# CVE-2026-84394: fast-uri host confusion via unclosed bracket in authority

Source: https://junglewise.ai/threats/cve-2026-84394-fast-uri-host-confusion-via-unclosed-bracket-in-authority  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-03. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-84394 · Severity: high · CVSS 7.5 · Published 2026-09-03

fast-uri is a JavaScript library for parsing and manipulating URLs used by many web applications to validate domains for security policies like SSRF denylists and redirect allowlists. The library fails to detect malformed hosts with unbalanced brackets (e.g., "[@127.0.0.1"), returning them without error while Node.js HTTP clients resolve the same URL to a different host (127.0.0.1). An attacker can craft URLs that bypass security checks, potentially allowing unauthorized requests to internal systems or arbitrary redirects.

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

## Quick answers

**Is CVE-2026-84394 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-84394 being exploited in the wild.

**How severe is CVE-2026-84394?** High severity, with a CVSS score of 7.5, published 3 September 2026.

**Which products and versions are affected?** fast-uri (npm) SEMVER:introduced=2.4.5,fixed=2.4.6.

**Is there a fix for CVE-2026-84394?** Yes. fast-uri (npm): upgrade to 2.4.6 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-84394 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-84394?** 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 2.4.6 or later

  ```sh
  npm install fast-uri@2.4.6
  ```


## Technical details

The vulnerability is an interpretation conflict (CWE-436) in fast-uri's host parsing. The library accepts hosts containing unbalanced or misplaced brackets ([ or ]) without validation or error reporting, treating them as literal domain names. However, Node.js URL and HTTP clients (http.get, axios, got) parse the same string differently, extracting the actual host from within or after the bracket. An attacker can exploit this desynchronization by crafting URLs like "http://[@127.0.0.1/path" that bypass host validation checks in security policies while the HTTP client resolves to 127.0.0.1. The issue affects parse(), normalize(), equal(), and resolve() functions. Patches are available in fast-uri 2.4.6, 3.1.7, and 4.1.4, where parse() now reports "URI host is malformed" for any bracketed host that is not a valid IPv6 literal.

## Affected products

- OpenJS Foundation fast-uri 2.4.5, 3.1.6, 4.1.3

## Identifiers

- CVE-2026-84394
- GHSA-58mr-gqgx-xq4g

## Timeline

- 2026-09-02: disclosed
- 2026-09-02: patched: Patches released: versions 2.4.6, 3.1.7, 4.1.4

## References

- [https://cna.openjsf.org/security-advisories.html](https://cna.openjsf.org/security-advisories.html)
- [https://github.com/fastify/fast-uri/security/advisories/GHSA-58mr-gqgx-xq4g](https://github.com/fastify/fast-uri/security/advisories/GHSA-58mr-gqgx-xq4g)

## Related threats

- [fast-uri vulnerable to scheme injection via an unescaped first-segment colon in suffix references](https://junglewise.ai/threats/cve-2026-110725-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [fast-uri URI scheme spoofing via percent-encoded scheme](https://junglewise.ai/threats/cve-2026-110306-fast-uri-is-a-dependency-free-rfc-3986-uri-parser-and-serializer)
- [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)