diff --git a/__tests__/api/security.test.ts b/__tests__/api/security.test.ts
index 465ca1b..e73d564 100644
--- a/__tests__/api/security.test.ts
+++ b/__tests__/api/security.test.ts
@@ -57,10 +57,16 @@ describe('Security: cross-user access', () => {
 
       expect(response.status).toBe(200)
       expect(data).toHaveLength(1)
-      // Verify the query filtered by user_id
+      // Verify the query includes owned products and products shared through membership.
       expect(mockPrisma.product.findMany).toHaveBeenCalledWith(
         expect.objectContaining({
-          where: expect.objectContaining({ user_id: 'user-1' }),
+          where: expect.objectContaining({
+            archived: false,
+            OR: expect.arrayContaining([
+              { user_id: 'user-1' },
+              { members: { some: { user_id: 'user-1' } } },
+            ]),
+          }),
         })
       )
     })
diff --git a/actions/sprints.ts b/actions/sprints.ts
index 31715d4..ed8857d 100644
--- a/actions/sprints.ts
+++ b/actions/sprints.ts
@@ -12,6 +12,10 @@ async function getSession() {
   return getIronSession<SessionData>(await cookies(), sessionOptions)
 }
 
+function hasDuplicateIds(ids: string[]) {
+  return new Set(ids).size !== ids.length
+}
+
 export async function createSprintAction(_prevState: unknown, formData: FormData) {
   const session = await getSession()
   if (!session.userId) return { error: 'Niet ingelogd' }
@@ -79,6 +83,7 @@ export async function addStoryToSprintAction(sprintId: string, storyId: string)
     where: { id: storyId, product: productAccessFilter(session.userId) },
   })
   if (!story) return { error: 'Story niet gevonden' }
+  if (story.product_id !== sprint.product_id) return { error: 'Story hoort niet bij deze Sprint' }
 
   const last = await prisma.story.findFirst({
     where: { sprint_id: sprintId },
@@ -120,12 +125,19 @@ export async function reorderSprintStoriesAction(sprintId: string, orderedIds: s
   const session = await getSession()
   if (!session.userId) return { error: 'Niet ingelogd' }
   if (session.isDemo) return { error: 'Niet beschikbaar in demo-modus' }
+  if (hasDuplicateIds(orderedIds)) return { error: 'Ongeldige Sprint Backlog-volgorde' }
 
   const sprint = await prisma.sprint.findFirst({
     where: { id: sprintId, product: productAccessFilter(session.userId) },
   })
   if (!sprint) return { error: 'Sprint niet gevonden' }
 
+  const stories = await prisma.story.findMany({
+    where: { id: { in: orderedIds }, sprint_id: sprintId, product_id: sprint.product_id },
+    select: { id: true },
+  })
+  if (stories.length !== orderedIds.length) return { error: 'Ongeldige Sprint Backlog-volgorde' }
+
   await prisma.$transaction(
     orderedIds.map((id, i) =>
       prisma.story.update({ where: { id }, data: { sort_order: i + 1.0 } })
@@ -149,8 +161,22 @@ export async function completeSprintAction(
   })
   if (!sprint) return { error: 'Sprint niet gevonden' }
 
+  const entries = Object.entries(decisions)
+  if (entries.some(([, status]) => status !== 'DONE' && status !== 'OPEN')) {
+    return { error: 'Ongeldige Sprint-afronding' }
+  }
+
+  const storyIds = entries.map(([storyId]) => storyId)
+  if (hasDuplicateIds(storyIds)) return { error: 'Ongeldige Sprint-afronding' }
+
+  const stories = await prisma.story.findMany({
+    where: { id: { in: storyIds }, sprint_id: sprintId, product_id: sprint.product_id },
+    select: { id: true },
+  })
+  if (stories.length !== storyIds.length) return { error: 'Ongeldige Sprint-afronding' }
+
   await prisma.$transaction([
-    ...Object.entries(decisions).map(([storyId, status]) =>
+    ...entries.map(([storyId, status]) =>
       prisma.story.update({
         where: { id: storyId },
         data: {
diff --git a/actions/stories.ts b/actions/stories.ts
index e7247d2..4813695 100644
--- a/actions/stories.ts
+++ b/actions/stories.ts
@@ -19,6 +19,10 @@ async function verifyStoryAccess(storyId: string, userId: string) {
   })
 }
 
