fast-uri vulnerable to host confusion via backslash authority introducer (CVE-2026-18446)

What is affected, what to do

A flaw was found in fast-uri. This vulnerability arises because fast-uri incorrectly parses Uniform Resource Identifiers (URIs) when a backslash is used in place of a forward slash to introduce the authority component.

Update Fast-uri to version 4.1.2, Fast-uri to version 3.1.5, Fast-uri to version 2.4.4.

Affected

• Fast-uri lt 4.0.0..4.1.2
• Fast-uri lt 3.0.0..3.1.5
• Fast-uri lt *..2.4.4

Fixed in

• Fast-uri 4.1.2
• Fast-uri 3.1.5
• Fast-uri 2.4.4

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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. This vulnerability arises because fast-uri incorrectly parses Uniform Resource Identifiers (URIs) when a backslash is used in place of a forward slash to introduce the authority component. This discrepancy with Node's native WHATWG URL parser can lead to host confusion. A remote attacker could exploit this to bypass security policies, such as allowlists or Server-Side Request Forgery (SSRF) filters, potentially redirecting applications to unintended hosts.

Source: redhat_csaf, BSI WID Portal, BSI CSAF, NVD (NIST), cvelistv5, OSV (Open Source Vulnerabilities)

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-7p8r-x3mc-p8w7web
• NVD (NIST)vuln/detail/CVE-2026-18446advisory
• github.comfastify/fast-uri/commit/f3c6c905f47831007490f466c5945012e905cc52web
• cna.openjsf.orgsecurity-advisories.htmlweb
• github.comfastify/fast-uripackage
• github.comfastify/fast-uri/releases/tag/v4.1.2web

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. 31.07.2026Record 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-2026-18446","role":"CISA Coordinator","options":[{"exploitation":"none"},{"automatable":"yes"},{"technicalImpact":"partial"}],"version":"2.0.3","timestamp":"2026-07-31T17:47:21.450996Z"}

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

2. 31.07.2026ReceivedOpenJS 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-7p8r-x3mc-p8w7
   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 before 4.1.2, 3.1.5, and 2.4.4 requires a literal double forward slash to recognize a URI authority, so a reference that uses a backslash based introducer in place of it (backslash backslash, forward slash backslash, or backslash forward slash) is parsed with no authority and folds into the path. Node's native WHATWG URL parser instead treats a backslash as interchangeable with a forward slash for special schemes, so the two parsers extract different hosts from the same input. Applications that use fast-uri to enforce host based policy such as allowlists, SSRF filtering, or redirect validation before passing the same URL into Node's URL or fetch consumers can be steered to an unintended host. Upgrade to fast-uri 4.1.2, 3.1.5, or 2.4.4.
   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-7p8r-x3mc-p8w7

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