CVE-2026-16221
fast-uri vulnerable to host confusion via literal backslash authority delimiter
Description
Impact: fast-uri versions from 2.3.1 through 4.1.0 (including the 3.x line up to 3.1.3 and the 2.x line up to 2.4.2) do not treat a literal backslash character (U+005C) as an authority delimiter. Node's native WHATWG URL parser, used by fetch, undici, and Node's http and https clients, normalizes the backslash to a forward slash for special schemes such as http, https, ws, wss, ftp, and file. As a result, the two parsers extract different hosts from the same input string. Applications that use fast-uri to enforce host-based policy such as allowlists, denylists, loopback or SSRF filtering, redirect validation, or outbound proxy routing before passing the same URL into Node's URL or fetch consumers can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts. Patches: upgrade to fast-uri 4.1.1, 3.1.4, or 2.4.3. Workarounds: none.
INFO
Published Date :
July 19, 2026, 3:16 p.m.
Last Modified :
Aug. 5, 2026, 7:46 p.m.
Remotely Exploit :
No
Source :
ce714d77-add3-4f53-aff5-83d477b104bb
CVSS Scores
| Score | Version | Severity | Vector | Exploitability Score | Impact Score | Source |
|---|---|---|---|---|---|---|
| CVSS | 134c704f-9b21-4f2e-91b3-4a467353bcc0 | |||||
| CVSS 3.1 | HIGH | ce714d77-add3-4f53-aff5-83d477b104bb |
Solution
- Upgrade fast-uri to version 4.1.1 or later.
- Upgrade fast-uri to version 3.1.4 or later.
- Upgrade fast-uri to version 2.4.3 or later.
References to Advisories, Solutions, and Tools
Here, you will find a curated list of external links that provide in-depth
information, practical solutions, and valuable tools related to
CVE-2026-16221.
| URL | Resource |
|---|---|
| https://cna.openjsf.org/security-advisories.html | Vendor Advisory |
| https://github.com/fastify/fast-uri/security/advisories/GHSA-v2hh-gcrm-f6hx | Vendor Advisory |
CWE - Common Weakness Enumeration
While CVE identifies
specific instances of vulnerabilities, CWE categorizes the common flaws or
weaknesses that can lead to vulnerabilities. CVE-2026-16221 is
associated with the following CWEs:
Common Attack Pattern Enumeration and Classification (CAPEC)
Common Attack Pattern Enumeration and Classification
(CAPEC)
stores attack patterns, which are descriptions of the common attributes and
approaches employed by adversaries to exploit the CVE-2026-16221
weaknesses.
We scan GitHub repositories to detect new proof-of-concept exploits. Following list is a collection of public exploits and proof-of-concepts, which have been published on GitHub (sorted by the most recently updated).
Results are limited to the first 15 repositories due to potential performance issues.
The following list is the news that have been mention
CVE-2026-16221 vulnerability anywhere in the article.
The following table lists the changes that have been made to the
CVE-2026-16221 vulnerability over time.
Vulnerability history details can be useful for understanding the evolution of a vulnerability, and for identifying the most recent changes that may impact the vulnerability's severity, exploitability, or other characteristics.
-
Initial Analysis by [email protected]
Aug. 05, 2026
Action Type Old Value New Value Added CPE Configuration OR *cpe:2.3:a:openjsf:fast-uri:*:*:*:*:*:node.js:*:* versions from (including) 2.3.1 up to (excluding) 2.4.3 *cpe:2.3:a:openjsf:fast-uri:*:*:*:*:*:node.js:*:* versions from (including) 3.0.0 up to (excluding) 3.1.4 *cpe:2.3:a:openjsf:fast-uri:*:*:*:*:*:node.js:*:* versions from (including) 4.0.0 up to (excluding) 4.1.1 Added Reference Type openjs: https://cna.openjsf.org/security-advisories.html Types: Vendor Advisory Added Reference Type openjs: https://github.com/fastify/fast-uri/security/advisories/GHSA-v2hh-gcrm-f6hx Types: Vendor Advisory -
CVE Modified by 134c704f-9b21-4f2e-91b3-4a467353bcc0
Jul. 20, 2026
Action Type Old Value New Value Added SSVC {'id': 'CVE-2026-16221', 'role': 'CISA Coordinator', 'options': [{'exploitation': 'none'}, {'automatable': 'yes'}, {'technicalImpact': 'partial'}], 'version': '2.0.3', 'timestamp': '2026-07-20T13:53:31.279563Z'} -
New CVE Received by ce714d77-add3-4f53-aff5-83d477b104bb
Jul. 19, 2026
Action Type Old Value New Value Added Affected [{'vendor': 'fast-uri', 'product': 'fast-uri', 'versions': [{'status': 'affected', 'version': '2.3.1', 'lessThan': '2.4.3', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '2.4.3', 'versionType': 'semver'}, {'status': 'affected', 'version': '3.0.0', 'lessThan': '3.1.4', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '3.1.4', 'versionType': 'semver'}, {'status': 'affected', 'version': '4.0.0', 'lessThan': '4.1.1', 'versionType': 'semver'}, {'status': 'unaffected', 'version': '4.1.1', 'versionType': 'semver'}], 'packageURL': 'pkg:npm/fast-uri', 'defaultStatus': 'unaffected'}] Added Description Impact: fast-uri versions from 2.3.1 through 4.1.0 (including the 3.x line up to 3.1.3 and the 2.x line up to 2.4.2) do not treat a literal backslash character (U+005C) as an authority delimiter. Node's native WHATWG URL parser, used by fetch, undici, and Node's http and https clients, normalizes the backslash to a forward slash for special schemes such as http, https, ws, wss, ftp, and file. As a result, the two parsers extract different hosts from the same input string. Applications that use fast-uri to enforce host-based policy such as allowlists, denylists, loopback or SSRF filtering, redirect validation, or outbound proxy routing before passing the same URL into Node's URL or fetch consumers can be steered to an unintended destination, including cloud metadata endpoints, loopback, or internal hosts. Patches: upgrade to fast-uri 4.1.1, 3.1.4, or 2.4.3. Workarounds: none. Added CVSS V3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N Added CWE CWE-436 Added Reference https://cna.openjsf.org/security-advisories.html Added Reference https://github.com/fastify/fast-uri/security/advisories/GHSA-v2hh-gcrm-f6hx