HONO · GHSA-3gg2-335m-rxc6 · Package
FIX → 4.13.16
The css helper in hono/css fails to escape a slash after a less-than sign when the < comes from the template or a previous value, so a value starting with /style> closes the style element early. An attacker who controls such a value can inject HTML, including script, into a page rendered on the server.
first public Nov 1, 2026 · 1 source
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Applications that use the css helper from hono/css and interpolate untrusted strings into a css template right after a < character, either written in the template or supplied by an earlier value, and render the result on the server. Other uses of the css helper are not affected according to the advisory.
Not affected if hono-above-4.13.15, hono-below-3.12.0, no-less-than-before-interpolation, no-untrusted-css-interpolation
Who cares hono, jsx, npm, server-side-rendering
npm ls honocss template literals from hono/css that interpolate user-controlled values directly after a <, including two adjacent interpolations like ${prefix}${name}Upgrade to 4.13.16
The css helper in hono/css escapes a / that follows < inside an interpolated value so that a value cannot close the <style> element. The check only looks inside each value. When the text before a value ends with <, whether that is the template itself or an earlier interpolated value, and the next value starts with /, the / is not escaped and the combined output contains an unescaped closing tag.
Interpolated string values are escaped one at a time before they are joined with the surrounding template text. The rule that neutralizes </ therefore cannot see a < that belongs to the template. A template such as a content declaration that opens with a literal < and interpolates a string right after it produces a raw when that string starts with . During server-side rendering the element is closed early and the rest of the value is interpreted as HTML.
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4.13.16</style>/style><Style />This issue arises only when < is the last character before an interpolation in a css template, either written in the template or supplied by a preceding value, and the interpolated value is controlled by a client.
A client that controls such a value can inject HTML, including script, into the page rendered on the server.
This may lead to:
This issue affects applications that interpolate untrusted strings into a css template immediately after a <, including two adjacent interpolations such as ${prefix}${name} where both are untrusted. Other uses of the css helper are not affected.
Do not interpolate untrusted values into css templates, or make sure the text before an interpolated value does not end with <.