diff --git a/src/app/api/v1/berths/[id]/waiting-list/route.ts b/src/app/api/v1/berths/[id]/waiting-list/route.ts
index 755a109..a702cc5 100644
--- a/src/app/api/v1/berths/[id]/waiting-list/route.ts
+++ b/src/app/api/v1/berths/[id]/waiting-list/route.ts
@@ -8,7 +8,7 @@ import { reorderWaitingListSchema } from '@/lib/validators/interests';
 import { getWaitingList, updateWaitingList } from '@/lib/services/berths.service';
 import { errorResponse, NotFoundError } from '@/lib/errors';
 import { db } from '@/lib/db';
-import { berthWaitingList } from '@/lib/db/schema/berths';
+import { berths, berthWaitingList } from '@/lib/db/schema/berths';
 
 // GET /api/v1/berths/[id]/waiting-list
 export const GET = withAuth(
@@ -47,11 +47,17 @@ export const PATCH = withAuth(
       const body = await parseBody(req, reorderWaitingListSchema);
       const berthId = params.id!;
 
+      // Tenant scope: refuse to reorder a foreign-port berth's waiting
+      // list. The route's URL id and the entry id are otherwise enough
+      // for any user with manage_waiting_list to mutate any tenant's
+      // queue ordering.
+      const berthRow = await db.query.berths.findFirst({
+        where: and(eq(berths.id, berthId), eq(berths.portId, ctx.portId)),
+      });
+      if (!berthRow) throw new NotFoundError('Berth');
+
       const entry = await db.query.berthWaitingList.findFirst({
-        where: and(
-          eq(berthWaitingList.id, body.entryId),
-          eq(berthWaitingList.berthId, berthId),
-        ),
+        where: and(eq(berthWaitingList.id, body.entryId), eq(berthWaitingList.berthId, berthId)),
       });
       if (!entry) throw new NotFoundError('Waiting list entry');
 
diff --git a/src/app/api/v1/custom-fields/[entityId]/route.ts b/src/app/api/v1/custom-fields/[entityId]/route.ts
index d137447..212dcfa 100644
--- a/src/app/api/v1/custom-fields/[entityId]/route.ts
+++ b/src/app/api/v1/custom-fields/[entityId]/route.ts
@@ -1,45 +1,55 @@
 import { NextRequest, NextResponse } from 'next/server';
 
-import { withAuth } from '@/lib/api/helpers';
+import { withAuth, withPermission } from '@/lib/api/helpers';
 import { errorResponse, NotFoundError } from '@/lib/errors';
 import { setValuesSchema } from '@/lib/validators/custom-fields';
 import { getValues, setValues } from '@/lib/services/custom-fields.service';
 
-export const GET = withAuth(async (_req: NextRequest, ctx, params) => {
-  try {
-    const { entityId } = params;
-    if (!entityId) throw new NotFoundError('Entity');
+// Custom-field values live on top of a port-scoped entity (client, yacht,
+// interest, berth, company). Reading the values is in scope for any role
+// that can view clients (the most common surface); writing requires the
+// equivalent edit permission. The service-layer also re-validates the
+// entityId against the field definition's entityType + portId so a
+// caller cannot poke values onto an arbitrary or foreign-port entity.
+export const GET = withAuth(
+  withPermission('clients', 'view', async (_req: NextRequest, ctx, params) => {
+    try {
+      const { entityId } = params;
+      if (!entityId) throw new NotFoundError('Entity');
 
-    const data = await getValues(entityId, ctx.portId);
-    return NextResponse.json({ data });
-  } catch (error) {
-    return errorResponse(error);
-  }
-});
+      const data = await getValues(entityId, ctx.portId);
+      return NextResponse.json({ data });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  }),
+);
 
-export const PUT = withAuth(async (req: NextRequest, ctx, params) => {
-  try {
-    const { entityId } = params;
-    if (!entityId) throw new NotFoundError('Entity');
+export const PUT = withAuth(
+  withPermission('clients', 'edit', async (req: NextRequest, ctx, params) => {
+    try {
+      const { entityId } = params;
+      if (!entityId) throw new NotFoundError('Entity');
 
-    const body = await req.json();
-    const { values } = setValuesSchema.parse(body);
+      const body = await req.json();
+      const { values } = setValuesSchema.parse(body);
 
