diff --git a/src/users/users.controller.spec.ts b/src/users/users.controller.spec.ts
new file mode 100644
index 0000000..f7b1b7d
--- /dev/null
+++ b/src/users/users.controller.spec.ts
@@ -0,0 +1,63 @@
+import { ForbiddenException } from '@nestjs/common';
+import { Test, TestingModule } from '@nestjs/testing';
+import { UsersController } from './users.controller';
+import { UsersService } from './users.service';
+
+describe('UsersController', () => {
+  let controller: UsersController;
+  let usersService: jest.Mocked<Partial<UsersService>>;
+
+  beforeEach(async () => {
+    usersService = {
+      list: jest.fn(),
+      findById: jest.fn(),
+      setStellarAddress: jest.fn(),
+    };
+
+    const module: TestingModule = await Test.createTestingModule({
+      controllers: [UsersController],
+      providers: [{ provide: UsersService, useValue: usersService }],
+    }).compile();
+
+    controller = module.get<UsersController>(UsersController);
+  });
+
+  describe('setStellarAddress (#39 IDOR guard)', () => {
+    const validDto = {
+      stellarAddress: 'GBBD47IF6LWK7P7MDEVSCWR7DPUWV3NY3DTQEVFL4NAT4AQH3ZLLFLA5',
+    };
+
+    it('allows a user to update their own payout address', async () => {
+      const userId = '11111111-1111-1111-1111-111111111111';
+      const req = { user: { userId } } as any;
+      const expectedUser = { id: userId, stellarAddress: validDto.stellarAddress } as any;
+      usersService.setStellarAddress.mockResolvedValue(expectedUser);
+
+      const result = await controller.setStellarAddress(userId, validDto, req);
+
+      expect(result).toEqual(expectedUser);
+      expect(usersService.setStellarAddress).toHaveBeenCalledWith(userId, validDto.stellarAddress);
+    });
+
+    it('throws ForbiddenException when user A attempts to overwrite user B address', async () => {
+      const victimId = '11111111-1111-1111-1111-111111111111';
+      const attackerId = '22222222-2222-2222-2222-222222222222';
+      const req = { user: { userId: attackerId } } as any;
+
+      expect(() => controller.setStellarAddress(victimId, validDto, req)).toThrow(
+        ForbiddenException,
+      );
+      expect(usersService.setStellarAddress).not.toHaveBeenCalled();
+    });
+
+    it('throws ForbiddenException when req.user is missing or lacks userId', async () => {
+      const targetId = '11111111-1111-1111-1111-111111111111';
+      const req = {} as any;
+
+      expect(() => controller.setStellarAddress(targetId, validDto, req)).toThrow(
+        ForbiddenException,
+      );
+      expect(usersService.setStellarAddress).not.toHaveBeenCalled();
+    });
+  });
+});
diff --git a/src/users/users.controller.ts b/src/users/users.controller.ts
index 21559de..f4841cf 100644
--- a/src/users/users.controller.ts
+++ b/src/users/users.controller.ts
@@ -1,13 +1,16 @@
 import {
   Body,
   Controller,
+  ForbiddenException,
   Get,
   Param,
   ParseUUIDPipe,
   Patch,
+  Req,
   UseGuards,
 } from '@nestjs/common';
 import { ApiBearerAuth, ApiProperty, ApiTags } from '@nestjs/swagger';
+import type { Request } from 'express';
 import { UsersService } from './users.service';
 import { JwtAuthGuard } from '../auth/guards/jwt-auth.guard';
 import { IsStellarAddress } from '../common/validators/stellar-address.validator';
@@ -48,7 +51,12 @@ export class UsersController {
   setStellarAddress(
     @Param('id', new ParseUUIDPipe()) id: string,
     @Body() dto: SetStellarAddressDto,
+    @Req() req: Request,
   ) {
+    const callerId = (req.user as any)?.userId;
+    if (!callerId || callerId !== id) {
+      throw new ForbiddenException("Cannot modify another user's payout address");
+    }
     return this.usersService.setStellarAddress(id, dto.stellarAddress);
   }
 }