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

References

• GHSA-p6j9-7xhc-rhwp
• https://nvd.nist.gov/vuln/detail/CVE-2021-27516
• medialize/URI.js@a1ad8bc
• https://github.com/medialize/URI.js/releases/tag/v1.19.6
• https://advisory.checkmarx.net/advisory/CX-2021-4305