-    const result = await setValues(
-      entityId,
-      ctx.portId,
-      ctx.userId,
-      values as Array<{ fieldId: string; value: unknown }>,
-      {
-        userId: ctx.userId,
-        portId: ctx.portId,
-        ipAddress: ctx.ipAddress,
-        userAgent: ctx.userAgent,
-      },
-    );
+      const result = await setValues(
+        entityId,
+        ctx.portId,
+        ctx.userId,
+        values as Array<{ fieldId: string; value: unknown }>,
+        {
+          userId: ctx.userId,
+          portId: ctx.portId,
+          ipAddress: ctx.ipAddress,
+          userAgent: ctx.userAgent,
+        },
+      );
 
-    return NextResponse.json({ data: result });
-  } catch (error) {
-    return errorResponse(error);
-  }
-});
+      return NextResponse.json({ data: result });
+    } catch (error) {
+      return errorResponse(error);
+    }
+  }),
+);
diff --git a/src/lib/services/berths.service.ts b/src/lib/services/berths.service.ts
index 9015d50..92fe229 100644
--- a/src/lib/services/berths.service.ts
+++ b/src/lib/services/berths.service.ts
@@ -2,10 +2,11 @@ import { and, eq, gte, lte, inArray } from 'drizzle-orm';
 
 import { db } from '@/lib/db';
 import { berths, berthTags, berthWaitingList, berthMaintenanceLog } from '@/lib/db/schema/berths';
+import { clients } from '@/lib/db/schema/clients';
 import { tags } from '@/lib/db/schema/system';
 import { createAuditLog, type AuditMeta } from '@/lib/audit';
 import { diffEntity } from '@/lib/entity-diff';
-import { NotFoundError } from '@/lib/errors';
+import { NotFoundError, ValidationError } from '@/lib/errors';
 import { buildListQuery } from '@/lib/db/query-builder';
 import { emitToRoom } from '@/lib/socket/server';
 import { setEntityTags } from '@/lib/services/entity-tags.helper';
@@ -401,6 +402,21 @@ export async function updateWaitingList(
   });
   if (!existing) throw new NotFoundError('Berth');
 
+  // Validate every supplied clientId belongs to portId. Without this
+  // check, a port-A admin could insert port-B clientIds into the
+  // waiting list — corrupting reportable data and creating a join
+  // surface that hydrates foreign-tenant client rows.
+  if (data.entries.length > 0) {
+    const clientIds = [...new Set(data.entries.map((e) => e.clientId))];
+    const validClients = await db
+      .select({ id: clients.id })
+      .from(clients)
+      .where(and(inArray(clients.id, clientIds), eq(clients.portId, portId)));
+    if (validClients.length !== clientIds.length) {
+      throw new ValidationError('One or more clients are not in this port');
+    }
+  }
+
   // Replace entire waiting list
   await db.delete(berthWaitingList).where(eq(berthWaitingList.berthId, id));
 
diff --git a/src/lib/services/custom-fields.service.ts b/src/lib/services/custom-fields.service.ts
index 605c31d..0e661de 100644
--- a/src/lib/services/custom-fields.service.ts
+++ b/src/lib/services/custom-fields.service.ts
@@ -266,6 +266,57 @@ export async function setValues(
     throw new ValidationError('Custom field validation failed', errors);
   }
 
