diff --git a/lib/extension/externalJS.ts b/lib/extension/externalJS.ts
index 119a18c9..528ce7c4 100644
--- a/lib/extension/externalJS.ts
+++ b/lib/extension/externalJS.ts
@@ -176,7 +176,13 @@ export default abstract class ExternalJSExtension<M> extends Extension {
         }
 
         const {name, code} = message;
+
+        if (!name.endsWith(".mjs") && !name.endsWith(".js") && !name.endsWith(".cjs")) {
+            return utils.getResponse(message, {}, "JavaScript file must have '.mjs', '.js' or '.cjs' extension");
+        }
+
         const filePath = this.getFilePath(name, true);
+
         try {
             fs.writeFileSync(filePath, code, "utf8");
             this.symlinkNodeModulesIfNecessary();
diff --git a/test/extensions/externalConverters.test.ts b/test/extensions/externalConverters.test.ts
index 7e733349..4406779c 100644
--- a/test/extensions/externalConverters.test.ts
+++ b/test/extensions/externalConverters.test.ts
@@ -488,6 +488,25 @@ describe("Extension: ExternalConverters", () => {
             expect(mockMQTTPublishAsync).toHaveBeenCalledWith("zigbee2mqtt/bridge/converters", stringify([]), {retain: true});
         });
 
+        it("returns error on invalid name", async () => {
+            const converterName = "foo1";
+
+            await resetExtension();
+            for (const mock of mocksClear) mock.mockClear();
+
+            await (controller.getExtension("ExternalConverters")! as ExternalConverters).onMQTTMessage({
+                topic: "zigbee2mqtt/bridge/request/converter/save",
+                message: {name: converterName, code: "a"},
+            });
+
+            expect(mockMQTTPublishAsync).toHaveBeenCalledWith(
+                "zigbee2mqtt/bridge/response/converter/save",
+                expect.stringContaining(`JavaScript file must have '.mjs', '.js' or '.cjs' extension`),
+                {},
+            );
+            expect(writeFileSyncSpy).toHaveBeenCalledTimes(0);
+        });
+
         it("returns error on invalid code", async () => {
             const converterName = "foo1.js";
             const converterCode = "definetly not a correct javascript code";
diff --git a/test/extensions/externalExtensions.test.ts b/test/extensions/externalExtensions.test.ts
index db72dc99..983cf0b0 100644
--- a/test/extensions/externalExtensions.test.ts
+++ b/test/extensions/externalExtensions.test.ts
@@ -299,6 +299,25 @@ describe("Extension: ExternalExtensions", () => {
             expect(mockMQTTPublishAsync).toHaveBeenCalledWith("zigbee2mqtt/bridge/extensions", stringify([]), {retain: true});
         });
 
+        it("returns error on invalid name", async () => {
+            const extensionName = "foo1";
+
+            await resetExtension();
+            for (const mock of mocksClear) mock.mockClear();
+
+            await (controller.getExtension("ExternalExtensions")! as ExternalExtensions).onMQTTMessage({
+                topic: "zigbee2mqtt/bridge/request/extension/save",
+                message: {name: extensionName, code: "a"},
+            });
+
+            expect(mockMQTTPublishAsync).toHaveBeenCalledWith(
+                "zigbee2mqtt/bridge/response/extension/save",
+                expect.stringContaining(`JavaScript file must have '.mjs', '.js' or '.cjs' extension`),
+                {},
+            );
+            expect(writeFileSyncSpy).toHaveBeenCalledTimes(0);
+        });
+
         it("returns error on invalid code", async () => {
             const extensionName = "foo1.js";
             const extensionCode = "definetly not a correct javascript code";