Astro: Reflected XSS via unescaped View Transition animation properties

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

Astro is a web framework for content-driven websites. From 2.9.0 until 7.1.0, Astro's server-side View Transition CSS generator interpolates animation properties into an inline style element without escaping them for CSS and HTML contexts. An attacker-controlled View Transition animation value such as duration can terminate the generated style element and inject arbitrary HTML or JavaScript. The affected code is packages/astro/src/runtime/server/transition.ts; renderTransition passes sheet.toString() into markHTMLString(), while addAnimationProperty serializes duration through toTimeValue() and also handles easing, direction, delay, fillMode, and name. Exploitation requires an on-demand or server-rendered route to pass attacker-controlled data into a View Transition animation definition and can execute arbitrary JavaScript in the affected application's origin, allowing access to sensitive page data and authenticated actions available to the victim. This issue is fixed in version 7.1.0.

Source published

12 Aug 2026 20:32:56 UTC

Source modified

14 Aug 2026 22:12:40 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

CVE Program Terms of Use

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CVSS 4.0 · 5.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

Astro is a web framework for content-driven websites. From 2.9.0 until 7.1.0, Astro's server-side View Transition CSS generator interpolates animation properties into an inline style element without escaping them for CSS and HTML contexts. An attacker-controlled View Transition animation value such as duration can terminate the generated style element and inject arbitrary HTML or JavaScript. The affected code is packages/astro/src/runtime/server/transition.ts; renderTransition passes sheet.toString() into markHTMLString(), while addAnimationProperty serializes duration through toTimeValue() and also handles easing, direction, delay, fillMode, and name. Exploitation requires an on-demand or server-rendered route to pass attacker-controlled data into a View Transition animation definition and can execute arbitrary JavaScript in the affected application's origin, allowing access to sensitive page data and authenticated actions available to the victim. This issue is fixed in version 7.1.0.

Source published

12 Aug 2026 21:17:41 UTC

Source modified

24 Oct 2026 15:10:00 UTC

Observed here

25 Oct 2026 15:19:09 UTC

Source terms

NIST NVD usage terms; underlying source attribution retained

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: nvd · NIST NVD usage terms; underlying source attribution retained. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS 4.0 · [email protected] · Secondary · 5.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

## Summary Astro's server-side View Transition CSS generator interpolates animation properties into an inline `<style>` element without escaping them for the CSS and HTML contexts. An attacker-controlled value passed to an animation property such as `duration` can contain a `</style>` sequence, terminate the generated style element, and inject arbitrary HTML or JavaScript. This is similar to GHSA-8hv8-536x-4wqp, but exploits a different injection point: unescaped View Transition animation values in a server-generated `<style>` element rather than an unescaped slot name in a hydration template. Like GHSA-8hv8-536x-4wqp, exploitation requires an application to pass attacker-controlled data to an Astro API. However, the value is subsequently inserted into the HTML response without context-appropriate escaping by Astro. ## Details `packages/astro/src/runtime/server/transition.ts` The generated stylesheet is wrapped in a `<style>` element and marked as HTML-safe: ```ts const css = sheet.toString(); result._metadata.extraHead.push(markHTMLString(`<style>${css}</style>`)); ``` Animation properties are added to the stylesheet without escaping: ```ts if (anim.duration) { addAnimationProperty(builder, 'animation-duration', toTimeValue(anim.duration)); } ``` For string values, `toTimeValue()` returns the input unchanged: ```ts export function toTimeValue(num: number | string) { return typeof num === 'number' ? num + 'ms' : num; } ``` As a result, a `duration` value containing `</style>` can escape from the generated style element. Other `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. ## PoC Using: - `[email protected]` - `@astrojs/[email protected]` ### `astro.config.mjs` ```js import node from '@astrojs/node'; import { defineConfig } from 'astro/config'; export default defineConfig({ output: 'server', adapter: node({ mode: 'standalone' }), }); ``` ### `src/pages/index.astro` ```astro --- import { fade } from 'astro:transitions'; const duration = Astro.url.searchParams.get('duration') ?? '300ms'; --- <html lang="en"> <head> <meta charset="utf-8" /> <title>PoC</title> </head> <body> <div transition:animate={fade({ duration })}> Animated content </div> </body> </html> ``` ### Payload: open: ```text http://localhost:4321/?duration=%3C%2Fstyle%3E%3Cscript%3Ealert(1)%3C%2Fscript%3E%3C!-- ``` The browser interprets `</style>` as the end of the generated style element and executes the injected script. An alert dialog is displayed when the page is opened. <img width="1307" height="681" alt="image" src="https://github.com/user-attachments/assets/0834236d-008f-415f-9f23-db509ab6f8f6" /> ## Impact An attacker who can control a View Transition animation value can execute arbitrary JavaScript in the origin of the affected Astro application. The query-based reflected XSS scenario affects on-demand/server-rendered routes, such as: - projects configured with `output: "server"`; - pages using `export const prerender = false`; - other server-side data flows that pass attacker-controlled values into a View Transition animation definition. Successful exploitation may allow access to sensitive page data and authenticated actions available to the victim. ## Suggested Fix Animation values should be serialized using context-appropriate CSS escaping or validation before being added to the generated stylesheet. Additionally, content inserted into a raw `<style>` element must not be able to contain an HTML end-tag sequence such as `</style>`. The final generated CSS should be made safe for the HTML raw-text context before it is passed to `markHTMLString()`.

Source published

20 Jul 2026 21:08:16 UTC

Source modified

12 Aug 2026 20:56:01 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N

Publisher rating · MODERATE