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: {% 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 Twig would like to thank Pierre Rudloff for reporting the issue.