diff --git a/apps/api/src/controllers/Events.ts b/apps/api/src/controllers/Events.ts
index d37ef113..8c28b334 100644
--- a/apps/api/src/controllers/Events.ts
+++ b/apps/api/src/controllers/Events.ts
@@ -1,7 +1,10 @@
 import {Controller, Delete, Get, Middleware, Post} from '@overnightjs/core';
 import type {NextFunction, Request, Response} from 'express';
 import signale from 'signale';
+import {prisma} from '../database/prisma.js';
+import {NotFound} from '../exceptions/index.js';
 import {requireAuth, requireEmailVerified} from '../middleware/auth.js';
+import {ContactService} from '../services/ContactService.js';
 import {EventService} from '../services/EventService.js';
 import {CatchAsync} from '../utils/asyncHandler.js';
 
@@ -22,6 +25,24 @@ export class Events {
       return res.status(400).json({error: 'Event name is required'});
     }
 
+    // Both IDs are client-supplied: they must belong to the authenticated project
+    // before they are recorded or used to start workflows (cross-tenant IDOR)
+    if (contactId) {
+      // Throws a 404 when the contact belongs to another project
+      await ContactService.get(auth.projectId!, contactId);
+    }
+
+    if (emailId) {
+      const email = await prisma.email.findFirst({
+        where: {id: emailId, projectId: auth.projectId!},
+        select: {id: true},
+      });
+
+      if (!email) {
+        throw new NotFound('email', emailId);
+      }
+    }
+
     const event = await EventService.trackEvent(auth.projectId!, name, contactId, emailId, data);
 
     return res.status(201).json(event);
diff --git a/apps/api/src/services/EventService.ts b/apps/api/src/services/EventService.ts
index af678fc7..0a06b15d 100644
--- a/apps/api/src/services/EventService.ts
+++ b/apps/api/src/services/EventService.ts
@@ -431,6 +431,20 @@ export class EventService {
         return;
       }
 
+      // Never run a workflow against a contact from another project.
+      // Queried directly instead of via ContactService, which imports this service.
+      const contact = await prisma.contact.findFirst({
+        where: {id: contactId, projectId: workflow.projectId},
+        select: {id: true},
+      });
+
+      if (!contact) {
+        signale.warn(
+          `[EVENT] Refusing to start workflow ${workflowId} for contact ${contactId}: contact does not belong to project ${workflow.projectId}`,
+        );
+        return;
+      }
+
       // Check re-entry rules
       if (!workflow.allowReentry) {
         // If re-entry is not allowed, check if contact has ANY execution (regardless of status)
diff --git a/apps/api/src/services/__tests__/EventService.test.ts b/apps/api/src/services/__tests__/EventService.test.ts
index c5c245e2..d7e66bf8 100644
--- a/apps/api/src/services/__tests__/EventService.test.ts
+++ b/apps/api/src/services/__tests__/EventService.test.ts
@@ -158,6 +158,27 @@ describe('EventService', () => {
       expect([WorkflowExecutionStatus.WAITING, WorkflowExecutionStatus.COMPLETED]).toContain(executions[0].status);
     });
 
+    it('should NOT start a workflow for a contact from another project', async () => {
+      const {project: otherProject} = await factories.createUserWithProject();
+      const foreignContact = await factories.createContact({projectId: otherProject.id});
+
+      const workflow = await factories.createWorkflow({
+        projectId,
+        enabled: true,
+        triggerType: WorkflowTriggerType.EVENT,
+        triggerConfig: {eventName: 'cross.tenant'},
+      });
+
+      // Event tracked in this project, but pointing at another tenant's contact
+      await EventService.trackEvent(projectId, 'cross.tenant', foreignContact.id);
+
+      const executions = await prisma.workflowExecution.findMany({
+        where: {workflowId: workflow.id},
+      });
+
+      expect(executions).toHaveLength(0);
+    });
+
     it('should NOT trigger disabled workflows', async () => {
       const contact = await factories.createContact({projectId});