# CVE-2026-84394: fast-uri vulnerable to host confusion via an unclosed bracket in the URI authority

> An attacker can bypass SSRF protections or redirect allowlists by exploiting host confusion in fast-uri, causing policy checks to evaluate a different host than the one actually requested.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Sep 2026
- **Updated:** 3 Sep 2026
- **Also known as:** DEBIAN-CVE-2026-84394, GHSA-58mr-gqgx-xq4g, MINI-4m8g-93c4-xcj4, MINI-7pfm-r9px-5979, MINI-83f6-7vxm-4gcg, MINI-96v9-g3j8-rjcp, MINI-j6cp-39mx-25w7, MINI-j6j2-35hj-qcx5, MINI-jhpm-j3v5-474f, MINI-mphj-vmwx-jr5r, MINI-qmx2-m939-2hv5, ROOT-APP-NPM-CVE-2026-84394, UBUNTU-CVE-2026-84394

## What is CVE-2026-84394?

CVE-2026-84394 is a high-severity server-side request forgery vulnerability in fast-uri, published on 2 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

fast-uri accepts a host that contains an unbalanced or misplaced authority bracket without reporting an error. A host that starts with an opening bracket but does not end with a closing bracket is neither validated as an IP literal nor canonicalized as a domain name, so parse() returns it as the host with error undefined, while Node's URL and the HTTP clients built on it resolve the same string to a different host. An application that reads the parsed host to make a host decision, such as an SSRF denylist, a redirect allowlist, or proxy routing, and then passes the original URL to an HTTP client evaluates its policy against a string that is not the host the request reaches. The same host is carried through normalize, equal, and resolve. This affects fast-uri versions 2.4.5, 3.1.6, and 4.1.3, and is fixed in 2.4.6, 3.1.7, and 4.1.4, where parse() reports a malformed host for any host that contains a bracket but is not a valid IPv6 literal.

Weakness: CWE-436 (Interpretation Conflict)

## Which versions are affected by CVE-2026-84394?

6 open source packages ship an affected version, including fast-uri. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `fast-uri` | npm | From 2.4.5 | 2.4.6 |
| `fast-uri` | npm | From 2.4.5 | 2.4.6 |
| `fast-uri` | npm | From 3.1.6 | 3.1.7 |
| `fast-uri` | npm | From 3.1.6 | 3.1.7 |
| `fast-uri` | npm | From 4.1.3 | 4.1.4 |
| `fast-uri` | npm | From 4.1.3 | 4.1.4 |

Products: Openjsf Fast-Uri.

## How do I fix CVE-2026-84394?

Upgrade fast-uri to a fixed version for your release line: 2.4.6, 3.1.7 or 4.1.4.

## Is CVE-2026-84394 being exploited?

There is no known exploitation of CVE-2026-84394 in the wild and no public exploit code.

## Other vulnerabilities in fast-uri

- [CVE-2026-84292](https://insights.sbomify.com/CVE-2026-84292/): fast-uri vulnerable to authority injection via an unvalidated port in serialize (high 7.5)
- [CVE-2026-75899](https://insights.sbomify.com/CVE-2026-75899/): fast-uri vulnerable to server-side request forgery via repeated hostname percent-decoding (high 7.5)
- [CVE-2026-75931](https://insights.sbomify.com/CVE-2026-75931/): fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references (high 7.5)
- [CVE-2026-75975](https://insights.sbomify.com/CVE-2026-75975/): fast-uri vulnerable to server-side request forgery via malformed IPv6 normalization (high 7.5)
- [CVE-2026-76172](https://insights.sbomify.com/CVE-2026-76172/): fast-uri vulnerable to host confusion via percent-encoded scheme normalization (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-84394 and fix it if it is affected.

What it is: CVE-2026-84394 is a high-severity server-side request forgery vulnerability in fast-uri, published on 2 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-84394 in the wild and no public exploit code.

Affected packages and fixed versions:
- fast-uri (npm): affected from 2.4.5; fixed in 2.4.6
- fast-uri (npm): affected from 2.4.5; fixed in 2.4.6
- fast-uri (npm): affected from 3.1.6; fixed in 3.1.7
- fast-uri (npm): affected from 3.1.6; fixed in 3.1.7
- fast-uri (npm): affected from 4.1.3; fixed in 4.1.4
- fast-uri (npm): affected from 4.1.3; fixed in 4.1.4

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-84394/ (Markdown: https://insights.sbomify.com/CVE-2026-84394/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-84394
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-84394
- OSV: https://osv.dev/vulnerability/CVE-2026-84394

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-84394/
Summaries are generated and may contain mistakes; the official description above is the record.