diff --git a/apps/api/src/services/WorkflowService.ts b/apps/api/src/services/WorkflowService.ts
index 47aafb48..34c58381 100644
--- a/apps/api/src/services/WorkflowService.ts
+++ b/apps/api/src/services/WorkflowService.ts
@@ -14,6 +14,21 @@ import {NtfyService} from './NtfyService.js';
 import {WorkflowExecutionService} from './WorkflowExecutionService.js';
 
 export class WorkflowService {
+  private static async validateStepTemplate(projectId: string, templateId?: string | null): Promise<void> {
+    if (templateId === undefined || templateId === null) {
+      return;
+    }
+
+    const template = await prisma.template.findFirst({
+      where: {id: templateId, projectId},
+      select: {id: true},
+    });
+
+    if (!template) {
+      throw new HttpException(404, 'Template not found');
+    }
+  }
+
   /**
    * Get all workflows for a project with pagination
    */
@@ -504,6 +519,8 @@ export class WorkflowService {
       }
     }
 
+    await this.validateStepTemplate(projectId, data.templateId);
+
     // Create the new step
     const newStep = await prisma.workflowStep.create({
       data: {
@@ -589,6 +606,8 @@ export class WorkflowService {
       }
     }
 
+    await this.validateStepTemplate(projectId, data.templateId);
+
     const updateData: Prisma.WorkflowStepUpdateInput = {};
 
     if (data.name !== undefined) updateData.name = data.name;
@@ -860,6 +879,8 @@ export class WorkflowService {
       );
     }
 
+    await this.validateStepTemplate(projectId, data.templateId);
+
     return prisma.$transaction(async tx => {
       const newStep = await tx.workflowStep.create({
         data: {
diff --git a/apps/api/src/services/__tests__/WorkflowService.test.ts b/apps/api/src/services/__tests__/WorkflowService.test.ts
index 412bb5c1..cee5a354 100644
--- a/apps/api/src/services/__tests__/WorkflowService.test.ts
+++ b/apps/api/src/services/__tests__/WorkflowService.test.ts
@@ -498,6 +498,44 @@ describe('WorkflowService', () => {
       expect(step.templateId).toBe(template.id);
     });
 
+    it('should reject foreign and missing templates without revealing which exists', async () => {
+      const workflow = await factories.createWorkflow({projectId});
+      const {project: otherProject} = await factories.createUserWithProject();
+      const foreignTemplate = await factories.createTemplate({projectId: otherProject.id});
+      const missingTemplateId = '00000000-0000-0000-0000-000000000000';
+
+      const addEmailStep = (templateId: string) =>
+        WorkflowService.addStep(projectId, workflow.id, {
+          type: WorkflowStepType.SEND_EMAIL,
+          name: 'Send welcome email',
+          position: {x: 200, y: 100},
+          config: {},
+          templateId,
+        });
+
+      await expect(addEmailStep(foreignTemplate.id)).rejects.toMatchObject({
+        code: 404,
+        message: 'Template not found',
+      });
+      await expect(addEmailStep(missingTemplateId)).rejects.toMatchObject({
+        code: 404,
+        message: 'Template not found',
+      });
+      await expect(
+        WorkflowService.addStep(projectId, workflow.id, {
+          type: WorkflowStepType.DELAY,
+          name: 'Wait 1 hour',
+          position: {x: 200, y: 100},
+          config: {delay: 3600},
+          templateId: foreignTemplate.id,
+        }),
+      ).rejects.toMatchObject({code: 404, message: 'Template not found'});
+
+      const addedSteps = await prisma.workflowStep.findMany({where: {workflowId: workflow.id}});
+      expect(addedSteps).toHaveLength(1);
+      expect(addedSteps[0]?.type).toBe(WorkflowStepType.TRIGGER);
+    });
+
     it('should auto-connect to previous step by default', async () => {
       const workflow = await factories.createWorkflow({projectId});
 
@@ -614,6 +652,33 @@ describe('WorkflowService', () => {
       expect(updated.templateId).toBe(template2.id);
     });
 
+    it('should reject updating an email step to foreign and missing templates', async () => {
+      const workflow = await factories.createWorkflow({projectId});
+      const template = await factories.createTemplate({projectId});
+      const {project: otherProject} = await factories.createUserWithProject();
+      const foreignTemplate = await factories.createTemplate({projectId: otherProject.id});
+      const missingTemplateId = '00000000-0000-0000-0000-000000000000';
+      const step = await factories.createWorkflowStep({
+        workflowId: workflow.id,
+        type: WorkflowStepType.SEND_EMAIL,
+        templateId: template.id,
+      });
+
+      await expect(
+        WorkflowService.updateStep(projectId, workflow.id, step.id, {
+          templateId: foreignTemplate.id,
+        }),
+      ).rejects.toMatchObject({code: 404, message: 'Template not found'});
+      await expect(
+        WorkflowService.updateStep(projectId, workflow.id, step.id, {
+          templateId: missingTemplateId,
+        }),
+      ).rejects.toMatchObject({code: 404, message: 'Template not found'});
+
+      const unchanged = await prisma.workflowStep.findUnique({where: {id: step.id}});
+      expect(unchanged?.templateId).toBe(template.id);
+    });
+
     it('should remove template reference when set to null', async () => {
       const workflow = await factories.createWorkflow({projectId});
       const template = await factories.createTemplate({projectId});
@@ -878,6 +943,31 @@ describe('WorkflowService', () => {
       expect(outgoing[0]?.condition).toBeNull();
     });
 
+    it('should reject inserting an email step with a foreign template', async () => {
+      const workflow = await factories.createWorkflow({projectId});
+      const stepA = await factories.createWorkflowStep({workflowId: workflow.id});
+      const stepB = await factories.createWorkflowStep({workflowId: workflow.id});
+      const transition = await prisma.workflowTransition.create({
+        data: {fromStepId: stepA.id, toStepId: stepB.id},
+      });
+      const {project: otherProject} = await factories.createUserWithProject();
+      const foreignTemplate = await factories.createTemplate({projectId: otherProject.id});
+      const stepCount = await prisma.workflowStep.count({where: {workflowId: workflow.id}});
+
+      await expect(
+        WorkflowService.insertStepOnTransition(projectId, workflow.id, transition.id, {
+          type: WorkflowStepType.SEND_EMAIL,
+          name: 'Inserted email',
+          config: {},
+          templateId: foreignTemplate.id,
+        }),
+      ).rejects.toMatchObject({code: 404, message: 'Template not found'});
+
+      const original = await prisma.workflowTransition.findUnique({where: {id: transition.id}});
+      expect(original?.toStepId).toBe(stepB.id);
+      expect(await prisma.workflowStep.count({where: {workflowId: workflow.id}})).toBe(stepCount);
+    });
+
     it('should attach the downstream step to the first branch when inserting a CONDITION', async () => {
       const workflow = await factories.createWorkflow({projectId});
       const stepA = await factories.createWorkflowStep({workflowId: workflow.id});