+function hasDuplicateIds(ids: string[]) {
+  return new Set(ids).size !== ids.length
+}
+
 const createStorySchema = z.object({
   pbiId: z.string(),
   productId: z.string(),
@@ -61,7 +65,7 @@ export async function createStoryAction(_prevState: unknown, formData: FormData)
   const story = await prisma.story.create({
     data: {
       pbi_id: parsed.data.pbiId,
-      product_id: parsed.data.productId,
+      product_id: pbi.product_id,
       title: parsed.data.title,
       priority: parsed.data.priority,
       sort_order,
@@ -69,7 +73,7 @@ export async function createStoryAction(_prevState: unknown, formData: FormData)
     },
   })
 
-  revalidatePath(`/products/${parsed.data.productId}`)
+  revalidatePath(`/products/${pbi.product_id}`)
   return { success: true, story }
 }
 
@@ -122,10 +126,17 @@ export async function reorderPbisAction(productId: string, orderedIds: string[])
   const session = await getSession()
   if (!session.userId) return { error: 'Niet ingelogd' }
   if (session.isDemo) return { error: 'Niet beschikbaar in demo-modus' }
+  if (hasDuplicateIds(orderedIds)) return { error: 'Ongeldige PBI-volgorde' }
 
   const product = await getAccessibleProduct(productId, session.userId)
   if (!product) return { error: 'Product niet gevonden' }
 
+  const pbis = await prisma.pbi.findMany({
+    where: { id: { in: orderedIds }, product_id: productId },
+    select: { id: true },
+  })
+  if (pbis.length !== orderedIds.length) return { error: 'Ongeldige PBI-volgorde' }
+
   await prisma.$transaction(
     orderedIds.map((id, i) =>
       prisma.pbi.update({ where: { id }, data: { sort_order: i + 1.0 } })
@@ -136,7 +147,7 @@ export async function reorderPbisAction(productId: string, orderedIds: string[])
   return { success: true }
 }
 
-export async function updatePbiPriorityAction(pbiId: string, priority: number, productId: string) {
+export async function updatePbiPriorityAction(pbiId: string, priority: number, _productId: string) {
   const session = await getSession()
   if (!session.userId) return { error: 'Niet ingelogd' }
   if (session.isDemo) return { error: 'Niet beschikbaar in demo-modus' }
@@ -145,9 +156,10 @@ export async function updatePbiPriorityAction(pbiId: string, priority: number, p
     where: { id: pbiId, product: productAccessFilter(session.userId) },
   })
   if (!pbi) return { error: 'PBI niet gevonden' }
+  if (!Number.isInteger(priority) || priority < 1 || priority > 4) return { error: 'Ongeldige prioriteit' }
 
   const last = await prisma.pbi.findFirst({
-    where: { product_id: productId, priority },
+    where: { product_id: pbi.product_id, priority },
     orderBy: { sort_order: 'desc' },
   })
 
@@ -156,7 +168,7 @@ export async function updatePbiPriorityAction(pbiId: string, priority: number, p
     data: { priority, sort_order: (last?.sort_order ?? 0) + 1.0 },
   })
 
-  revalidatePath(`/products/${productId}`)
+  revalidatePath(`/products/${pbi.product_id}`)
   return { success: true }
 }
 
@@ -199,12 +211,22 @@ export async function reorderStoriesAction(
   const session = await getSession()
   if (!session.userId) return { error: 'Niet ingelogd' }
   if (session.isDemo) return { error: 'Niet beschikbaar in demo-modus' }
+  if (hasDuplicateIds(orderedIds)) return { error: 'Ongeldige story-volgorde' }
+  if (newPriority !== undefined && (!Number.isInteger(newPriority) || newPriority < 1 || newPriority > 4)) {
+    return { error: 'Ongeldige prioriteit' }
+  }
 
   const pbi = await prisma.pbi.findFirst({
     where: { id: pbiId, product: productAccessFilter(session.userId) },
   })
   if (!pbi) return { error: 'PBI niet gevonden' }
 