+  // Tenant scope: verify entityId actually points at a port-scoped row of
+  // the entity type the field definitions target. Without this gate, any
+  // authenticated user could write custom-field rows pointing at arbitrary
+  // entityIds (or none at all) — polluting customFieldValues and creating
+  // a join surface that could later leak data.
+  const entityTypes = new Set(
+    values
+      .map((v) => definitionMap.get(v.fieldId)?.entityType)
+      .filter((t): t is string => Boolean(t)),
+  );
+  for (const entityType of entityTypes) {
+    const { eq: drizzleEq, and: drizzleAnd } = await import('drizzle-orm');
+    let exists = false;
+    if (entityType === 'client') {
+      const { clients } = await import('@/lib/db/schema/clients');
+      const row = await db.query.clients.findFirst({
+        where: drizzleAnd(drizzleEq(clients.id, entityId), drizzleEq(clients.portId, portId)),
+      });
+      exists = Boolean(row);
+    } else if (entityType === 'interest') {
+      const { interests } = await import('@/lib/db/schema/interests');
+      const row = await db.query.interests.findFirst({
+        where: drizzleAnd(drizzleEq(interests.id, entityId), drizzleEq(interests.portId, portId)),
+      });
+      exists = Boolean(row);
+    } else if (entityType === 'berth') {
+      const { berths } = await import('@/lib/db/schema/berths');
+      const row = await db.query.berths.findFirst({
+        where: drizzleAnd(drizzleEq(berths.id, entityId), drizzleEq(berths.portId, portId)),
+      });
+      exists = Boolean(row);
+    } else if (entityType === 'yacht') {
+      const { yachts } = await import('@/lib/db/schema/yachts');
+      const row = await db.query.yachts.findFirst({
+        where: drizzleAnd(drizzleEq(yachts.id, entityId), drizzleEq(yachts.portId, portId)),
+      });
+      exists = Boolean(row);
+    } else if (entityType === 'company') {
+      const { companies } = await import('@/lib/db/schema/companies');
+      const row = await db.query.companies.findFirst({
+        where: drizzleAnd(drizzleEq(companies.id, entityId), drizzleEq(companies.portId, portId)),
+      });
+      exists = Boolean(row);
+    } else {
+      throw new ValidationError(`Unsupported custom-field entity type: ${entityType}`);
+    }
+    if (!exists) {
+      throw new ValidationError(`${entityType} not found in this port`);
+    }
+  }
+
   // Upsert all values
   const results = await Promise.all(
     values.map(async ({ fieldId, value }) => {
diff --git a/src/lib/services/entity-tags.helper.ts b/src/lib/services/entity-tags.helper.ts
index acf5279..c79b3f6 100644
--- a/src/lib/services/entity-tags.helper.ts
+++ b/src/lib/services/entity-tags.helper.ts
@@ -13,11 +13,14 @@
  * the audit-log payload to `newValue: { tagIds }` across all entity types.
  */
 
-import { eq, type Column } from 'drizzle-orm';
+import { and, eq, inArray, type Column } from 'drizzle-orm';
 import type { PgTable } from 'drizzle-orm/pg-core';
 
+import { db } from '@/lib/db';
+import { tags } from '@/lib/db/schema/system';
 import { withTransaction } from '@/lib/db/utils';
 import { createAuditLog, type AuditMeta } from '@/lib/audit';
+import { ValidationError } from '@/lib/errors';
 import { emitToRoom } from '@/lib/socket/server';
 
 interface SetEntityTagsArgs<TJoin extends PgTable> {
@@ -51,6 +54,23 @@ export async function setEntityTags<TJoin extends PgTable>(
 ): Promise<{ entityId: string; tagIds: string[] }> {
   const { joinTable, entityColumn, tagColumn, entityId, portId, tagIds, meta, entityType } = args;
 
+  // Tenant scope: every supplied tagId must belong to the caller's port.
+  // The tags table is per-port (`tags.port_id`) but the join tables only
+  // have a single-column FK to `tags.id` — without this guard, a port-A
+  // caller could splice a port-B tag UUID onto their own entity. The
+  // entity's GET handler joins `tags ON join.tag_id = tags.id` with no
+  // port filter, so the foreign tag's name and color render in port A.
+  if (tagIds.length > 0) {
+    const uniqueTagIds = [...new Set(tagIds)];
+    const validTags = await db
+      .select({ id: tags.id })
+      .from(tags)
+      .where(and(inArray(tags.id, uniqueTagIds), eq(tags.portId, portId)));
+    if (validTags.length !== uniqueTagIds.length) {
+      throw new ValidationError('One or more tags are not in this port');
+    }
+  }
+
   await withTransaction(async (tx) => {
     await tx.delete(joinTable).where(eq(entityColumn, entityId));
     if (tagIds.length > 0) {
diff --git a/src/lib/services/recommendations.ts b/src/lib/services/recommendations.ts
index f6bb94e..70f3c64 100644
--- a/src/lib/services/recommendations.ts
+++ b/src/lib/services/recommendations.ts
@@ -147,7 +147,16 @@ export async function generateRecommendations(interestId: string, portId: string
 
