical details

The vulnerability is an interpretation conflict (CWE-436) in fast-uri's IDN canonicalization logic. When parsing HTTP URLs with Unicode/IDN hostnames, fast-uri attempts to call URL.domainToASCII() on the global WHATWG URL constructor; however, this method does not exist globally, resulting in a TypeError. The error is silently caught and stored in parsed.error, but the parse(), normalize(), and equal() functions return with the host left uncanonicalized in its original Unicode form. This causes a semantic desynchronization: fast-uri treats http://127。0。0。1/ (with fullwidth periods) as host "127。0。0。1", while Node's native WHATWG URL parser and fetch() canonicalize the same input to "127.0.0.1". Applications relying on fast-uri for host-based access control (denylists, loopback filtering, redirect validation, outbound proxy routing) before passing URLs to Node's URL or fetch() consumers are vulnerable to policy bypass. The vulnerability affects versions >=2.3.1 and <=4.0.0. Patches are available in v2.4.2, v3.1.3, and v4.0.1, which fix the issue by using WHATWG URL hostname parsing instead of the non-existent domainToASCII helper.

## Affected products

- Fastify fast-uri >=2.3.1, <=4.0.0 (unfixed); 2.4.2, 3.1.3, 4.0.1 (patched)

## Identifiers

- CVE-2026-13676
- GHSA-4c8g-83qw-93j6

## Timeline

- 2026-06-29: disclosed
- 2026-07-21: advisory
- 2026-07-17: patched: v2.4.2, v3.1.3, v4.0.1 released

## References

- [https://github.com/fastify/fast-uri/security/advisories/GHSA-4c8g-83qw-93j6](https://github.com/fastify/fast-uri/security/advisories/GHSA-4c8g-83qw-93j6)
- [https://github.com/fastify/fast-uri/pull/188](https://github.com/fastify/fast-uri/pull/188)
- [https://github.com/fastify/fast-uri/commit/2a6d357a18a68e6d812824379fd3388a1ae50d05](https://github.com/fastify/fast-uri/commit/2a6d357a18a68e6d812824379fd3388a1ae50d05)
- [https://github.com/fastify/fast-uri/commit/21ea1f9d70495c931f55dff893a8fa38f4f2e6bd](https://github.com/fastify/fast-uri/commit/21ea1f9d70495c931f55dff893a8fa38f4f2e6bd)
- [https://github.com/fastify/fast-uri/commit/01db48010f594b98f7b323be18b393791c66ed1d](https://github.com/fastify/fast-uri/commit/01db48010f594b98f7b323be18b393791c66ed1d)
- [https://access.redhat.com/errata/RHSA-2026:63371](https://access.redhat.com/errata/RHSA-2026:63371)

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- [fast-uri host confusion via unchecked WHATWG canonicalization](https://junglewise.ai/threats/cve-2026-110180-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [fast-uri vulnerable to improper path equivalence in relative reference normalization](https://junglewise.ai/threats/cve-2026-110095-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [fast-uri mailto header injection via percent-encoded field-name desynchronization](https://junglewise.ai/threats/cve-2026-86818-fast-uri-mailto-header-injection-via-percent-encoded-field-name)