diff --git a/backend/internal/db/postgres/category_repo.go b/backend/internal/db/postgres/category_repo.go
index ea97bbd..509ac2a 100644
--- a/backend/internal/db/postgres/category_repo.go
+++ b/backend/internal/db/postgres/category_repo.go
@@ -116,6 +116,12 @@ func (r *CategoryRepo) List(ctx context.Context, params port.OffsetParams) (*dom
 		args = append(args, "%"+params.Search+"%")
 		n++
 	}
+	// Restrict to categories the viewer may see (private-by-default), unless admin.
+	if !params.ViewerIsAdmin {
+		var aclCond string
+		aclCond, n, args = aclVisibilityCond("c", objTypeCategory, params.ViewerID, n, args)
+		conditions = append(conditions, aclCond)
+	}
 
 	where := ""
 	if len(conditions) > 0 {
diff --git a/backend/internal/db/postgres/file_repo.go b/backend/internal/db/postgres/file_repo.go
index 57e11ee..3a227c9 100644
--- a/backend/internal/db/postgres/file_repo.go
+++ b/backend/internal/db/postgres/file_repo.go
@@ -607,6 +607,13 @@ func (r *FileRepo) List(ctx context.Context, params domain.FileListParams) (*dom
 		}
 	}
 
+	// Restrict to files the viewer may see (private-by-default), unless admin.
+	if !params.ViewerIsAdmin {
+		var aclCond string
+		aclCond, n, args = aclVisibilityCond("f", objTypeFile, params.ViewerID, n, args)
+		conds = append(conds, aclCond)
+	}
+
 	var orderBy string
 	if hasCursor {
 		ksWhere, ksOrder, nextN, ksArgs := buildKeysetCond(
diff --git a/backend/internal/db/postgres/pool_repo.go b/backend/internal/db/postgres/pool_repo.go
index e5b373e..e1fc848 100644
--- a/backend/internal/db/postgres/pool_repo.go
+++ b/backend/internal/db/postgres/pool_repo.go
@@ -183,6 +183,12 @@ func (r *PoolRepo) List(ctx context.Context, params port.OffsetParams) (*domain.
 		args = append(args, "%"+params.Search+"%")
 		n++
 	}
+	// Restrict to pools the viewer may see (private-by-default), unless admin.
+	if !params.ViewerIsAdmin {
+		var aclCond string
+		aclCond, n, args = aclVisibilityCond("p", objTypePool, params.ViewerID, n, args)
+		conditions = append(conditions, aclCond)
+	}
 
 	where := ""
 	if len(conditions) > 0 {
diff --git a/backend/internal/db/postgres/postgres.go b/backend/internal/db/postgres/postgres.go
index c8c4bdd..86e9d11 100644
--- a/backend/internal/db/postgres/postgres.go
+++ b/backend/internal/db/postgres/postgres.go
@@ -65,3 +65,29 @@ func connOrTx(ctx context.Context, pool *pgxpool.Pool) db.Querier {
 	}
 	return pool
 }
+
+// Object type IDs as seeded in core.object_types (007_seed_data.sql).
+const (
+	objTypeFile     int16 = 1
+	objTypeTag      int16 = 2
+	objTypeCategory int16 = 3
+	objTypePool     int16 = 4
+)
+
+// aclVisibilityCond returns a SQL boolean fragment that is true when the viewer
+// may see the row at <alias>.id of the given object type under the
+// private-by-default model: the row is public, the viewer created it, or the
+// viewer holds an explicit can_view grant. objectTypeID is a trusted constant
+// and is inlined; viewerID is bound as $n (referenced twice). Returns the
+// fragment, the next free parameter index, and the extended args.
+//
+// Callers skip this entirely for admins (who bypass ACL).
+func aclVisibilityCond(alias string, objectTypeID int16, viewerID int16, n int, args []any) (string, int, []any) {
+	cond := fmt.Sprintf(
+		"(%[1]s.is_public OR %[1]s.creator_id = $%[2]d OR EXISTS ("+
+			"SELECT 1 FROM acl.permissions p "+
+			"WHERE p.object_type_id = %[3]d AND p.object_id = %[1]s.id "+
+			"AND p.user_id = $%[2]d AND p.can_view))",
+		alias, n, objectTypeID)
+	return cond, n + 1, append(args, viewerID)
+}
diff --git a/backend/internal/db/postgres/tag_repo.go b/backend/internal/db/postgres/tag_repo.go
index 42dd990..742e552 100644
--- a/backend/internal/db/postgres/tag_repo.go
+++ b/backend/internal/db/postgres/tag_repo.go
@@ -169,6 +169,12 @@ func (r *TagRepo) listTags(ctx context.Context, params port.OffsetParams, catego
 		args = append(args, *categoryID)
 		n++
 	}
+	// Restrict to tags the viewer may see (private-by-default), unless admin.
+	if !params.ViewerIsAdmin {
+		var aclCond string
+		aclCond, n, args = aclVisibilityCond("t", objTypeTag, params.ViewerID, n, args)
+		conditions = append(conditions, aclCond)
+	}
 
