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chore: load katex with async stylesheet #4680

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Merged
merged 4 commits into from
Nov 4, 2025
Merged

chore: load katex with async stylesheet #4680

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01bb2d1
chore: load katex with async stylesheet
chdeskur Nov 3, 2025
5baba78
preload
chdeskur Nov 3, 2025
61f88ee
format
chdeskur Nov 3, 2025
2671194
use client
chdeskur Nov 3, 2025
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16 changes: 2 additions & 14 deletions packages/fern-docs/bundle/src/app/layout.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ import { Providers } from "@fern-docs/components/providers/providers";
import type { Metadata, Viewport } from "next/types";
import { experimental_taintUniqueValue } from "react";

import { AsyncStylesheet } from "@/components/AsyncStylesheet";
import { ConsoleMessage } from "@/components/console-message";
import { WebSocketRefresh } from "@/components/websocket-refresh";

Expand Down Expand Up @@ -56,23 +57,10 @@ export default function Layout({ children }: { children: React.ReactNode }) {
<link rel="dns-prefetch" href="https://cdn.jsdelivr.net" />

{/* Non-blocking KaTeX CSS loading */}
<link
rel="preload"
as="style"
<AsyncStylesheet
href="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.css"
crossOrigin="anonymous"
/>
<link
rel="stylesheet"
href="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.css"
media="print"
onload="this.media='all'"
/>
<noscript
dangerouslySetInnerHTML={{
__html: '<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/katex.min.css" />'
}}
/>
</head>
);

Expand Down
41 changes: 41 additions & 0 deletions packages/fern-docs/bundle/src/components/AsyncStylesheet.tsx
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Original file line number Diff line number Diff line change
@@ -0,0 +1,41 @@
"use client";

/**
* AsyncStylesheet - Non-blocking CSS loading component
*
* Loads a stylesheet asynchronously without blocking page render.
* Uses the media="print" + onLoad trick for optimal performance while
* maintaining React/TypeScript compatibility.
*/

interface AsyncStylesheetProps {
href: string;
crossOrigin?: "anonymous" | "use-credentials";
}

export function AsyncStylesheet({ href, crossOrigin = "anonymous" }: AsyncStylesheetProps) {
return (
<>
{/* Preload hint for early resource discovery */}
<link rel="preload" as="style" href={href} crossOrigin={crossOrigin} />

{/* Async loading with media swap trick - React's onLoad works here */}
<link
rel="stylesheet"
href={href}
media="print"
crossOrigin={crossOrigin}
onLoad={(e) => {
(e.target as HTMLLinkElement).media = "all";
}}
/>

{/* Fallback for users with JavaScript disabled */}
<noscript
dangerouslySetInnerHTML={{
__html: `<link rel="stylesheet" href="${href}" crossorigin="${crossOrigin}" />`
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@vercel vercel bot Nov 3, 2025

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The href and crossOrigin props are directly interpolated into HTML without escaping, creating an XSS vulnerability through dangerouslySetInnerHTML.

View Details
📝 Patch Details 
diff --git a/packages/fern-docs/bundle/src/components/AsyncStylesheet.tsx b/packages/fern-docs/bundle/src/components/AsyncStylesheet.tsx
index dd91da19d..785fd357e 100644
--- a/packages/fern-docs/bundle/src/components/AsyncStylesheet.tsx
+++ b/packages/fern-docs/bundle/src/components/AsyncStylesheet.tsx
@@ -8,32 +8,69 @@
  * maintaining React/TypeScript compatibility.
  */
 
+/**
+ * Validates and sanitizes a URL to prevent XSS attacks
+ * Returns null if the URL is invalid or potentially malicious
+ */
+function validateStylesheetUrl(href: string): string | null {
+    try {
+        // Only allow http/https URLs
+        const url = new URL(href, typeof window !== 'undefined' ? window.location.href : 'http://localhost');
+        if (url.protocol !== 'http:' && url.protocol !== 'https:') {
+            return null;
+        }
+        return url.toString();
+    } catch {
+        return null;
+    }
+}
+
+/**
+ * Validates crossOrigin value to ensure it's one of the allowed values
+ */
+function validateCrossOrigin(crossOrigin: string): "anonymous" | "use-credentials" | null {
+    if (crossOrigin === "anonymous" || crossOrigin === "use-credentials") {
+        return crossOrigin;
+    }
+    return null;
+}
+
 interface AsyncStylesheetProps {
     href: string;
     crossOrigin?: "anonymous" | "use-credentials";
 }
 
 export function AsyncStylesheet({ href, crossOrigin = "anonymous" }: AsyncStylesheetProps) {
+    // Validate and sanitize inputs to prevent XSS
+    const validatedHref = validateStylesheetUrl(href);
+    const validatedCrossOrigin = validateCrossOrigin(crossOrigin);
+
+    // If validation fails, don't render anything to prevent XSS
+    if (!validatedHref || !validatedCrossOrigin) {
+        console.warn('AsyncStylesheet: Invalid href or crossOrigin prop detected, skipping render for security');
+        return null;
+    }
+
     return (
         <>
             {/* Preload hint for early resource discovery */}
-            <link rel="preload" as="style" href={href} crossOrigin={crossOrigin} />
+            <link rel="preload" as="style" href={validatedHref} crossOrigin={validatedCrossOrigin} />
 
             {/* Async loading with media swap trick - React's onLoad works here */}
             <link
                 rel="stylesheet"
-                href={href}
+                href={validatedHref}
                 media="print"
-                crossOrigin={crossOrigin}
+                crossOrigin={validatedCrossOrigin}
                 onLoad={(e) => {
                     (e.target as HTMLLinkElement).media = "all";
                 }}
             />
 
-            {/* Fallback for users with JavaScript disabled */}
+            {/* Fallback for users with JavaScript disabled - now safe because we validated inputs */}
             <noscript
                 dangerouslySetInnerHTML={{
-                    __html: `<link rel="stylesheet" href="${href}" crossorigin="${crossOrigin}" />`
+                    __html: `<link rel="stylesheet" href="${validatedHref}" crossorigin="${validatedCrossOrigin}" />`
                 }}
             />
         </>

Analysis

XSS vulnerability in AsyncStylesheet component via dangerouslySetInnerHTML

What fails: AsyncStylesheet component in packages/fern-docs/bundle/src/components/AsyncStylesheet.tsx directly interpolates untrusted href and crossOrigin props into HTML via dangerouslySetInnerHTML, allowing script execution

How to reproduce:

<AsyncStylesheet 
  href='x" onload="alert(1)'
  crossOrigin="anonymous" 
/>

Result: Generated HTML contains executable JavaScript: <link rel="stylesheet" href="x" onload="alert(1)" crossorigin="anonymous" /> which executes when parsed

Expected: Input should be validated/sanitized to prevent XSS attacks per React security guidelines

}}
/>
</>
);
}