Summary

Loofah's HTML5 sanitizer restricts the href and xlink:href attributes of SVG reference elements to local references. That check matched the attribute value against /^\s*[^#\s].*/m. In Ruby ^ anchors to every line start and \s matches newlines, so an all-whitespace value made the engine rescan the value from each of its line starts, taking time quadratic in its length.

CRuby 3.1 and earlier, and JRuby, are affected. CRuby 3.2 added linear-time regexp matching, which makes the pattern linear. JRuby's regexp engine does not.

Impact

An attacker who supplies HTML to be sanitized can consume CPU out of proportion to the input size, by sending an SVG reference element whose href or xlink:href is a long run of whitespace.

The default safelist allows the elements in SVG_ALLOW_LOCAL_HREF, so no configuration change is required.

Mitigation

Upgrade to Loofah >= 2.25.3. Applications on CRuby that cannot upgrade should run CRuby 3.2 or later, which matches the pattern in linear time.

References

• CWE-1333: Inefficient Regular Expression Complexity
• Ruby 3.2.0 release notes

Credit

Reported by scyoon through the Rails HackerOne program.