 	where := ""
 	if len(conditions) > 0 {
diff --git a/backend/internal/domain/file.go b/backend/internal/domain/file.go
index 45f72f9..d5569e4 100644
--- a/backend/internal/domain/file.go
+++ b/backend/internal/domain/file.go
@@ -49,6 +49,12 @@ type FileListParams struct {
 	Filter string // filter DSL expression
 	Search string // substring match on original_name
 	Trash  bool   // if true, return only soft-deleted files
+
+	// Visibility — populated by the service from the request context. When
+	// ViewerIsAdmin is false the repository restricts results to files the
+	// viewer may see (public, owned, or explicitly granted).
+	ViewerID      int16
+	ViewerIsAdmin bool
 }
 
 // FilePage is the result of a cursor-based file listing.
diff --git a/backend/internal/integration/server_test.go b/backend/internal/integration/server_test.go
index 27556f5..aa50b3b 100644
--- a/backend/internal/integration/server_test.go
+++ b/backend/internal/integration/server_test.go
@@ -828,6 +828,121 @@ func TestBlockRevokesActiveSessions(t *testing.T) {
 	assert.Equal(t, http.StatusUnauthorized, resp.StatusCode, resp.String())
 }
 
+// fileListIDs returns the set of file IDs visible to token via GET /files.
+func (h *harness) fileListIDs(token string) map[string]bool {
+	h.t.Helper()
+	resp := h.doJSON("GET", "/files", nil, token)
+	require.Equal(h.t, http.StatusOK, resp.StatusCode, resp.String())
+	var page map[string]any
+	resp.decode(h.t, &page)
+	ids := map[string]bool{}
+	if items, ok := page["items"].([]any); ok {
+		for _, it := range items {
+			if m, ok := it.(map[string]any); ok {
+				if id, ok := m["id"].(string); ok {
+					ids[id] = true
+				}
+			}
+		}
+	}
+	return ids
+}
+
+// TestPrivateByDefaultVisibility verifies that listings only return rows the
+// caller may see: private files are hidden from non-owners, public files are
+// visible to all, an explicit grant reveals a private file, and admins see all.
+func TestPrivateByDefaultVisibility(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+	h := setupSuite(t)
+	adminToken := h.login("admin", "admin")
+
+	resp := h.doJSON("POST", "/users", map[string]any{"name": "alice", "password": "alicepass"}, adminToken)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+	resp = h.doJSON("POST", "/users", map[string]any{"name": "bob", "password": "bobpass"}, adminToken)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+	var bob map[string]any
+	resp.decode(t, &bob)
+	bobID := bob["id"].(float64)
+
+	aliceToken := h.login("alice", "alicepass")
+	bobToken := h.login("bob", "bobpass")
+
+	file := h.uploadJPEG(aliceToken, "alice-secret.jpg")
+	fileID := file["id"].(string)
+
+	// Owner and admin see it; the unrelated user does not.
+	assert.True(t, h.fileListIDs(aliceToken)[fileID], "owner should see own file")
+	assert.True(t, h.fileListIDs(adminToken)[fileID], "admin should see all files")
+	assert.False(t, h.fileListIDs(bobToken)[fileID], "private file must not appear for a non-owner")
+
+	// Making it public reveals it to everyone.
+	resp = h.doJSON("PATCH", "/files/"+fileID, map[string]any{"is_public": true}, aliceToken)
+	require.Equal(t, http.StatusOK, resp.StatusCode, resp.String())
+	assert.True(t, h.fileListIDs(bobToken)[fileID], "public file should be visible to all")
+
+	// Private again → hidden; an explicit view grant reveals it.
+	resp = h.doJSON("PATCH", "/files/"+fileID, map[string]any{"is_public": false}, aliceToken)
+	require.Equal(t, http.StatusOK, resp.StatusCode, resp.String())
+	assert.False(t, h.fileListIDs(bobToken)[fileID])
+
+	resp = h.doJSON("PUT", "/acl/file/"+fileID, map[string]any{
