diff --git a/packages/backend/src/server/FileServerService.ts b/packages/backend/src/server/FileServerService.ts
index 18ee0dbbd..47cc77da2 100644
--- a/packages/backend/src/server/FileServerService.ts
+++ b/packages/backend/src/server/FileServerService.ts
@@ -319,6 +319,12 @@ export class FileServerService {
 			);
 		}
 
+		if (!request.headers['user-agent']) {
+			throw new StatusError('User-Agent is required', 400, 'User-Agent is required');
+		} else if (request.headers['user-agent'].toLowerCase().indexOf('misskey/') !== -1) {
+			throw new StatusError('Refusing to proxy a request from another proxy', 403, 'Proxy is recursive');
+		}
+
 		// Create temp file
 		const file = await this.getStreamAndTypeFromUrl(url);
 		if (file === '404') {