Sanitizing untrusted input with regular expressions is a common technique. However, it is error-prone to match untrusted input against regular expressions without anchors such as ^ or $. Malicious input can bypass such security checks by embedding one of the allowed patterns in an unexpected location.

Even if the matching is not done in a security-critical context, it may still cause undesirable behavior when the regular expression accidentally matches.

Use anchors to ensure that regular expressions match at the expected locations.

The following example code checks that a URL redirection will reach the example.com domain, or one of its subdomains, and not some malicious site.

The check with the regular expression match is, however, easy to bypass. For example by embedding http://example.com/ in the query string component: http://evil-example.net/?x=http://example.com/. Address these shortcomings by using anchors in the regular expression instead:

A related mistake is to write a regular expression with multiple alternatives, but to only include an anchor for one of the alternatives. As an example, the regular expression /^www\.example\.com|beta\.example\.com/ will match the host evil.beta.example.com because the regular expression is parsed as /(^www\.example\.com)|(beta\.example\.com)/

MDN: Regular Expressions

OWASP: SSRF

OWASP: XSS Unvalidated Redirects and Forwards Cheat Sheet.