+		"permissions": []map[string]any{{"user_id": bobID, "can_view": true, "can_edit": false}},
+	}, aliceToken)
+	require.Equal(t, http.StatusOK, resp.StatusCode, resp.String())
+	assert.True(t, h.fileListIDs(bobToken)[fileID], "granted file should be visible in the listing")
+}
+
+// TestPoolOperationsRequireACL verifies that a non-owner cannot view or modify
+// another user's private pool's contents.
+func TestPoolOperationsRequireACL(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping integration test in short mode")
+	}
+	h := setupSuite(t)
+	adminToken := h.login("admin", "admin")
+
+	resp := h.doJSON("POST", "/users", map[string]any{"name": "alice", "password": "alicepass"}, adminToken)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+	resp = h.doJSON("POST", "/users", map[string]any{"name": "bob", "password": "bobpass"}, adminToken)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+
+	aliceToken := h.login("alice", "alicepass")
+	bobToken := h.login("bob", "bobpass")
+
+	file := h.uploadJPEG(aliceToken, "f.jpg")
+	fileID := file["id"].(string)
+
+	resp = h.doJSON("POST", "/pools", map[string]any{"name": "alice pool", "is_public": false}, aliceToken)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+	var pool map[string]any
+	resp.decode(t, &pool)
+	poolID := pool["id"].(string)
+
+	resp = h.doJSON("POST", "/pools/"+poolID+"/files", map[string]any{"file_ids": []string{fileID}}, aliceToken)
+	require.Equal(t, http.StatusCreated, resp.StatusCode, resp.String())
+
+	// Bob cannot view the pool, list its files, or mutate its membership.
+	for _, c := range []struct {
+		method, path string
+		body         any
+	}{
+		{"GET", "/pools/" + poolID, nil},
+		{"GET", "/pools/" + poolID + "/files", nil},
+		{"POST", "/pools/" + poolID + "/files", map[string]any{"file_ids": []string{fileID}}},
+		{"POST", "/pools/" + poolID + "/files/remove", map[string]any{"file_ids": []string{fileID}}},
+		{"PUT", "/pools/" + poolID + "/files/reorder", map[string]any{"file_ids": []string{fileID}}},
+	} {
+		resp = h.doJSON(c.method, c.path, c.body, bobToken)
+		assert.Equal(t, http.StatusForbidden, resp.StatusCode, "%s %s: %s", c.method, c.path, resp)
+	}
+
+	// The owner can still list the pool's files.
+	resp = h.doJSON("GET", "/pools/"+poolID+"/files", nil, aliceToken)
+	assert.Equal(t, http.StatusOK, resp.StatusCode, resp.String())
+}
+
 // ---------------------------------------------------------------------------
 // Test helpers
 // ---------------------------------------------------------------------------
diff --git a/backend/internal/port/repository.go b/backend/internal/port/repository.go
index a7d3649..e578aa2 100644
--- a/backend/internal/port/repository.go
+++ b/backend/internal/port/repository.go
@@ -22,6 +22,13 @@ type OffsetParams struct {
 	Search string
 	Offset int
 	Limit  int
+
+	// Visibility — populated by the service from the request context. When
+	// ViewerIsAdmin is false the repository restricts results to rows the viewer
+	// may see (public, owned, or explicitly granted). Ignored by user listing,
+	// which is admin-only.
+	ViewerID      int16
+	ViewerIsAdmin bool
 }
 
 // PoolFileListParams holds parameters for listing files inside a pool.
diff --git a/backend/internal/service/category_service.go b/backend/internal/service/category_service.go
index 2034245..31c9809 100644
--- a/backend/internal/service/category_service.go
+++ b/backend/internal/service/category_service.go
@@ -49,14 +49,27 @@ func NewCategoryService(
 // CRUD
 // ---------------------------------------------------------------------------
 
