CVE-2026-126205

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.3, the Loofah HTML5 sanitizer checks href and xlink:href on elements in SVG_ALLOW_LOCAL_HREF with a regular expression whose line anchor and newline-matching whitespace cause an all-whitespace value to be rescanned from every line start. An attacker can submit an SVG reference element with a long multiline-whitespace attribute and consume CPU quadratically during sanitization. CRuby 3.1 and earlier and JRuby are affected, while CRuby 3.2 and later uses linear-time regular-expression matching. The default safelist reaches the vulnerable check without a configuration change. This issue is fixed in version 2.25.3.

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.3, the Loofah HTML5 sanitizer checks href and xlink:href on elements in SVG_ALLOW_LOCAL_HREF with a regular expression whose line anchor and newline-matching whitespace cause an all-whitespace value to be rescanned from every line start. An attacker can submit an SVG reference element with a long multiline-whitespace attribute and consume CPU quadratically during sanitization. CRuby 3.1 and earlier and JRuby are affected, while CRuby 3.2 and later uses linear-time regular-expression matching. The default safelist reaches the vulnerable check without a configuration change. This issue is fixed in version 2.25.3.

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.

Only Ruby 3.1 and earlier are affected. Ruby 3.2 added linear-time regexp matching, which makes the pattern linear.

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 that cannot upgrade should run Ruby 3.2 or later, which matches the pattern in linear time.

Credit

Reported by scyoon through the Rails HackerOne program.

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.3, the Loofah HTML5 sanitizer checks href and xlink:href on elements in SVG_ALLOW_LOCAL_HREF with a regular expression whose line anchor and newline-matching whitespace cause an all-whitespace value to be rescanned from every line start. An attacker can submit an SVG reference element with a long multiline-whitespace attribute and consume CPU quadratically during sanitization. CRuby 3.1 and earlier and JRuby are affected, while CRuby 3.2 and later uses linear-time regular-expression matching. The default safelist reaches the vulnerable check without a configuration change. This issue is fixed in version 2.25.3.

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.3, the Loofah HTML5 sanitizer checks href and xlink:href on elements in SVG_ALLOW_LOCAL_HREF with a regular expression whose line anchor and newline-matching whitespace cause an all-whitespace value to be rescanned from every line start. An attacker can submit an SVG reference element with a long multiline-whitespace attribute and consume CPU quadratically during sanitization. CRuby 3.1 and earlier and JRuby are affected, while CRuby 3.2 and later uses linear-time regular-expression matching. The default safelist reaches the vulnerable check without a configuration change. This issue is fixed in version 2.25.3.

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.

Only Ruby 3.1 and earlier are affected. Ruby 3.2 added linear-time regexp matching, which makes the pattern linear.

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 that cannot upgrade should run Ruby 3.2 or later, which matches the pattern in linear time.

Credit

Reported by scyoon through the Rails HackerOne program.

Loofah is a general library for manipulating and transforming HTML/XML documents and fragments, built on top of Nokogiri. Prior to 2.25.3, the Loofah HTML5 sanitizer checks href and xlink:href on elements in SVG_ALLOW_LOCAL_HREF with a regular expression whose line anchor and newline-matching whitespace cause an all-whitespace value to be rescanned from every line start. An attacker can submit an SVG reference element with a long multiline-whitespace attribute and consume CPU quadratically during sanitization. CRuby 3.1 and earlier and JRuby are affected, while CRuby 3.2 and later uses linear-time regular-expression matching. The default safelist reaches the vulnerable check without a configuration change. This issue is fixed in version 2.25.3.