{"id":"GHSA-4g3v-8h47-v7g6","title":"Astro: Reflected XSS via unescaped View Transition animation properties","summary":"Astro: Reflected XSS via unescaped View Transition animation properties","severity":"medium","cwe":["CWE-79"],"vendor":"astro","product":"astro","ecosystem":"npm","affected":["astro >= 2.9.0, <= 7.0.9"],"patched":["astro 7.1.0"],"published":"2026-07-20","updated":"2026-07-20","source":"GHSA","sourceUrl":"https://github.com/advisories/GHSA-4g3v-8h47-v7g6","references":[{"url":"https://github.com/withastro/astro/security/advisories/GHSA-4g3v-8h47-v7g6"},{"url":"https://github.com/withastro/astro/pull/17393"},{"url":"https://github.com/withastro/astro/commit/092da560eea77ee63a3e2c583c80d8238544e42b"},{"url":"https://github.com/withastro/astro/releases/tag/astro@7.1.0"},{"url":"https://github.com/advisories/GHSA-4g3v-8h47-v7g6"}],"tags":["ghsa","npm"],"ingestedAt":"2026-07-20T21:43:15.657Z","slug":"GHSA-4g3v-8h47-v7g6","body":"## Overview\n\n## Summary\n\nAstro's server-side View Transition CSS generator interpolates animation properties into an inline `` sequence, terminate the generated style element, and inject arbitrary HTML or JavaScript.\n\nThis is similar to GHSA-8hv8-536x-4wqp, but exploits a different injection point: unescaped View Transition animation values in a server-generated ``));\n```\n\nAnimation properties are added to the stylesheet without escaping:\n\n```ts\nif (anim.duration) {\n addAnimationProperty(builder, 'animation-duration', toTimeValue(anim.duration));\n}\n```\n\nFor string values, `toTimeValue()` returns the input unchanged:\n\n```ts\nexport function toTimeValue(num: number | string) {\n return typeof num === 'number' ? num + 'ms' : num;\n}\n```\n\nAs a result, a `duration` value containing `` can escape from the generated style element.\n\nOther `TransitionAnimation` properties, including `easing`, `direction`, `delay`, `fillMode`, and `name`, are serialized by the same animation builder. The following PoC only relies on the official `fade()` helper and its `duration` option.\n\n\n## PoC\n\nUsing:\n\n- `astro@7.0.9`\n- `@astrojs/node@11.0.2`\n\n### `astro.config.mjs`\n\n```js\nimport node from '@astrojs/node';\nimport { defineConfig } from 'astro/config';\n\nexport default defineConfig({\n output: 'server',\n adapter: node({ mode: 'standalone' }),\n});\n```\n\n### `src/pages/index.astro`\n\n```astro\n---\nimport { fade } from 'astro:transitions';\n\nconst duration = Astro.url.searchParams.get('duration') ?? '300ms';\n---\n\n\n \n \n PoC\n \n \n
\n Animated content\n
\n \n\n```\n\n### Payload:\n\nopen:\n\n```text\nhttp://localhost:4321/?duration=%3C%2Fstyle%3E%3Cscript%3Ealert(1)%3C%2Fscript%3E%3C!--\n```\n\nThe browser interprets `` as the end of the generated style element and executes the injected script. An alert dialog is displayed when the page is opened.\n\n\"image\"\n\n## Impact\n\nAn attacker who can control a View Transition animation value can execute arbitrary JavaScript in the origin of the affected Astro application.\n\nThe query-based reflected XSS scenario affects on-demand/server-rendered routes, such as:\n\n- projects configured with `output: \"server\"`;\n- pages using `export const prerender = false`;\n- other server-side data flows that pass attacker-controlled values into a View Transition animation definition.\n\nSuccessful exploitation may allow access to sensitive page data and authenticated actions available to the victim.\n\n## Suggested Fix\n\nAnimation values should be serialized using context-appropriate CSS escaping or validation before being added to the generated stylesheet.\n\nAdditionally, content inserted into a raw ``. The final generated CSS should be made safe for the HTML raw-text context before it is passed to `markHTMLString()`.\n\n## Affected packages\n\n- `astro >= 2.9.0, <= 7.0.9`\n\n## Remediation\n\nUpgrade to a patched release:\n\n- `astro 7.1.0`","depth":"sunlit","depthScore":28,"depthScoreParts":{"impact":27.5,"likelihood":0,"exploitation":0,"ransomware":0},"changes":[]}