---
id: "CVE-2026-114127"
kind: "vulnerability"
aliases: ["CVE-2026-114127", "GHSA-mpr7-6m5w-76rw"]
title: "Astro: Arbitrary CSS injection in Astro View Transitions"
severity: "low"
cvss: 2.3
epss: 0.003289999905973673
exploited: false
status: "open"
category: "package"
products: [{"name": "astro", "fixed": ["7.3.3"], "affected": ">= 7.1.0, <= 7.3.2", "ecosystem": "npm"}]
ecosystems: ["npm"]
tags: ["astro", "cwe-79", "next.js", "package", "self-hosted", "ssr"]
not_affected_if: ["default-config", "static-export", "trusted-transition-values", "versions-before-7.1.0"]
published_at: "2026-10-21T18:17:25+00:00"
first_seen_at: "2026-10-23T19:11:31+00:00"
updated_at: "2026-10-23T21:57:29+00:00"
sources: [{"source": "ghsa", "url": "https://github.com/advisories/GHSA-mpr7-6m5w-76rw", "first_seen_at": "2026-10-23T19:11:31+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-114127", "first_seen_at": "2026-10-23T21:57:29+00:00"}]
---

# Astro: Arbitrary CSS injection in Astro View Transitions

Astro's View Transitions stylesheet generator did not escape CSS delimiters in string animation properties such as duration, so an application that puts request-controlled data into a transition animation value lets an attacker add arbitrary CSS to the rendered page. The injected CSS cannot break out of the style element or run HTML or JavaScript.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- npm:astro affected >= 7.1.0, <= 7.3.2 fixed 7.3.3

## How to check

Search the project for transition:animate or custom transition objects that take values from Astro.url, request params, cookies or user data.
Check the installed version with: npm ls astro

## Fix

Upgrade astro to 7.3.3 or later.
Until upgraded, do not pass untrusted values into View Transition animation properties; validate duration and delay as finite numbers or strict CSS time values.

## Timeline

- 2026-10-23T19:11:31+00:00 ghsa https://github.com/advisories/GHSA-mpr7-6m5w-76rw
- 2026-10-23T21:57:29+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-114127

## From the sources

### 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()`:

```ts
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:

```ts
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.

### Proof of Concept

Using `astro@7.3.2`, create an on-demand rendered page:

```astro
---
import { fade } from "astro:transitions";
const duration = Astro.url.searchParams.get("duration") ?? "1ms";
---
<div id="target" transition:animate={fade({ duration })}>SAFE</div>
```

Request the page with:

```text
/?duration=1ms%3B%7D%23target%7Bcolor%3Argb(1%2C2%2C3)%3Bfont-size%3A37px%7Dx%7Banimation-duration%3A1ms
```

Before the fix, the generated stylesheet contains an active rule equivalent to:

```css
#target {
  color: rgb(1, 2, 3);
  font-size: 37px;
}
```

In Chromium, the target's computed styles change to `rgb(1, 2, 3)` and `37px`. This behavior is reproducible in a production standalone SSR build, not only in the development server.

### Impact

An attacker can inject CSS into the affected application's rendered page when the application places attacker-controlled data into a transition animation value. The CSS can restyle or conceal content, reposition existing controls, add generated text, or otherwise misrepresent the page's UI.

CSS selectors and conditional resource loads may expose data already present in selectable DOM attributes in some applications, depending on the page and its Content Security Policy. The issue does not expose cookies, server-only values, arbitrary DOM text, or cross-origin response data by itself.

### Affected Versions

`astro >= 7.1.0, <= 7.3.2`.

Versions through 7.0.9 are covered by GHSA-4g3v-8h47-v7g6 / CVE-2026-73422, which had the stronger impact of escaping the `<style>` element and injecting HTML or JavaScript.

### Patched Versions

This issue is patched in `astro@7.3.3`. Fix: https://github.com/withastro/astro/pull/17970

### Workarounds

Do not pass untrusted or request-derived values into View Transition animation properties. If dynamic values are required, validate them according to the expected CSS grammar before constructing the transition. For `duration` and `delay`, prefer finite numbers or strictly validated CSS time values.

### Credits

Reported by @avrlab233.

---

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