Open redirect is a security flaw in an app or a web page that causes it to fail to properly authenticate URLs.
When apps and web pages have requests for URLs, they are supposed to verify that those URLs are part of the intended page’s domain. Open redirect is a failure in that process that makes it possible for attackers to steer users to malicious third-party websites. Sites or apps that fail to authenticate URLs can become a vector for malicious redirects to convincing fake sites for identity theft or sites that install malware.
Normally, redirection is a technique for shifting users to a different web page than the URL they requested. Webmasters use redirection for valid reasons, such as dealing with resources that are no longer available or have been moved to a different location. Web users often encounter redirection when they visit the Web site of a company whose name has been changed or which has been acquired by another company.
The Heartbleed vulnerability, originally reported to be enabled by covert redirects, was eventually discovered to be the result of the less serious -- but still irresponsible -- enabling of open redirect.