Validate 'next-action' header format in server action requests

packages/gitbook/src/middleware.ts

@@ -64,21 +64,35 @@ export async function middleware(request: NextRequest) {
 }
 
 async function validateServerActionRequest(request: NextRequest) {
-    // We need to reject incorrect server actions requests
-    // We do not do it in cloudflare workers as there is a bug that prevents us from reading the request body.
-    if (request.headers.has('next-action') && process.env.GITBOOK_RUNTIME !== 'cloudflare') {
-        // We just test that the json body is parseable
-        try {
-            const clonedRequest = request.clone();
-            await clonedRequest.json();
-        } catch (e) {
-            console.warn('Invalid server action request', e);
-            // If the body is not parseable, we reject the request
+    // First thing we need to do is validate that the header is in a correct format.
+    if (request.headers.has('next-action')) {
+        // A server action id is a 1-byte hex string (2 chars) followed by a 20-byte SHA1 hash (40 chars) = 42 total characters.
+        // For ref https://github.com/vercel/next.js/blob/db561cb924cbea0f3384e89f251fc443a8aec1ae/crates/next-custom-transforms/src/transforms/server_actions.rs#L266-L268
+        const regex = /^[a-fA-F0-9]{42}$/;
+        const match = request.headers.get('next-action')?.match(regex);
+        if (!match) {
             return new Response('Invalid request', {
                 status: 400,
                 headers: { 'content-type': 'text/plain' },
             });
         }
+
+        // We need to reject incorrect server actions requests
+        // We do not do it in cloudflare workers as there is a bug that prevents us from reading the request body.
+        if (process.env.GITBOOK_RUNTIME !== 'cloudflare') {
+            // We just test that the json body is parseable
+            try {
+                const clonedRequest = request.clone();
+                await clonedRequest.json();
+            } catch (e) {
+                console.warn('Invalid server action request', e);
+                // If the body is not parseable, we reject the request
+                return new Response('Invalid request', {
+                    status: 400,
+                    headers: { 'content-type': 'text/plain' },
+                });
+            }
+        }
     }
 }