diff --git a/src/fiddle.ts b/src/fiddle.ts
index 44ec179..ae36459 100644
--- a/src/fiddle.ts
+++ b/src/fiddle.ts
@@ -92,19 +92,26 @@ export class FiddleFactory {
     const md5sum = createHash('md5');
     for (const content of map.values()) md5sum.update(content);
     const hash = md5sum.digest('hex');
-    const folder = path.join(this.fiddles, hash);
+    const folder = path.resolve(this.fiddles, hash);
     await fs.promises.mkdir(folder, { recursive: true });
     d({ folder });
 
     // save content to that temp directory
     await Promise.all(
-      [...map.entries()].map(([filename, content]) =>
-        util.promisify(fs.writeFile)(
-          path.join(folder, filename),
-          content,
-          'utf8',
-        ),
-      ),
+      [...map.entries()].map(([filename, content]) => {
+        const filePath = path.resolve(folder, filename);
+        const relative = path.relative(folder, filePath);
+        if (
+          !relative ||
+          relative.startsWith('..') ||
+          path.isAbsolute(relative)
+        ) {
+          throw new Error(
+            `Refusing to write file outside of fiddle: "${filename}"`,
+          );
+        }
+        return util.promisify(fs.writeFile)(filePath, content, 'utf8');
+      }),
     );
 
     return new Fiddle(path.join(folder, 'main.js'), 'entries');
diff --git a/src/installer.ts b/src/installer.ts
index ba9cef0..330e27d 100644
--- a/src/installer.ts
+++ b/src/installer.ts
@@ -14,6 +14,12 @@ function getZipName(version: string): string {
   return `electron-v${version}-${process.platform}-${process.arch}.zip`;
 }
 
+function assertValidVersion(version: string): void {
+  if (!semver.valid(version)) {
+    throw new Error(`Invalid Electron version: "${version}"`);
+  }
+}
+
 export type ProgressObject = { percent: number };
 
 /**
@@ -163,6 +169,7 @@ export class Installer extends EventEmitter {
   public async remove(version: string): Promise<void> {
     const d = debug('fiddle-core:Installer:remove');
     d(version);
+    assertValidVersion(version);
     let isBinaryDeleted = false;
     // utility to re-run removal functions upon failure
     // due to windows filesystem lockfile jank
@@ -270,6 +277,7 @@ export class Installer extends EventEmitter {
     opts?: Partial<InstallerParams>,
   ): Promise<ElectronBinary> {
     const d = debug(`fiddle-core:Installer:${version}:ensureDownloadedImpl`);
+    assertValidVersion(version);
     const { electronDownloads } = this.paths;
     const zipFile = path.join(electronDownloads, getZipName(version));
     const zipFileExists = fs.existsSync(zipFile);
diff --git a/tests/fiddle.test.ts b/tests/fiddle.test.ts
index 27b4375..df6db24 100644
--- a/tests/fiddle.test.ts
+++ b/tests/fiddle.test.ts
@@ -84,6 +84,17 @@ describe('FiddleFactory', () => {
       }
     });
 
+    it('rejects entries with filenames that escape the fiddle directory', async () => {
+      const files: [string, string][] = [
+        ['main.js', '"use strict";'],
+        [path.join('..', '..', 'escaped.txt'), 'pwned'],
+      ];
+      await expect(fiddleFactory.create(files)).rejects.toThrow(
+        /outside of fiddle/,
+      );
+      expect(fs.existsSync(path.join(tmpdir, 'escaped.txt'))).toBe(false);
+    });
+
     it('reads fiddles from gists', async () => {
       const gistId = '642fa8daaebea6044c9079e3f8a46390';
       const fiddle = await fiddleFactory.create(gistId);
diff --git a/tests/installer.test.ts b/tests/installer.test.ts
index f16297b..c66300d 100644
--- a/tests/installer.test.ts
+++ b/tests/installer.test.ts
@@ -243,6 +243,12 @@ describe('Installer', () => {
       expect(nockScope.isDone());
     });
 
+    it('rejects versions that are not valid semver', async () => {
+      await expect(
+        installer.ensureDownloaded(path.join('..', 'x')),
+      ).rejects.toThrow(/Invalid Electron version/);
+    });
+
     it('resets install state on error', async () => {
       // setup: version is not installed
       expect(installer.state(version)).toBe(missing);
@@ -297,6 +303,15 @@ describe('Installer', () => {
       expect(fs.existsSync(extractDir)).toBe(false);
       expect(events).toStrictEqual([{ version, state: missing }]);
     });
+
+    it('rejects versions that are not valid semver', async () => {
+      const canary = path.join(tmpdir, 'canary');
+      await fs.promises.mkdir(canary);
+      await expect(installer.remove(path.join('..', 'canary'))).rejects.toThrow(
+        /Invalid Electron version/,
+      );
+      expect(fs.existsSync(canary)).toBe(true);
+    });
   });
 
   describe('install()', () => {