To reduce the risk of cross-site scripting attacks, templating systems, such as Twig, Django, Smarty,
Groovy's template engine, allow configuration of automatic variable escaping before rendering templates. When escape occurs, characters
that make sense to the browser (eg: <a>) will be transformed/replaced with escaped/sanitized values (eg: & lt;a& gt; ).
Auto-escaping is not a magic feature to annihilate all cross-site scripting attacks, it depends on the strategy applied and the context, for example a "html auto-escaping" strategy
(which only transforms html characters into html entities) will not be relevant
when variables are used in a html attribute because ':' character is not
escaped and thus an attack as below is possible:
<a href="{{ myLink }}">link</a> // myLink = javascript:alert(document.cookie)
<a href="javascript:alert(document.cookie)">link</a> // JS injection (XSS attack)
Ask Yourself Whether
- Templates are used to render web content and
- dynamic variables in templates come from untrusted locations or are user-controlled inputs
- there is no local mechanism in place to sanitize or validate the inputs.
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
Enable auto-escaping by default and continue to review the use of inputs in order to be sure that the chosen auto-escaping strategy is the right
one.
Sensitive Code Example
With JMustache by samskivert:
Mustache.compiler().escapeHTML(false).compile(template).execute(context); // Sensitive
Mustache.compiler().withEscaper(Escapers.NONE).compile(template).execute(context); // Sensitive
With Freemarker:
freemarker.template.Configuration configuration = new freemarker.template.Configuration();
configuration.setAutoEscapingPolicy(DISABLE_AUTO_ESCAPING_POLICY); // Sensitive
Compliant Solution
With JMustache by samskivert:
Mustache.compiler().compile(template).execute(context); // Compliant, auto-escaping is enabled by default
Mustache.compiler().escapeHTML(true).compile(template).execute(context); // Compliant
With Freemarker. See "setAutoEscapingPolicy"
documentation for more details.
freemarker.template.Configuration configuration = new freemarker.template.Configuration();
configuration.setAutoEscapingPolicy(ENABLE_IF_DEFAULT_AUTO_ESCAPING_POLICY); // Compliant
See
