Astro: Arbitrary CSS injection in Astro View Transitions

Summary

Astro's server-side View Transition CSS generator did not keep string animation properties within their generated CSS declarations. In an application that passes attacker-controlled data into an animation property such as duration, a value containing CSS delimiters could terminate the declaration and rule, then add attacker-selected CSS to the server-rendered page.

The injected content remains inside the generated <style> element. It cannot terminate that element or inject HTML or JavaScript because Astro escapes < before emitting the stylesheet.

Severity

Exploitation requires an application to pass untrusted, request-derived, or user-controlled content directly into a transition animation value. This is an uncommon application pattern, and applications using static or otherwise trusted transition values are unaffected.

The resulting capability is limited to CSS in the rendered document. The proof of concept does not demonstrate JavaScript execution, HTML injection, an origin-boundary bypass, or access to server-only data.

Details

Animation values are collected by stringifyAnimations() in packages/astro/src/runtime/server/transition.ts and passed to addAnimationProperty(). String values were converted directly with toString():

function addAnimationProperty(builder: AnimationBuilder, prop: string, value: string | number) {
  let arr = builder[prop];
  if (Array.isArray(arr)) {
    arr.push(value.toString());
  } else {
    builder[prop] = [value.toString()];
  }
}

The animation builder then interpolated each value into a declaration:

out += `\n\t${k}: ${value.join(', ')};`;

The completed stylesheet passes through escapeStyleText(), which escapes literal < characters to keep the content inside its HTML <style> element. CSS delimiters such as ;, {, and } remained active, allowing a value to escape its intended declaration and add another rule.

The shared serializer handles duration, delay, easing, direction, fillMode, and animation names, so custom transition objects using those string properties followed the same path.

The fix encodes CSS declaration delimiters while serializing each animation value. Supported CSS functions and strings, including calc() and var(), remain unchanged.