agent-skills/skills/nestjs-best-practices/rules/security-validate-all-input.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Validate All Input with DTOs and Pipes	HIGH	First line of defense against attacks	security, validation, dto, pipes

Validate All Input with DTOs and Pipes

Always validate incoming data using class-validator decorators on DTOs and the global ValidationPipe. Never trust user input. Validate all request bodies, query parameters, and route parameters before processing.

Incorrect (trust raw input without validation):

// Trust raw input without validation
@Controller('users')
export class UsersController {
  @Post()
  create(@Body() body: any) {
    // body could contain anything - SQL injection, XSS, etc.
    return this.usersService.create(body);
  }

  @Get()
  findAll(@Query() query: any) {
    // query.limit could be "'; DROP TABLE users; --"
    return this.usersService.findAll(query.limit);
  }
}

// DTOs without validation decorators
export class CreateUserDto {
  name: string;    // No validation
  email: string;   // Could be "not-an-email"
  age: number;     // Could be "abc" or -999
}

Correct (validated DTOs with global ValidationPipe):

// Enable ValidationPipe globally in main.ts
async function bootstrap() {
  const app = await NestFactory.create(AppModule);

  app.useGlobalPipes(
    new ValidationPipe({
      whitelist: true,              // Strip unknown properties
      forbidNonWhitelisted: true,   // Throw on unknown properties
      transform: true,              // Auto-transform to DTO types
      transformOptions: {
        enableImplicitConversion: true,
      },
    }),
  );

  await app.listen(3000);
}

// Create well-validated DTOs
import {
  IsString,
  IsEmail,
  IsInt,
  Min,
  Max,
  IsOptional,
  MinLength,
  MaxLength,
  Matches,
  IsNotEmpty,
} from 'class-validator';
import { Transform, Type } from 'class-transformer';

export class CreateUserDto {
  @IsString()
  @IsNotEmpty()
  @MinLength(2)
  @MaxLength(100)
  @Transform(({ value }) => value?.trim())
  name: string;

  @IsEmail()
  @Transform(({ value }) => value?.toLowerCase().trim())
  email: string;

  @IsInt()
  @Min(0)
  @Max(150)
  age: number;

  @IsString()
  @MinLength(8)
  @MaxLength(100)
  @Matches(/^(?=.*[a-z])(?=.*[A-Z])(?=.*\d)/, {
    message: 'Password must contain uppercase, lowercase, and number',
  })
  password: string;
}

// Query DTO with defaults and transformation
export class FindUsersQueryDto {
  @IsOptional()
  @IsString()
  @MaxLength(100)
  search?: string;

  @IsOptional()
  @Type(() => Number)
  @IsInt()
  @Min(1)
  @Max(100)
  limit: number = 20;

  @IsOptional()
  @Type(() => Number)
  @IsInt()
  @Min(0)
  offset: number = 0;
}

// Param validation
export class UserIdParamDto {
  @IsUUID('4')
  id: string;
}

@Controller('users')
export class UsersController {
  @Post()
  create(@Body() dto: CreateUserDto): Promise<User> {
    // dto is guaranteed to be valid
    return this.usersService.create(dto);
  }

  @Get()
  findAll(@Query() query: FindUsersQueryDto): Promise<User[]> {
    // query.limit is a number, query.search is sanitized
    return this.usersService.findAll(query);
  }

  @Get(':id')
  findOne(@Param() params: UserIdParamDto): Promise<User> {
    // params.id is a valid UUID
    return this.usersService.findById(params.id);
  }
}

Reference: NestJS Validation