+  const stories = await prisma.story.findMany({
+    where: { id: { in: orderedIds }, pbi_id: pbiId, product_id: pbi.product_id },
+    select: { id: true },
+  })
+  if (stories.length !== orderedIds.length) return { error: 'Ongeldige story-volgorde' }
+
   await prisma.$transaction(
     orderedIds.map((id, i) =>
       prisma.story.update({
@@ -217,6 +239,6 @@ export async function reorderStoriesAction(
     )
   )
 
-  revalidatePath(`/products/${productId}`)
+  revalidatePath(`/products/${pbi.product_id}`)
   return { success: true }
 }
diff --git a/actions/todos.ts b/actions/todos.ts
index 2561d9f..ab9ea7e 100644
--- a/actions/todos.ts
+++ b/actions/todos.ts
@@ -6,6 +6,7 @@ import { getIronSession } from 'iron-session'
 import { z } from 'zod'
 import { prisma } from '@/lib/prisma'
 import { SessionData, sessionOptions } from '@/lib/session'
+import { productAccessFilter } from '@/lib/product-access'
 
 async function getSession() {
   return getIronSession<SessionData>(await cookies(), sessionOptions)
@@ -81,6 +82,11 @@ export async function promoteTodoToPbiAction(_prevState: unknown, formData: Form
   })
   if (!product) return { error: 'Product niet gevonden' }
 
+  const todo = await prisma.todo.findFirst({
+    where: { id: parsed.data.todoId, user_id: session.userId, product_id: parsed.data.productId },
+  })
+  if (!todo) return { error: 'Todo niet gevonden' }
+
   const last = await prisma.pbi.findFirst({
     where: { product_id: parsed.data.productId, priority: parsed.data.priority },
     orderBy: { sort_order: 'desc' },
@@ -95,7 +101,7 @@ export async function promoteTodoToPbiAction(_prevState: unknown, formData: Form
         sort_order: (last?.sort_order ?? 0) + 1.0,
       },
     }),
-    prisma.todo.delete({ where: { id: parsed.data.todoId } }),
+    prisma.todo.deleteMany({ where: { id: parsed.data.todoId, user_id: session.userId } }),
   ])
 
   revalidatePath('/todos')
@@ -125,10 +131,16 @@ export async function promoteTodoToStoryAction(_prevState: unknown, formData: Fo
   })
   if (!parsed.success) return { error: parsed.error.flatten().fieldErrors }
 
+  const todo = await prisma.todo.findFirst({
+    where: { id: parsed.data.todoId, user_id: session.userId },
+  })
+  if (!todo) return { error: 'Todo niet gevonden' }
+
   const pbi = await prisma.pbi.findFirst({
-    where: { id: parsed.data.pbiId, product: { user_id: session.userId } },
+    where: { id: parsed.data.pbiId, product: productAccessFilter(session.userId) },
   })
   if (!pbi) return { error: 'PBI niet gevonden' }
+  if (todo.product_id !== pbi.product_id) return { error: 'Todo hoort niet bij dit product' }
 
   const last = await prisma.story.findFirst({
     where: { pbi_id: parsed.data.pbiId, priority: parsed.data.priority },
@@ -139,18 +151,18 @@ export async function promoteTodoToStoryAction(_prevState: unknown, formData: Fo
     prisma.story.create({
       data: {
         pbi_id: parsed.data.pbiId,
-        product_id: parsed.data.productId,
+        product_id: pbi.product_id,
         title: parsed.data.title,
         priority: parsed.data.priority,
         sort_order: (last?.sort_order ?? 0) + 1.0,
         status: 'OPEN',
       },
     }),
-    prisma.todo.delete({ where: { id: parsed.data.todoId } }),
+    prisma.todo.deleteMany({ where: { id: parsed.data.todoId, user_id: session.userId } }),
   ])
 
