CVE-2021-27516: Input Validation
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
Other sources
A flaw was found in nodejs-urijs where URI.js (urijs) mishandles certain uses of the backslash such as http:\/ and interprets the URI as a relative path. The highest threat from this vulnerability is to confidentiality.
URI.js (aka urijs) before 1.19.6 mishandles certain uses of backslash such as http:\/ and interprets the URI as a relative path.
Affected Software
Remediation
Event History
Frequently Asked Questions
What is CVE-2021-27516?
CVE-2021-27516 is a vulnerability in URI.js library versions before 1.19.6 that mishandles the use of a backslash character in the scheme delimiter, leading to potential hostname spoofing.
How does CVE-2021-27516 impact URL hostname determination?
If affected versions of URI.js are used to determine a URL's hostname, an attacker can spoof the hostname by using a backslash character as part of the scheme delimiter.
What could be the consequences of CVE-2021-27516?
Depending on how the library is used, if the hostname is used in security decisions, those decisions may be incorrect due to potential spoofing.
How can I fix CVE-2021-27516?
To fix CVE-2021-27516, update to URI.js version 1.19.6 or later.
What is the severity of CVE-2021-27516?
CVE-2021-27516 has a severity keyword of 'high' and a severity value of 7.5 (out of 10).