URIjs Hostname spoofing via backslashes in URL
Impact
If using affected versions to determine a URL's hostname, the hostname can be spoofed by using a backslash (\) character as part of the scheme delimiter, e.g. scheme:/\hostname. If the hostname is used in security decisions, the decision may be incorrect.
Depending on library usage and attacker intent, impacts may include allow/block list bypasses, SSRF attacks, open redirects, or other undesired behavior.
Example URL: https:/\expected-example.com/path
Escaped string: https:/\\expected-example.com/path (JavaScript strings must escape backslash)
Affected versions incorrectly return no hostname. Patched versions correctly return expected-example.com. Patched versions match the behavior of other parsers which implement the WHATWG URL specification, including web browsers and Node's built-in URL class.
Patches
Version 1.19.6 is patched against all known payload variants.
References
https://github.com/medialize/URI.js/releases/tag/v1.19.6 (fix for this particular bypass) https://github.com/medialize/URI.js/releases/tag/v1.19.4 (fix for related bypass) https://github.com/medialize/URI.js/releases/tag/v1.19.3 (fix for related bypass) PR #233 (initial fix for backslash handling)
For more information
If you have any questions or comments about this advisory, open an issue in https://github.com/medialize/URI.js
Reporter credit
Yaniv Nizry from the CxSCA AppSec team at Checkmarx
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | urijs | < 1.19.6 | 1.19.6 |
References
- https://github.com/medialize/URI.js/security/advisories/GHSA-p6j9-7xhc-rhwp
- https://nvd.nist.gov/vuln/detail/CVE-2021-27516
- https://github.com/medialize/URI.js/commit/a1ad8bcbc39a4d136d7e252e76e957f3ece70839
- https://advisory.checkmarx.net/advisory/CX-2021-4305
- https://github.com/medialize/URI.js
- https://github.com/medialize/URI.js/releases/tag/v1.19.6
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N- Weakness (CWE)
CWE-20- Platform
- npm
- Package
urijs- Disclosed
- 01.03.2021
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-p6j9-7xhc-rhwp - CC BY 4.0
Vulnerability history: urijs
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 8
- First → latest
- 12.2020 04.2022
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 2
- Medium 6
Disclosures per year
- 2020: 1
- 2021: 2
- 2022: 5
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 0
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