fast-uri vulnerable to host confusion via skipped IDN canonicalization on scheme-relative references (CVE-2026-75931)

What is affected, what to do

A flaw was found in fast-uri, a URI parser for Node.js. This vulnerability arises because the parser fails to consistently convert internationalized domain names (IDN) to their standard ASCII form when processing scheme-relative references.

Update Fast-uri to version 2.4.5, Fast-uri to version 3.1.6, Fast-uri to version 4.1.3.

Affected

• Fast-uri lt 2.4.2..2.4.5
• Fast-uri lt 3.1.3..3.1.6
• Fast-uri lt 4.0.1..4.1.3

Fixed in

• Fast-uri 2.4.5
• Fast-uri 3.1.6
• Fast-uri 4.1.3

Not sure whether and how you are affected? Ask a question about this vulnerability

Metrics

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2

NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.

Read in glossary

High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.

CRA

The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.

Read in glossary

From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.

ISO/IEC 27001

ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security.

Read in glossary

Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

A flaw was found in fast-uri, a URI parser for Node.js. This vulnerability arises because the parser fails to consistently convert internationalized domain names (IDN) to their standard ASCII form when processing scheme-relative references. This inconsistency can lead to host confusion, where an application might validate a URI against a security policy using one hostname, but then resolve to a different, potentially malicious, hostname. This discrepancy could allow an attacker to bypass security policies and access unauthorized resources.

Source: redhat_csaf, NVD (NIST), OSV (Open Source Vulnerabilities), cvelistv5, GitHub Security Advisories

Vendors and products

• Fast-uri Fast-uri
• Openjsf OpenJS Foundation fast-uri

Weakness classes (CWE)

• CWE-436Interpretation Conflict

  Class

  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.

  Entry at MITRE CWE-436

References & sources

• github.comfastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8
• cna.openjsf.orgsecurity-advisories.html
• NVD (NIST)vuln/detail/CVE-2026-75931advisory
• github.comfastify/fast-uri/commit/0256bc8d1f28b5d0ac657faf67e2411a189dfcb5web
• github.comfastify/fast-uri/commit/444ecdad447db2cc23c4d422acc6f0daa6fa8eefweb
• github.comfastify/fast-uri/commit/4e4ebd8b245f6ca16e448203559eea3e545453a3web
• github.comfastify/fast-uripackage
• github.comfastify/fast-uri/releases/tag/v2.4.5web
• github.comfastify/fast-uri/releases/tag/v3.1.6web
• github.comfastify/fast-uri/releases/tag/v4.1.3web

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

1. 02.09.2026Initial analysis by NVDNVD

   Newly listed as affected:

   • Openjsf OpenJS Foundation fast-uri (2.4.2 – before 2.4.5; 3.1.3 – before 3.1.6; 4.0.1 – before 4.1.3)

   New references:

   • https://cna.openjsf.org/security-advisories.html
   • https://github.com/fastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8
   Show 3 original changes

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   Product configuration	added	—	OR *cpe:2.3:a:openjsf:fast-uri:*:*:*:*:*:node.js:*:* versions from (including) 2.4.2 up to (excluding) 2.4.5 *cpe:2.3:a:openjsf:fast-uri:*:*:*:*:*:node.js:*:* versions from (including) 3.1.3 up to (excluding) 3.1.6 *cpe:2.3:a:openjsf:fast-uri:*:*:*:*:*:node.js:*:* versions from (including) 4.0.1 up to (excluding) 4.1.3
   Reference type	added	—	openjs: https://cna.openjsf.org/security-advisories.html Types: Vendor Advisory
   Reference type	added	—	openjs: https://github.com/fastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8 Types: Mitigation, Vendor Advisory

   Source identifier: nvd@nist.gov

2. 24.08.2026Record modifiedCISA ADP

   CISA assessment (SSVC): exploitation: none known, automatable: yes

   Show the original change

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   SSVC assessment	added	—	{"id":"CVE-2026-75931","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-08-24T16:28:35.705177Z"}

   Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

3. 24.08.2026ReceivedOpenJS Foundation

   CVSS V3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

   Weakness type: CWE-436

   Description added or changed

   New references:

   • https://cna.openjsf.org/security-advisories.html
   • https://github.com/fastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8
   Show 6 original changes

   Individual changes of this event as published by NVD

   Type	Action	Before	After
   Affected versions (publisher)	added	—	fast-uri
   Description	added	—	fast-uri is a URI parser for Node.js. It canonicalizes a host to its ASCII form only when the input carries an explicit scheme, so a scheme-relative reference such as a host preceded by two slashes is returned with its host verbatim and no error set. As a result fast-uri's own entry points disagree with each other: parse, resolve, normalize, and equal can yield different hosts for the same input depending only on whether a scheme is written out, and equal can return opposite verdicts for the same pair of hosts. An application that extracts a host with fast-uri to check it against a policy list and then resolves the same reference can make its decision on one host while the destination is another, enabling host confusion and policy bypass. The affected versions are 2.4.2 up to but not including 2.4.5, 3.1.3 up to but not including 3.1.6, and 4.0.1 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which canonicalize the host consistently across the resolve path. Users should upgrade to a patched version.
   CVSS	added	—	AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
   Weakness type (CWE)	added	—	CWE-436
   Reference	added	—	https://cna.openjsf.org/security-advisories.html
   Reference	added	—	https://github.com/fastify/fast-uri/security/advisories/GHSA-5jgf-p345-68v8

   Source identifier: ce714d77-add3-4f53-aff5-83d477b104bb

One NVD translation into another language is hidden.