260223:1415 20260223 nextJS & nestJS Best pratices

2026-02-23 14:15:06 +07:00
parent c90a664f53
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--- a/.agents/skills/nestjs-best-practices/rules/security-sanitize-output.md
+++ b/.agents/skills/nestjs-best-practices/rules/security-sanitize-output.md
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+---
+title: Sanitize Output to Prevent XSS
+impact: HIGH
+impactDescription: XSS vulnerabilities can compromise user sessions and data
+tags: security, xss, sanitization, html
+---
+
+## Sanitize Output to Prevent XSS
+
+While NestJS APIs typically return JSON (which browsers don't execute), XSS risks exist when rendering HTML, storing user content, or when frontend frameworks improperly handle API responses. Sanitize user-generated content before storage and use proper Content-Type headers.
+
+**Incorrect (storing raw HTML without sanitization):**
+
+```typescript
+// Store raw HTML from users
+@Injectable()
+export class CommentsService {
+  async create(dto: CreateCommentDto): Promise<Comment> {
+    // User can inject: <script>steal(document.cookie)</script>
+    return this.repo.save({
+      content: dto.content, // Raw, unsanitized
+      authorId: dto.authorId,
+    });
+  }
+}
+
+// Return HTML without sanitization
+@Controller('pages')
+export class PagesController {
+  @Get(':slug')
+  @Header('Content-Type', 'text/html')
+  async getPage(@Param('slug') slug: string): Promise<string> {
+    const page = await this.pagesService.findBySlug(slug);
+    // If page.content contains user input, XSS is possible
+    return `<html><body>${page.content}</body></html>`;
+  }
+}
+
+// Reflect user input in errors
+@Get(':id')
+async findOne(@Param('id') id: string): Promise<User> {
+  const user = await this.repo.findOne({ where: { id } });
+  if (!user) {
+    // XSS if id contains malicious content and error is rendered
+    throw new NotFoundException(`User ${id} not found`);
+  }
+  return user;
+}
+```
+
+**Correct (sanitize content and use proper headers):**
+
+```typescript
+// Sanitize HTML content before storage
+import * as sanitizeHtml from 'sanitize-html';
+
+@Injectable()
+export class CommentsService {
+  private readonly sanitizeOptions: sanitizeHtml.IOptions = {
+    allowedTags: ['b', 'i', 'em', 'strong', 'a', 'p', 'br'],
+    allowedAttributes: {
+      a: ['href', 'title'],
+    },
+    allowedSchemes: ['http', 'https', 'mailto'],
+  };
+
+  async create(dto: CreateCommentDto): Promise<Comment> {
+    return this.repo.save({
+      content: sanitizeHtml(dto.content, this.sanitizeOptions),
+      authorId: dto.authorId,
+    });
+  }
+}
+
+// Use validation pipe to strip HTML
+import { Transform } from 'class-transformer';
+
+export class CreatePostDto {
+  @IsString()
+  @MaxLength(1000)
+  @Transform(({ value }) => sanitizeHtml(value, { allowedTags: [] }))
+  title: string;
+
+  @IsString()
+  @Transform(({ value }) =>
+    sanitizeHtml(value, {
+      allowedTags: ['p', 'br', 'b', 'i', 'a'],
+      allowedAttributes: { a: ['href'] },
+    }),
+  )
+  content: string;
+}
+
+// Set proper Content-Type headers
+@Controller('api')
+export class ApiController {
+  @Get('data')
+  @Header('Content-Type', 'application/json')
+  async getData(): Promise<DataResponse> {
+    // JSON response - browser won't execute scripts
+    return this.service.getData();
+  }
+}
+
+// Sanitize error messages
+@Get(':id')
+async findOne(@Param('id', ParseUUIDPipe) id: string): Promise<User> {
+  const user = await this.repo.findOne({ where: { id } });
+  if (!user) {
+    // UUID validation ensures safe format
+    throw new NotFoundException('User not found');
+  }
+  return user;
+}
+
+// Use Helmet for CSP headers
+import helmet from 'helmet';
+
+async function bootstrap() {
+  const app = await NestFactory.create(AppModule);
+
+  app.use(
+    helmet({
+      contentSecurityPolicy: {
+        directives: {
+          defaultSrc: ["'self'"],
+          scriptSrc: ["'self'"],
+          styleSrc: ["'self'", "'unsafe-inline'"],
+          imgSrc: ["'self'", 'data:', 'https:'],
+        },
+      },
+    }),
+  );
+
+  await app.listen(3000);
+}
+```
+
+Reference: [OWASP XSS Prevention](https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html)