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()`: ```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 `[email protected]`, 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 `[email protected]`. 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.

Source published

22 Oct 2026 16:29:28 UTC

Source modified

22 Oct 2026 16:45:03 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:P/PR:N/UI:P/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

Publisher rating · LOW