vibe-security

Secure Coding Guide for Web Applications

Comprehensive secure coding practices for web applications. Approach code from a bug hunter's perspective and make applications as secure as possible without breaking functionality.

When to Use This Skill

• Writing new web application endpoints or API routes
• Reviewing PRs that handle user input, authentication, or file uploads
• Implementing authentication, authorization, or session management
• Working with file uploads, redirects, or URL-based features
• Adding security headers or CSP policies
• Avoid using for infrastructure/network security — use defense-in-depth instead

Workflow

Step 1: Identify Attack Surface

Determine which security domains apply to the code under review:

| Domain | Trigger | |--------|---------| | Access Control | Any authenticated endpoint, multi-tenant data | | XSS | User input rendered in HTML, JavaScript, or CSS | | CSRF | State-changing endpoints (POST, PUT, DELETE) | | SSRF | Server makes requests to user-provided URLs | | SQL Injection | Dynamic database queries | | File Upload | Any file upload functionality | | Path Traversal | User input in file paths |

Step 2: Apply Domain-Specific Checks

Use the relevant sections below as checklists for each identified domain.

Step 3: Verify Security Headers

Ensure all responses include the required headers (see Security Headers Checklist).

Step 4: Review and Test

• Verify fixes don't break functionality
• Test with bypass techniques listed in each section
• Run automated security scanning if available

Key Principles:

• Defense in depth: Never rely on a single security control
• Fail securely: When something fails, fail closed (deny access)
• Least privilege: Grant minimum permissions necessary
• Input validation: Never trust user input, validate everything server-side
• Output encoding: Encode data appropriately for the context it's rendered in

Access Control Issues

Access control vulnerabilities occur when users can access resources or perform actions beyond their intended permissions.

Core Requirements

For every data point and action that requires authentication:

1. User-Level Authorization
2. Each user must only access/modify their own data
3. No user should access data from other users or organizations
4. Always verify ownership at the data layer, not just the route level

1. Use UUIDs Instead of Sequential IDs
2. Use UUIDv4 or similar non-guessable identifiers
3. Exception: Only use sequential IDs if explicitly requested by user

1. Account Lifecycle Handling
2. When a user is removed from an organization: immediately revoke all access tokens and sessions
3. When an account is deleted/deactivated: invalidate all active sessions and API keys
4. Implement token revocation lists or short-lived tokens with refresh mechanisms

Authorization Checks Checklist

• [ ] Verify user owns the resource on every request (don't trust client-side data)
• [ ] Check organization membership for multi-tenant apps
• [ ] Validate role permissions for role-based actions
• [ ] Re-validate permissions after any privilege change
• [ ] Check parent resource ownership (e.g., if accessing a comment, verify user owns the parent post)

Common Pitfalls to Avoid

• IDOR (Insecure Direct Object Reference): Always verify the requesting user has permission to access the requested resource ID
• Privilege Escalation: Validate role changes server-side; never trust role info from client
• Horizontal Access: User A accessing User B's resources with the same privilege level
• Vertical Access: Regular user accessing admin functionality
• Mass Assignment: Filter which fields users can update; don't blindly accept all request body fields

Implementation Pattern

# Pseudocode for secure resource access
function getResource(resourceId, currentUser):
    resource = database.find(resourceId)
    
    if resource is null:
        return 404  # Don't reveal if resource exists
    
    if resource.ownerId != currentUser.id:
        if not currentUser.hasOrgAccess(resource.orgId):
            return 404  # Return 404, not 403, to prevent enumeration
    
    return resource

Client-Side Bugs

Cross-Site Scripting (XSS)

Every input controllable by the user—whether directly or indirectly—must be sanitized against XSS.

Input Sources to Protect

Direct Inputs:

• Form fields (email, name, bio, comments, etc.)
• Search queries
• File names during upload
• Rich text editors / WYSIWYG content

Indirect Inputs:

• URL parameters and query strings
• URL fragments (hash values)
• HTTP headers used in the application (Referer, User-Agent if displayed)
• Data from third-party APIs displayed to users
• WebSocket messages
• postMessage data from iframes
• LocalStorage/SessionStorage values if rendered

Often Overlooked:

• Error messages that reflect user input
• PDF/document generators that accept HTML
• Email templates with user data
• Log viewers in admin panels
• JSON responses rendered as HTML
• SVG file uploads (can contain JavaScript)
• Markdown rendering (if allowing HTML)

Protection Strategies

1. Output Encoding (Context-Specific)
2. HTML context: HTML entity encode (< → <)
3. JavaScript context: JavaScript escape
4. URL context: URL encode
5. CSS context: CSS escape
6. Use framework's built-in escaping (React's JSX, Vue's {{ }}, etc.)

1. Content Security Policy (CSP)

`` Content-Security-Policy: default-src 'self'; script-src 'self'; style-src 'self' 'unsafe-inline'; img-src 'self' data: https:; font-src 'self'; connect-src 'self' https://api.yourdomain.com; frame-ancestors 'none'; base-uri 'self'; form-action 'self'; ``

• Avoid 'unsafe-inline' and 'unsafe-eval' for scripts
• Use nonces or hashes for inline scripts when necessary
• Report violations: report-uri /csp-report

1. Input Sanitization
2. Use established libraries (DOMPurify for HTML)
3. Whitelist allowed tags/attributes for rich text
4. Strip or encode dangerous patterns

1. Additional Headers
2. X-Content-Type-Options: nosniff
3. X-Frame-Options: DENY (or use CSP frame-ancestors)

Cross-Site Request Forgery (CSRF)

Every state-changing endpoint must be protected against CSRF attacks.

Endpoints Requiring CSRF Protection

Authenticated Actions:

• All POST, PUT, PATCH, DELETE requests
• Any GET request that changes state (fix these to use proper HTTP methods)
• File uploads
• Settings changes
• Payment/transaction endpoints

Pre-Authentication Actions:

• Login endpoints (prevent login CSRF)
• Signup endpoints
• Password reset request endpoints
• Password change endpoints
• Email/phone verification endpoints
• OAuth callback endpoints

Protection Mechanisms

1. CSRF Tokens
2. Generate cryptographically random tokens
3. Tie token to user session
4. Validate on every state-changing request
5. Regenerate after login (prevent session fixation combo)

1. SameSite Cookies

`` Set-Cookie: session=abc123; SameSite=Strict; Secure; HttpOnly ``

• Strict: Cookie never sent cross-site (best security)
• Lax: Cookie sent on top-level navigations (good balance)
• Always combine with CSRF tokens for defense in depth

1. Double Submit Cookie Pattern
2. Send CSRF token in both cookie and request body/header
3. Server validates they match

Edge Cases and Common Mistakes

• Token presence check: CSRF validation must NOT depend on whether the token is present, always require it
• Token per form: Consider unique tokens per form for sensitive operations
• JSON APIs: Don't assume JSON content-type prevents CSRF; validate Origin/Referer headers AND use tokens
• CORS misconfiguration: Overly permissive CORS can bypass SameSite cookies
• Subdomains: CSRF tokens should b