Affected versions
Twig versions <3.26.0 are affected by this security issue.
The issue has been fixed in Twig 3.26.0.
Description
The spaceless filter is registered with is_safe => ['html'], which
means Twig's autoescaper does not escape its output in an HTML context. As
a result, applying spaceless to attacker-controlled input that contains
markup emits the markup unescaped even when the developer never wrote
|raw and autoescape is enabled.
Example:
1 2 3
{% set payload = '<script>alert()</script>' %}
{{ payload }} {# escaped #}
{{ payload|spaceless }} {# not escaped #}The filter is deprecated but still functional. With the deprecation, some
downstream projects (e.g. Drupal modules) have duplicated the filter and
inherited the same is_safe flag.
Resolution
The spaceless filter no longer marks its output as safe. Documentation
has been updated to warn that spaceless should not be applied to
unsanitised user input.
Credits
We would like to thank Pierre Rudloff for reporting the issue.