Template::Plugin::HTML versions through 3.102 for Perl allows HTML and JavaScript to be injected.

The html_filter function did not escape single quotes. HTML attributes inside of single quotes could be have code injected. For example, the variable "var" in

<a id='ref' title='[% var | html %]'>

would not be properly escaped. An attacker could insert some limited HTML and JavaScript, for example,

var = " ' onclick='while (true) { alert(1) }'"

Note that arbitrary HTML and JavaScript would be difficult to inject, because angle brackets, ampersands and double-quotes would still be escaped.