-// List returns a paginated, optionally filtered list of categories.
+// List returns a paginated list of categories the caller may see.
 func (s *CategoryService) List(ctx context.Context, params port.OffsetParams) (*domain.CategoryOffsetPage, error) {
+	params.ViewerID, params.ViewerIsAdmin, _ = domain.UserFromContext(ctx)
 	return s.categories.List(ctx, params)
 }
 
-// Get returns a category by ID.
+// Get returns a category by ID, enforcing view ACL.
 func (s *CategoryService) Get(ctx context.Context, id uuid.UUID) (*domain.Category, error) {
-	return s.categories.GetByID(ctx, id)
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+	c, err := s.categories.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	ok, err := s.acl.CanView(ctx, userID, isAdmin, c.CreatorID, c.IsPublic, categoryObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+	return c, nil
 }
 
 // Create inserts a new category record.
@@ -158,7 +171,9 @@ func (s *CategoryService) Delete(ctx context.Context, id uuid.UUID) error {
 // Tags in category
 // ---------------------------------------------------------------------------
 
-// ListTags returns a paginated list of tags belonging to this category.
+// ListTags returns a paginated list of tags in this category that the caller
+// may see.
 func (s *CategoryService) ListTags(ctx context.Context, categoryID uuid.UUID, params port.OffsetParams) (*domain.TagOffsetPage, error) {
+	params.ViewerID, params.ViewerIsAdmin, _ = domain.UserFromContext(ctx)
 	return s.tags.ListByCategory(ctx, categoryID, params)
 }
\ No newline at end of file
diff --git a/backend/internal/service/file_service.go b/backend/internal/service/file_service.go
index 5c56ce7..05acd10 100644
--- a/backend/internal/service/file_service.go
+++ b/backend/internal/service/file_service.go
@@ -406,8 +406,10 @@ func (s *FileService) Replace(ctx context.Context, id uuid.UUID, p UploadParams)
 	return updated, nil
 }
 
-// List delegates to FileRepo with the given params.
+// List delegates to FileRepo with the given params, restricting results to
+// files the caller may see (unless they are an admin).
 func (s *FileService) List(ctx context.Context, params domain.FileListParams) (*domain.FilePage, error) {
+	params.ViewerID, params.ViewerIsAdmin, _ = domain.UserFromContext(ctx)
 	return s.files.List(ctx, params)
 }
 
diff --git a/backend/internal/service/pool_service.go b/backend/internal/service/pool_service.go
index 98552df..d1890f0 100644
--- a/backend/internal/service/pool_service.go
+++ b/backend/internal/service/pool_service.go
@@ -41,14 +41,63 @@ func NewPoolService(
 // CRUD
 // ---------------------------------------------------------------------------
 
-// List returns a paginated list of pools.
+// List returns a paginated list of pools the caller may see.
 func (s *PoolService) List(ctx context.Context, params port.OffsetParams) (*domain.PoolOffsetPage, error) {
+	params.ViewerID, params.ViewerIsAdmin, _ = domain.UserFromContext(ctx)
 	return s.pools.List(ctx, params)
 }
 
-// Get returns a pool by ID.
+// Get returns a pool by ID, enforcing view ACL.
 func (s *PoolService) Get(ctx context.Context, id uuid.UUID) (*domain.Pool, error) {
-	return s.pools.GetByID(ctx, id)
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+	p, err := s.pools.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	ok, err := s.acl.CanView(ctx, userID, isAdmin, p.CreatorID, p.IsPublic, poolObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+	return p, nil
+}
+
+// authorizeView returns nil if the caller may view the pool, else ErrForbidden
+// (or ErrNotFound if the pool does not exist).
+func (s *PoolService) authorizeView(ctx context.Context, poolID uuid.UUID) error {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+	p, err := s.pools.GetByID(ctx, poolID)
+	if err != nil {
+		return err
+	}
+	ok, err := s.acl.CanView(ctx, userID, isAdmin, p.CreatorID, p.IsPublic, poolObjectTypeID, poolID)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return domain.ErrForbidden
+	}
+	return nil
+}
+
+// authorizeEdit returns nil if the caller may edit the pool, else ErrForbidden
+// (or ErrNotFound if the pool does not exist).
+func (s *PoolService) authorizeEdit(ctx context.Context, poolID uuid.UUID) error {
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+	p, err := s.pools.GetByID(ctx, poolID)
+	if err != nil {
+		return err
+	}
+	ok, err := s.acl.CanEdit(ctx, userID, isAdmin, p.CreatorID, poolObjectTypeID, poolID)
+	if err != nil {
+		return err
+	}
+	if !ok {
+		return domain.ErrForbidden
+	}
+	return nil
 }
 
