CVE-2021-27516
URIjs Hostname spoofing via backslashes in URL
URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:/ and interprets the URI as a relative path.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- Low
- Privileges needed
- None
- User interaction
- None
What an attacker can affect
- Confidentiality
- High
- Integrity
- None
- Availability
- None
Exploit likelihood
What each source reports
NVD Score used
URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:/ and interprets the URI as a relative path.
- Record
CVE-2021-27516- CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N - CVSS 2.0
- 5.0 Medium
AV:N/AC:L/Au:N/C:P/I:N/A:N - Updated
- 2026-11-02
CVE List No score
URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:/ and interprets the URI as a relative path.
- Record
CVE-2021-27516- Updated
- 2026-11-02
EUVDEUVD-2021-0686 No score
URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:/ and interprets the URI as a relative path.
- Record
EUVD-2021-0686- Updated
- 2026-11-03
GitHub AdvisoriesGHSA-p6j9-7xhc-rhwp
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
- Record
GHSA-p6j9-7xhc-rhwp- CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N - Weakness
- CWE-20
- Updated
- 2026-11-02
OSV
URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:/ and interprets the URI as a relative path.
- Record
CVE-2021-27516- CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N - Updated
- 2026-11-02
OSVGHSA-p6j9-7xhc-rhwp
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
- Record
GHSA-p6j9-7xhc-rhwp- CVSS 3.1
- 7.5 High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N - Weakness
- CWE-20
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
urijsnpm |
before 1.19.6 | 1.19.6 |
History
| Recorded | Changed | Sources |
|---|---|---|
| aliases | EUVD | |
| first indexed | CVE List, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2021-27516 is a high vulnerability (CVSS 7.5) in Uri.js Project Uri.js, published February 21, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 2.48%, the 84th percentile. Fixed in urijs 1.19.6.
References
github.com/medialize/URI.js/commit/a1ad8bcbc39a4d136d7e252e76e957f3ece70839
advisory.checkmarx.net/advisory/CX-2021-4305
nvd.nist.gov/vuln/detail/CVE-2021-27516