SWC HTML minifier may allow script element breakout when minifying embedded JSON

Impact

@swc/html minifies JSON contained in script elements such as
application/json and application/ld+json by parsing and serializing the
JSON value.

Before the patched versions, JSON serialization could convert escaped
less-than signs such as \u003C into literal < characters. If the JSON
contained an escaped </script> sequence, the generated HTML could terminate
the containing script element early because HTML tokenization occurs before
the JSON is consumed.

Applications that minify HTML containing attacker-controlled JSON data could
therefore transform inert data into active markup. A crafted payload could
execute script in the origin of the generated page.

Patches

The issue is fixed in:

• @swc/html 1.15.47
• swc_html_minifier 59.0.0

The minifier now re-escapes less-than signs after JSON serialization, preserving
the script element boundary.

Workarounds

Users who cannot upgrade can disable JSON minification with:

await minify(html, {
  minifyJson: false,
});