 // Create inserts a new pool.
@@ -146,13 +195,21 @@ func (s *PoolService) Delete(ctx context.Context, id uuid.UUID) error {
 // Pool–file operations
 // ---------------------------------------------------------------------------
 
-// ListFiles returns cursor-paginated files within a pool ordered by position.
+// ListFiles returns cursor-paginated files within a pool ordered by position,
+// enforcing view ACL on the pool.
 func (s *PoolService) ListFiles(ctx context.Context, poolID uuid.UUID, params port.PoolFileListParams) (*domain.PoolFilePage, error) {
+	if err := s.authorizeView(ctx, poolID); err != nil {
+		return nil, err
+	}
 	return s.pools.ListFiles(ctx, poolID, params)
 }
 
-// AddFiles adds files to a pool at the given position (nil = append).
+// AddFiles adds files to a pool at the given position (nil = append), enforcing
+// edit ACL on the pool.
 func (s *PoolService) AddFiles(ctx context.Context, poolID uuid.UUID, fileIDs []uuid.UUID, position *int) error {
+	if err := s.authorizeEdit(ctx, poolID); err != nil {
+		return err
+	}
 	if err := s.pools.AddFiles(ctx, poolID, fileIDs, position); err != nil {
 		return err
 	}
@@ -161,8 +218,11 @@ func (s *PoolService) AddFiles(ctx context.Context, poolID uuid.UUID, fileIDs []
 	return nil
 }
 
-// RemoveFiles removes files from a pool.
+// RemoveFiles removes files from a pool, enforcing edit ACL on the pool.
 func (s *PoolService) RemoveFiles(ctx context.Context, poolID uuid.UUID, fileIDs []uuid.UUID) error {
+	if err := s.authorizeEdit(ctx, poolID); err != nil {
+		return err
+	}
 	if err := s.pools.RemoveFiles(ctx, poolID, fileIDs); err != nil {
 		return err
 	}
@@ -171,7 +231,11 @@ func (s *PoolService) RemoveFiles(ctx context.Context, poolID uuid.UUID, fileIDs
 	return nil
 }
 
-// Reorder sets the full ordered sequence of file IDs within a pool.
+// Reorder sets the ordered sequence of file IDs within a pool, enforcing edit
+// ACL on the pool.
 func (s *PoolService) Reorder(ctx context.Context, poolID uuid.UUID, fileIDs []uuid.UUID) error {
+	if err := s.authorizeEdit(ctx, poolID); err != nil {
+		return err
+	}
 	return s.pools.Reorder(ctx, poolID, fileIDs)
 }
\ No newline at end of file
diff --git a/backend/internal/service/tag_service.go b/backend/internal/service/tag_service.go
index 9de7e9a..195d6fd 100644
--- a/backend/internal/service/tag_service.go
+++ b/backend/internal/service/tag_service.go
@@ -54,14 +54,27 @@ func NewTagService(
 // Tag CRUD
 // ---------------------------------------------------------------------------
 
-// List returns a paginated, optionally filtered list of tags.
+// List returns a paginated, optionally filtered list of tags the caller may see.
 func (s *TagService) List(ctx context.Context, params port.OffsetParams) (*domain.TagOffsetPage, error) {
+	params.ViewerID, params.ViewerIsAdmin, _ = domain.UserFromContext(ctx)
 	return s.tags.List(ctx, params)
 }
 
-// Get returns a tag by ID.
+// Get returns a tag by ID, enforcing view ACL.
 func (s *TagService) Get(ctx context.Context, id uuid.UUID) (*domain.Tag, error) {
-	return s.tags.GetByID(ctx, id)
+	userID, isAdmin, _ := domain.UserFromContext(ctx)
+	t, err := s.tags.GetByID(ctx, id)
+	if err != nil {
+		return nil, err
+	}
+	ok, err := s.acl.CanView(ctx, userID, isAdmin, t.CreatorID, t.IsPublic, tagObjectTypeID, id)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, domain.ErrForbidden
+	}
+	return t, nil
 }
 
 // Create inserts a new tag record.