   revalidatePath('/todos')
-  revalidatePath(`/products/${parsed.data.productId}`)
+  revalidatePath(`/products/${pbi.product_id}`)
   return { success: true }
 }
 
diff --git a/app/api/products/route.ts b/app/api/products/route.ts
index fda4582..89c7c9e 100644
--- a/app/api/products/route.ts
+++ b/app/api/products/route.ts
@@ -1,5 +1,6 @@
 import { authenticateApiRequest } from '@/lib/api-auth'
 import { prisma } from '@/lib/prisma'
+import { productAccessFilter } from '@/lib/product-access'
 
 export async function GET(request: Request) {
   const auth = await authenticateApiRequest(request)
@@ -8,7 +9,7 @@ export async function GET(request: Request) {
   }
 
   const products = await prisma.product.findMany({
-    where: { user_id: auth.userId, archived: false },
+    where: { archived: false, ...productAccessFilter(auth.userId) },
     orderBy: { created_at: 'desc' },
     select: { id: true, name: true, repo_url: true },
   })
diff --git a/package-lock.json b/package-lock.json
index ea58c02..183d1c5 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -28,6 +28,7 @@
         "react": "19.2.4",
         "react-dom": "19.2.4",
         "shadcn": "^4.4.0",
+        "sharp": "^0.34.5",
         "sonner": "^1.7.4",
         "tailwind-merge": "^3.5.0",
         "tw-animate-css": "^1.4.0",
@@ -1567,7 +1568,6 @@
       "resolved": "https://registry.npmjs.org/@img/colour/-/colour-1.1.0.tgz",
       "integrity": "sha512-Td76q7j57o/tLVdgS746cYARfSyxk8iEfRxewL9h4OMzYhbW4TAcppl0mT4eyqXddh6L/jwoM75mo7ixa/pCeQ==",
       "license": "MIT",
-      "optional": true,
       "engines": {
         "node": ">=18"
       }
@@ -5781,7 +5781,6 @@
       "version": "2.1.2",
       "resolved": "https://registry.npmjs.org/detect-libc/-/detect-libc-2.1.2.tgz",
       "integrity": "sha512-Btj2BOOO83o3WyH59e8MgXsxEQVcarkUOpEYrubB0urwnN10yQ364rsiByU11nZlqWYZm05i/of7io4mzihBtQ==",
-      "devOptional": true,
       "license": "Apache-2.0",
       "engines": {
         "node": ">=8"
@@ -11081,7 +11080,6 @@
       "integrity": "sha512-Ou9I5Ft9WNcCbXrU9cMgPBcCK8LiwLqcbywW3t4oDV37n1pzpuNLsYiAV8eODnjbtQlSDwZ2cUEeQz4E54Hltg==",
       "hasInstallScript": true,
       "license": "Apache-2.0",
-      "optional": true,
       "dependencies": {
         "@img/colour": "^1.0.0",
         "detect-libc": "^2.1.2",
@@ -11125,7 +11123,6 @@
       "resolved": "https://registry.npmjs.org/semver/-/semver-7.7.4.tgz",
       "integrity": "sha512-vFKC2IEtQnVhpT78h1Yp8wzwrf8CM+MzKMHGJZfBtzhZNycRFnXsHk6E5TxIkkMsgNS7mdX3AGB7x2QM2di4lA==",
       "license": "ISC",
-      "optional": true,
       "bin": {
         "semver": "bin/semver.js"
       },
diff --git a/package.json b/package.json
index 5ccccf6..dae3058 100644
--- a/package.json
+++ b/package.json
@@ -32,6 +32,7 @@
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "shadcn": "^4.4.0",
+    "sharp": "^0.34.5",
     "sonner": "^1.7.4",
     "tailwind-merge": "^3.5.0",
     "tw-animate-css": "^1.4.0",