The NextGen Gallery plugin for WordPress is vulnerable to a type of cyber attack known as Reflected Cross-Site Scripting. This type of attack occurs when a hacker is able to embed malicious code into a page viewed by a user. In this case, vulnerable versions of the NextGen Gallery plugin up to and including 1.9.7 are susceptible to this attack, as the plugin does not properly sanitize input or escape output, making it easy for hackers to trick users into clicking on a malicious link or performing an action.