ExpressionEngine is prone to an HTML-injection vulnerability because the application fails to properly sanitize user-supplied input before using it in dynamically generated content.
Attacker-supplied HTML and script code would run in the context of the affected browser, potentially allowing the attacker to steal cookie-based authentication credentials or to control how the site is rendered to the user. Other attacks are also possible.
ExpressionEngine 1.6.4 through 1.6.6 are affected. Other versions may also be vulnerable.