Secure Coding Guide for Web Applications

Overview

This guide provides 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.

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

   • Each user must only access/modify their own data
   • No user should access data from other users or organizations
   • Always verify ownership at the data layer, not just the route level

2. Use UUIDs Instead of Sequential IDs

   • Use UUIDv4 or similar non-guessable identifiers
   • Exception: Only use sequential IDs if explicitly requested by user

3. Account Lifecycle Handling

   • When a user is removed from an organization: immediately revoke all access tokens and sessions
   • When an account is deleted/deactivated: invalidate all active sessions and API keys
   • 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)

   • HTML context: HTML entity encode (< → <)
   • JavaScript context: JavaScript escape
   • URL context: URL encode
   • CSS context: CSS escape
   • Use framework's built-in escaping (React's JSX, Vue's {{ }}, etc.)

2. Content Security Policy (CSP)

   Copy

   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

3. Input Sanitization

   • Use established libraries (DOMPurify for HTML)
   • Whitelist allowed tags/attributes for rich text
   • Strip or encode dangerous patterns

4. Additional Headers

   • X-Content-Type-Options: nosniff
   • 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

   • Generate cryptographically random tokens
   • Tie token to user session
   • Validate on every state-changing request
   • Regenerate after login (prevent session fixation combo)

2. SameSite Cookies

   Copy

   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

3. Double Submit Cookie Pattern

   • Send CSRF token in both cookie and request body/header
   • 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 be scoped because subdomain takeover can lead to CSRF
• GET requests with side effects: Never perform state changes on GET
• Token leakage: Don't include CSRF tokens in URLs
• Token in URL vs Header: Prefer custom headers (X-CSRF-Token) over URL parameters

Verification Checklist

• Token is cryptographically random (use secure random generator)
• Token is tied to user session
• Token is validated server-side on all state-changing requests
• Missing token = rejected request
• Token regenerated on authentication state change
• SameSite cookie attribute is set
• Secure and HttpOnly flags on session cookies

Secret Keys and Sensitive Data Exposure

No secrets or sensitive information should be accessible to client-side code.

Never Expose in Client-Side Code

API Keys and Secrets:

• Third-party API keys (Stripe, AWS, etc.)
• Database connection strings
• JWT signing secrets
• Encryption keys
• OAuth client secrets
• Internal service URLs/credentials

Sensitive User Data:

• Full credit card numbers
• Social Security Numbers
• Passwords (even hashed)
• Security questions/answers
• Full phone numbers (mask them: --1234)
• Sensitive PII that isn't needed for display

Infrastructure Details:

• Internal IP addresses
• Database schemas
• Debug information
• Stack traces in production
• Server software versions

Where Secrets Hide (Check These!)

• JavaScript bundles (including source maps)
• HTML comments
• Hidden form fields
• Data attributes
• LocalStorage/SessionStorage
• Initial state/hydration data in SSR apps
• Environment variables exposed via build tools (NEXT_PUBLIC_, REACT_APP_)

Best Practices

1. Environment Variables: Store secrets in .env files
2. Server-Side Only: Make API calls requiring secrets from backend only

Open Redirect

Any endpoint accepting a URL for redirection must be protected against open redirect attacks.

Protection Strategies

1. Allowlist Validation

   Copy

   allowed_domains = ['yourdomain.com', 'app.yourdomain.com']
   
   function isValidRedirect(url):
       parsed = parseUrl(url)
       return parsed.hostname in allowed_domains
   

2. Relative URLs Only

   • Only accept paths (e.g., /dashboard) not full URLs
   • Validate the path starts with / and doesn't contain //

3. Indirect References

   • Use a mapping instead of raw URLs: ?redirect=dashboard → lookup to /dashboard

Bypass Techniques to Block

Password Security

Password Requirements

• Minimum 8 characters (12+ recommended)
• No maximum length (or very high, e.g., 128 chars)
• Allow all characters including special chars
• Don't require specific character types (let users choose strong passwords)

Storage

• Use Argon2id, bcrypt, or scrypt
• Never MD5, SHA1, or plain SHA256

Server-Side Bugs

Server-Side Request Forgery (SSRF)

Any functionality where the server makes requests to URLs provided or influenced by users must be protected.

Potential Vulnerable Features

• Webhooks (user provides callback URL)
• URL previews
• PDF generators from URLs
• Image/file fetching from URLs
• Import from URL features
• RSS/feed readers
• API integrations with user-provided endpoints
• Proxy functionality

Protection Strategies

1. Allowlist Approach (Preferred)

   • Only allow requests to pre-approved domains
   • Maintain a strict allowlist for integrations

2. Network Segmentation

   • Run URL-fetching services in isolated network
   • Block access to internal network, cloud metadata

Cloud Metadata Protection

Block access to cloud metadata endpoints:

• AWS: 169.254.169.254
• GCP: metadata.google.internal, 169.254.169.254
• Azure: 169.254.169.254
• DigitalOcean: 169.254.169.254

Insecure File Upload

File uploads must validate type, content, and size to prevent various attacks.

Validation Requirements

1. File Type Validation - Check extension against allowlist, validate magic bytes
2. File Content Validation - Read and verify magic bytes, process with image library for images
3. File Size Limits - Set maximum file size server-side, configure web server limits

Secure Upload Handling

1. Rename files: Use random UUID names, discard original
2. Store outside webroot: Or use separate domain for uploads
3. Serve with correct headers: Content-Disposition: attachment, X-Content-Type-Options: nosniff
4. Use CDN/separate domain: Isolate uploaded content from main app
5. Set restrictive permissions: Uploaded files should not be executable

SQL Injection

Prevention: Parameterized Queries (Prepared Statements) — PRIMARY DEFENSE

-- VULNERABLE
query = "SELECT * FROM users WHERE id = " + userId

-- SECURE
query = "SELECT * FROM users WHERE id = ?"
execute(query, [userId])

Additional: Use ORM methods, validate data types, whitelist for ORDER BY and table/column names.

Path Traversal

Never use user input directly in file paths. Canonicalize paths and validate against base directory.

import os

def safe_join(base_directory, user_path):
    base = os.path.abspath(os.path.realpath(base_directory))
    target = os.path.abspath(os.path.realpath(os.path.join(base, user_path)))
    if os.path.commonpath([base, target]) != base:
        raise ValueError("Error!")
    return target

Security Headers Checklist

Include these headers in all responses:

Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
Content-Security-Policy: [see XSS section]
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
Referrer-Policy: strict-origin-when-cross-origin
Cache-Control: no-store (for sensitive pages)

General Security Principles

When generating code, always:

1. Validate all input server-side — Never trust client-side validation alone
2. Use parameterized queries — Never concatenate user input into queries
3. Encode output contextually — HTML, JS, URL, CSS contexts need different encoding
4. Apply authentication checks — On every endpoint, not just at routing
5. Apply authorization checks — Verify the user can access the specific resource
6. Use secure defaults
7. Handle errors securely — Don't leak stack traces or internal details to users
8. Keep dependencies updated — Use tools to track vulnerable dependencies

When unsure, choose the more restrictive/secure option and document the security consideration in comments.