 // ─── List Recommendations ─────────────────────────────────────────────────────
 
-export async function listRecommendations(interestId: string, _portId: string) {
+export async function listRecommendations(interestId: string, portId: string) {
+  // Verify the interest belongs to the caller's port. Without this gate,
+  // any user with `interests:view` could pass a foreign-port interestId
+  // and receive that tenant's recommended berths (mooring numbers,
+  // dimensions, status — operational data they should not see).
+  const interest = await db.query.interests.findFirst({
+    where: and(eq(interests.id, interestId), eq(interests.portId, portId)),
+  });
+  if (!interest) throw new NotFoundError('Interest');
+
   const rows = await db
     .select({
       id: berthRecommendations.id,
@@ -167,7 +176,7 @@ export async function listRecommendations(interestId: string, _portId: string) {
     })
     .from(berthRecommendations)
     .innerJoin(berths, eq(berthRecommendations.berthId, berths.id))
-    .where(eq(berthRecommendations.interestId, interestId))
+    .where(and(eq(berthRecommendations.interestId, interestId), eq(berths.portId, portId)))
     .orderBy(berthRecommendations.matchScore);
 
   return rows.reverse(); // highest score first
diff --git a/src/lib/services/reminders.service.ts b/src/lib/services/reminders.service.ts
index e40cecf..8a13241 100644
--- a/src/lib/services/reminders.service.ts
+++ b/src/lib/services/reminders.service.ts
@@ -2,6 +2,7 @@ import { and, eq, lte, gte, desc, asc, inArray, sql, isNull } from 'drizzle-orm'
 
 import { db } from '@/lib/db';
 import { reminders, interests, clients } from '@/lib/db/schema';
+import { berths } from '@/lib/db/schema/berths';
 import { createAuditLog, type AuditMeta } from '@/lib/audit';
 import { NotFoundError, ValidationError } from '@/lib/errors';
 import { emitToRoom } from '@/lib/socket/server';
@@ -107,6 +108,50 @@ export async function getUpcomingReminders(portId: string, days: number = 14) {
     .orderBy(asc(reminders.dueAt));
 }
 
+/**
+ * Validate that the supplied subject FKs (clientId / interestId / berthId)
+ * all point at rows inside the caller's port. Without this guard, a
+ * reminder created with a foreign-port FK would later be hydrated with
+ * `with: { client, interest, berth }` joins (no port filter on the
+ * relation) — leaking the foreign-tenant rows back to the attacker.
+ */
+async function assertReminderFksInPort(
+  portId: string,
+  fks: { clientId?: string | null; interestId?: string | null; berthId?: string | null },
+): Promise<void> {
+  const checks: Array<Promise<void>> = [];
+  if (fks.clientId) {
+    checks.push(
+      db.query.clients
+        .findFirst({ where: and(eq(clients.id, fks.clientId), eq(clients.portId, portId)) })
+        .then((row) => {
+          if (!row) throw new ValidationError('clientId not found in this port');
+        }),
+    );
+  }
+  if (fks.interestId) {
+    checks.push(
+      db.query.interests
+        .findFirst({
+          where: and(eq(interests.id, fks.interestId), eq(interests.portId, portId)),
+        })
+        .then((row) => {
+          if (!row) throw new ValidationError('interestId not found in this port');
+        }),
+    );
+  }
+  if (fks.berthId) {
+    checks.push(
+      db.query.berths
+        .findFirst({ where: and(eq(berths.id, fks.berthId), eq(berths.portId, portId)) })
+        .then((row) => {
+          if (!row) throw new ValidationError('berthId not found in this port');
+        }),
+    );
+  }
+  await Promise.all(checks);
+}
+
 // ─── CRUD ────────────────────────────────────────────────────────────────────
 
 export async function getReminder(id: string, portId: string) {
@@ -119,6 +164,12 @@ export async function getReminder(id: string, portId: string) {
 }
 
 export async function createReminder(portId: string, data: CreateReminderInput, meta: AuditMeta) {
+  await assertReminderFksInPort(portId, {
+    clientId: data.clientId,
+    interestId: data.interestId,
+    berthId: data.berthId,
+  });
+
   const [reminder] = await db
     .insert(reminders)
     .values({
@@ -186,6 +237,13 @@ export async function updateReminder(
   if (data.interestId !== undefined) updates.interestId = data.interestId;
   if (data.berthId !== undefined) updates.berthId = data.berthId;
 
