diff --git a/.jules/sentinel.md b/.jules/sentinel.md
index 501ee8f..03026c1 100644
--- a/.jules/sentinel.md
+++ b/.jules/sentinel.md
@@ -7,3 +7,8 @@
 **Vulnerability:** The application used a hardcoded string ('ffui-webui-2025') as a salt for HMAC signatures on session tokens. This makes session tokens predictable if the password is known or weak, and allows rainbow table attacks if the source code is public.
 **Learning:** Hardcoded salts in open-source projects defeat the purpose of salting. Even if combined with a password, they don't provide per-installation uniqueness.
 **Prevention:** Use a random secret generated at runtime (or installation time) and persist it in the application configuration.
+
+## 2026-01-22 - Path Traversal in API Schemas
+**Vulnerability:** The `JobStartRequestSchema` validated filenames only by length (`min(1)`), allowing path traversal characters (e.g., `../../etc/passwd`). If the backend naively concatenates this filename to a path, it allows arbitrary file read/write.
+**Learning:** Zod's string validation is basic. For file paths, explicit validation against directory traversal (e.g., forbidding `..`) is essential, especially when inputs are passed to filesystem operations.
+**Prevention:** Use `.refine()` in Zod schemas to reject strings containing `..` path segments: `/(^|[\/])\.\.([\/]|$)/`.
diff --git a/src/main/webui/schemas/__tests__/web-api.schemas.test.ts b/src/main/webui/schemas/__tests__/web-api.schemas.test.ts
new file mode 100644
index 0000000..8b08c4a
--- /dev/null
+++ b/src/main/webui/schemas/__tests__/web-api.schemas.test.ts
@@ -0,0 +1,34 @@
+
+import { JobStartRequestSchema } from '../web-api.schemas';
+
+describe('JobStartRequestSchema', () => {
+  it('should accept valid filenames', () => {
+    const validInputs = [
+      { filename: 'test.gcode' },
+      { filename: 'folder/test.gcode' },
+      { filename: 'my_print_job.gcode' },
+      { filename: 'job.gcode', startNow: false },
+    ];
+
+    validInputs.forEach((input) => {
+      const result = JobStartRequestSchema.safeParse(input);
+      expect(result.success).toBe(true);
+    });
+  });
+
+  it('should reject path traversal attempts', () => {
+    const invalidInputs = [
+      { filename: '../system.conf' },
+      { filename: '../../etc/passwd' },
+      { filename: 'folder/../hack.txt' },
+      { filename: '/../root.txt' },
+      { filename: '..\\windows\\system32' },
+    ];
+
+    invalidInputs.forEach((input) => {
+      const result = JobStartRequestSchema.safeParse(input);
+      // We expect validation to fail for these insecure inputs
+      expect(result.success).toBe(false);
+    });
+  });
+});
diff --git a/src/main/webui/schemas/web-api.schemas.ts b/src/main/webui/schemas/web-api.schemas.ts
index acff167..6bd918f 100644
--- a/src/main/webui/schemas/web-api.schemas.ts
+++ b/src/main/webui/schemas/web-api.schemas.ts
@@ -65,7 +65,10 @@ export const TemperatureDataSchema = z.object({
  * Job start data validation
  */
 export const JobStartDataSchema = z.object({
-  filename: z.string().min(1),
+  filename: z
+    .string()
+    .min(1)
+    .refine((val) => !/(^|[\\/])\.\.([\\/]|$)/.test(val), 'Path traversal characters (..) are not allowed'),
   leveling: z.boolean().optional().default(false),
   startNow: z.boolean().optional().default(true),
 });
@@ -93,7 +96,10 @@ const MaterialMappingSchema = z.object({
 });
 
 export const JobStartRequestSchema = z.object({
-  filename: z.string().min(1, 'Filename is required'),
+  filename: z
+    .string()
+    .min(1, 'Filename is required')
+    .refine((val) => !/(^|[\\/])\.\.([\\/]|$)/.test(val), 'Path traversal characters (..) are not allowed'),
   leveling: z.boolean().optional().default(false),
   startNow: z.boolean().optional().default(true),
   materialMappings: z