Broken Access Control in Nestjs

Broken Access Control in Nestjs applications often stems from improper implementation of guards, missing role checks, or inconsistent authorization logic across different endpoints. The modular nature of Nestjs, while powerful, can create scenarios where developers inadvertently expose sensitive functionality.

One common pattern is the misuse of @UseGuards decorators. Developers frequently apply guards at the controller level but forget to secure individual methods that handle sensitive operations. For example, a controller might have a RolesGuard protecting most endpoints, but a delete operation without proper authorization slips through:

@Controller('users')
@UseGuards(RolesGuard)
export class UsersController {
  @Get()
  @Roles('admin')
  findAll() { /* ... */ }

  @Delete(':id')
  remove(@Param('id') id: string) { /* missing @Roles decorator */ }
}

In this scenario, the remove method would be accessible to any authenticated user, not just admins, because the guard applies to the controller but the specific role requirement is missing.

Another Nestjs-specific manifestation occurs with dynamic routing and parameter-based authorization. When authorization decisions depend on route parameters, developers must implement custom logic in guards or interceptors. A common mistake is to check only the authenticated user's ID without validating ownership or permissions:

@Controller('users/:userId/posts')
export class PostsController {
  @Get()
  findAll(@Param('userId') userId: string) {
    // No check that userId matches req.user.id
    return this.postsService.findAllByUser(userId);
  }
}

This allows any authenticated user to view posts from any other user by simply changing the userId parameter—a classic IDOR (Insecure Direct Object Reference) vulnerability.

Nestjs's dependency injection can also introduce subtle access control issues. When services are shared across different parts of the application, authorization logic might be bypassed if developers directly inject and use services without proper guards:

@Injectable()
export class UserService {
  async delete(id: string) {
    // No authorization check here
    return this.userRepository.delete(id);
  }
}

// Controller that bypasses guards
@Get('bypass-delete/:id')
async bypassDelete(@Param('id') id: string) {
  return this.userService.delete(id); // No guard applied
}

Finally, Nestjs applications often integrate with external services or microservices where authentication tokens aren't properly propagated, leading to privilege escalation across service boundaries. A user authenticated in one service might gain elevated privileges in another if JWT tokens or session data aren't correctly validated.

Detecting Broken Access Control in Nestjs applications requires both static analysis of the codebase and dynamic testing of the running application. middleBrick's API security scanner is particularly effective at identifying these vulnerabilities in Nestjs applications.

middleBrick performs black-box scanning that tests the unauthenticated attack surface of your Nestjs API. It automatically sends requests with modified parameters to detect IDOR vulnerabilities, attempts privilege escalation by changing user IDs in URLs, and tests for missing authorization controls. The scanner runs 12 parallel security checks including Authentication bypass, BOLA/IDOR, and BFLA/Privilege Escalation—all of which are relevant to Nestjs applications.

For Nestjs-specific detection, middleBrick analyzes the OpenAPI/Swagger specification generated by your application. It cross-references the documented endpoints with actual runtime behavior, identifying discrepancies between what's documented and what's actually protected. The scanner can detect when @UseGuards decorators are missing from critical endpoints or when role-based access controls aren't properly implemented.

middleBrick's LLM/AI Security checks are also relevant for modern Nestjs applications that use AI features. The scanner tests for system prompt leakage, prompt injection vulnerabilities, and excessive agency in LLM endpoints—vulnerabilities that can lead to broken access control in AI-powered Nestjs applications.

Beyond automated scanning, developers should implement runtime monitoring in their Nestjs applications. Using Nestjs's built-in interceptors and guards, you can log authorization failures and suspicious access patterns. The AuthGuard and custom guards can be instrumented to track which endpoints are being accessed without proper authorization.

middleBrick's GitHub Action integration allows you to add API security checks to your CI/CD pipeline. For Nestjs applications, this means you can automatically scan your staging API before deployment and fail the build if Broken Access Control vulnerabilities are detected. The Pro plan's continuous monitoring feature can scan your production Nestjs API on a configurable schedule, alerting you to new vulnerabilities as they emerge.

The CLI tool (npm install -g middlebrick) provides another detection avenue, allowing you to scan your Nestjs API from the terminal during development. This is particularly useful for catching access control issues before they reach production.

Remediating Broken Access Control in Nestjs applications requires a systematic approach using the framework's built-in security features. The most effective strategy combines proper guard implementation, consistent authorization patterns, and comprehensive testing.

Start by implementing a robust RolesGuard that checks both authentication and authorization:

import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common';
import { Reflector } from '@nestjs/core';

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private reflector: Reflector) {}

  canActivate(context: ExecutionContext): boolean {
    const requiredRoles = this.reflector.get<string[]>('roles', context.getHandler());
    if (!requiredRoles) {
      return true;
    }

    const { user } = context.switchToHttp().getRequest();
    return user && user.roles.some(role => requiredRoles.includes(role));
  }
}

// Apply globally
@Module({
  providers: [RolesGuard],
  exports: [RolesGuard]
})
export class AuthModule {}

Always use the @Roles decorator consistently across all sensitive endpoints:

@Controller('users')
@UseGuards(RolesGuard)
export class UsersController {
  @Get()
  @Roles('admin')
  findAll() { /* admin-only access */ }

  @Get(':id')
  @Roles('user', 'admin')
  findOne(@Param('id') id: string, @Request() req) {
    // Check if user is accessing their own profile or has admin role
    if (req.user.id !== id && !req.user.roles.includes('admin')) {
      throw new UnauthorizedException();
    }
    return this.userService.findOne(id);
  }

  @Delete(':id')
  @Roles('admin')
  remove(@Param('id') id: string) {
    return this.userService.delete(id);
  }
}

For parameter-based authorization, create a custom guard that validates ownership:

@Injectable()
export class OwnershipGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const { user } = context.switchToHttp().getRequest();
    const params = context.switchToHttp().getRequest().params;
    
    // Example: check if user owns the resource
    return user.id === params.userId || user.roles.includes('admin');
  }
}

// Apply to specific routes
@Get(':userId/posts')
@UseGuards(OwnershipGuard)
findAllByUser(@Param('userId') userId: string) {
  return this.postsService.findAllByUser(userId);
}

Implement consistent error handling to avoid information leakage:

@Catch(UnauthorizedException)
export class UnauthorizedExceptionFilter implements ExceptionFilter {
  catch(exception: UnauthorizedException, host: ExecutionContext) {
    const ctx = host.switchToHttp();
    const response = ctx.getResponse();
    response.status(403).json({
      statusCode: 403,
      message: 'Access denied'
    });
  }
}

// Apply globally
app.useGlobalFilters(new UnauthorizedExceptionFilter());

For microservice communication in Nestjs, ensure JWT tokens are properly validated across service boundaries:

@Injectable()
export class JwtStrategy extends PassportStrategy(Strategy) {
  validate(payload: JwtPayload) {
    // Validate token and extract user
    return { userId: payload.sub, email: payload.email, roles: payload.roles };
  }
}

// Apply to all microservice messages
app.connectMicroservice({
  strategy: new GrpcOptionsFactory().createClientOptions(),
  context: (metadata: Metadata) => {
    const token = metadata.get('authorization')[0];
    // Validate token here
    return { token };
  }
});

Finally, integrate middleBrick's continuous monitoring into your security strategy. The Pro plan's scheduled scans can catch new Broken Access Control vulnerabilities as your Nestjs application evolves, while the GitHub Action integration ensures security testing is part of your development workflow.