+  // Re-validate any subject-FK changes against the caller's port.
+  await assertReminderFksInPort(portId, {
+    clientId: data.clientId,
+    interestId: data.interestId,
+    berthId: data.berthId,
+  });
+
   const [updated] = await db
     .update(reminders)
     .set(updates)
diff --git a/tests/unit/custom-field-validation.test.ts b/tests/unit/custom-field-validation.test.ts
index 0e81097..fd583db 100644
--- a/tests/unit/custom-field-validation.test.ts
+++ b/tests/unit/custom-field-validation.test.ts
@@ -12,6 +12,13 @@ vi.mock('@/lib/db', () => ({
   db: {
     query: {
       customFieldDefinitions: { findMany: vi.fn(), findFirst: vi.fn() },
+      // Entity-port-scope checks added by the security fix; default to a
+      // truthy row so existing assertions still focus on validation logic.
+      clients: { findFirst: vi.fn().mockResolvedValue({ id: 'entity-1', portId: 'port-1' }) },
+      interests: { findFirst: vi.fn().mockResolvedValue({ id: 'entity-1', portId: 'port-1' }) },
+      berths: { findFirst: vi.fn().mockResolvedValue({ id: 'entity-1', portId: 'port-1' }) },
+      yachts: { findFirst: vi.fn().mockResolvedValue({ id: 'entity-1', portId: 'port-1' }) },
+      companies: { findFirst: vi.fn().mockResolvedValue({ id: 'entity-1', portId: 'port-1' }) },
     },
     insert: vi.fn(),
     update: vi.fn(),
@@ -20,6 +27,12 @@ vi.mock('@/lib/db', () => ({
   },
 }));
 
+vi.mock('@/lib/db/schema/clients', () => ({ clients: {} }));
+vi.mock('@/lib/db/schema/interests', () => ({ interests: {} }));
+vi.mock('@/lib/db/schema/berths', () => ({ berths: {} }));
+vi.mock('@/lib/db/schema/yachts', () => ({ yachts: {} }));
+vi.mock('@/lib/db/schema/companies', () => ({ companies: {} }));
+
 vi.mock('@/lib/audit', () => ({
   createAuditLog: vi.fn().mockResolvedValue(undefined),
 }));
@@ -84,7 +97,11 @@ beforeEach(() => {
 });
 
 /** Convenience: call setValues with a single field/value pair. */
-async function validate(fieldType: string, value: unknown, extras?: { isRequired?: boolean; selectOptions?: string[] }) {
+async function validate(
+  fieldType: string,
+  value: unknown,
+  extras?: { isRequired?: boolean; selectOptions?: string[] },
+) {
   (db.query.customFieldDefinitions.findMany as ReturnType<typeof vi.fn>).mockResolvedValue([
     makeDefinition(fieldType, extras),
   ]);
@@ -182,7 +199,9 @@ describe('custom field validation — select', () => {
   });
 
   it('rejects an option not in the list', async () => {
-    await expect(validate('select', 'XL', { selectOptions: options })).rejects.toBeInstanceOf(ValidationError);
+    await expect(validate('select', 'XL', { selectOptions: options })).rejects.toBeInstanceOf(
+      ValidationError,
+    );
   });
 
   it('error message lists the valid options', async () => {
@@ -203,11 +222,15 @@ describe('custom field validation — select', () => {
 
 describe('custom field validation — required vs optional null', () => {
   it('required field: null value → throws ValidationError', async () => {
-    await expect(validate('text', null, { isRequired: true })).rejects.toBeInstanceOf(ValidationError);
+    await expect(validate('text', null, { isRequired: true })).rejects.toBeInstanceOf(
+      ValidationError,
+    );
   });
 
   it('required field: undefined value → throws ValidationError', async () => {
-    await expect(validate('text', undefined, { isRequired: true })).rejects.toBeInstanceOf(ValidationError);
+    await expect(validate('text', undefined, { isRequired: true })).rejects.toBeInstanceOf(
+      ValidationError,
+    );
   });
 
   it('non-required field: null value → succeeds (no error)', async () => {