Secure Coding

Secure Coding

Comprehensive guidance for writing secure code, covering OWASP Top 10 2025, CWE Top 25, and language-specific security patterns.

When to Use This Skill

Use this skill when:

• Reviewing code for security vulnerabilities
• Implementing input validation or output encoding
• Learning about common security weaknesses (OWASP, CWE)
• Fixing identified security issues
• Writing security-sensitive code (authentication, authorization, data handling)
• Conducting security code reviews

OWASP Top 10 2025 Quick Reference

For detailed mitigations: See OWASP Top 10 2025 Reference

Core Secure Coding Principles

1. Input Validation

Never trust user input. Validate all inputs on the server side.

using System.Text.RegularExpressions;

// Good: Server-side validation with allowlist
public static partial class InputValidation
{
    [GeneratedRegex(@"^[a-zA-Z0-9_]{3,20}$")]
    private static partial Regex UsernamePattern();

    /// <summary>
    /// Validate username against allowlist pattern.
    /// </summary>
    public static bool ValidateUsername(string username) =>
        !string.IsNullOrEmpty(username) && UsernamePattern().IsMatch(username);
}

// Bad: No validation
public string ProcessUsername(string username) => username;  // Dangerous!

Validation strategies:

• Allowlist validation: Define what IS allowed (preferred)
• Blocklist validation: Define what is NOT allowed (less secure)
• Type checking: Ensure correct data types
• Range checking: Verify values within expected bounds
• Length limits: Prevent buffer overflows and DoS

2. Output Encoding

Encode output based on context to prevent injection attacks.

3. Parameterized Queries (Injection Prevention)

Always use parameterized queries for database operations.

// Good: Parameterized query with SqlCommand
using var cmd = new SqlCommand(
    "SELECT * FROM Users WHERE Username = @username AND Status = @status",
    connection);
cmd.Parameters.AddWithValue("@username", username);
cmd.Parameters.AddWithValue("@status", status);

// Good: Parameterized query with Dapper
var users = await connection.QueryAsync<User>(
    "SELECT * FROM Users WHERE Username = @Username AND Status = @Status",
    new { Username = username, Status = status });

// Bad: String interpolation (SQL Injection vulnerable)
var query = $"SELECT * FROM Users WHERE Username = '{username}'";  // VULNERABLE

4. Authentication Security

• Use strong password hashing: Argon2id, bcrypt, scrypt (see cryptography skill)
• Implement MFA: Time-based OTP, hardware keys, passkeys
• Secure session management: HttpOnly cookies, secure flag, short expiration
• Account lockout: Prevent brute force attacks
• Credential storage: Never store plaintext passwords

5. Authorization Security

• Deny by default: Require explicit permission grants
• Server-side checks: Never rely on client-side authorization
• Verify object ownership: Check user can access requested resource
• Use indirect references: Map internal IDs to user-specific references
• Implement RBAC/ABAC: Use structured access control models

6. Error Handling

// Good: Generic error message to user, detailed logging
public async Task<IActionResult> ProcessData([FromBody] DataRequest request)
{
    try
    {
        await _dataService.ProcessSensitiveDataAsync(request.Data);
        return Ok();
    }
    catch (DbException ex)
    {
        _logger.LogError(ex, "Database error processing request for user {UserId}", User.GetUserId());
        return StatusCode(500, new { error = "An error occurred" });
    }
}

// Bad: Exposing internal details
catch (DbException ex)
{
    return StatusCode(500, new { error = ex.Message });  // VULNERABLE - exposes internals
}

Error handling rules:

• Return generic error messages to users
• Log detailed errors server-side
• Never expose stack traces, database errors, or internal paths
• Fail securely (deny access on error)

Language-Specific Patterns

JavaScript/TypeScript

// XSS Prevention - use textContent, not innerHTML
element.textContent = userInput;  // Safe
element.innerHTML = userInput;    // VULNERABLE

// Use DOMPurify for HTML that must be rendered
import DOMPurify from 'dompurify';
element.innerHTML = DOMPurify.sanitize(userInput);

// Avoid eval() and Function()
eval(userInput);           // VULNERABLE
new Function(userInput)(); // VULNERABLE

// Use strict mode
'use strict';

C# / .NET

// Safe process execution - use argument list, avoid shell
using System.Diagnostics;

public static async Task<string> SafeExecuteAsync(string command, params string[] args)
{
    using var process = new Process
    {
        StartInfo = new ProcessStartInfo
        {
            FileName = command,
            RedirectStandardOutput = true,
            RedirectStandardError = true,
            UseShellExecute = false,  // Safe: no shell interpretation
            CreateNoWindow = true
        }
    };

    foreach (var arg in args)
        process.StartInfo.ArgumentList.Add(arg);  // Safe: no shell escaping needed

    process.Start();
    var output = await process.StandardOutput.ReadToEndAsync();
    await process.WaitForExitAsync();
    return output;
}

// Bad: Shell injection vulnerable
Process.Start("cmd", $"/c dir {userInput}");  // VULNERABLE

// Safe file operations - prevent path traversal
public static class SafeFileAccess
{
    /// <summary>
    /// Safely read a file, preventing path traversal attacks.
    /// </summary>
    public static string SafeReadFile(string baseDir, string filename)
    {
        var basePath = Path.GetFullPath(baseDir);
        var filePath = Path.GetFullPath(Path.Combine(baseDir, filename));

        if (!filePath.StartsWith(basePath, StringComparison.OrdinalIgnoreCase))
            throw new UnauthorizedAccessException("Path traversal detected");

        return File.ReadAllText(filePath);
    }
}

// Use parameterized queries with Entity Framework
var users = context.Users
    .Where(u => u.Username == username)  // Safe - parameterized
    .ToList();

// Avoid raw SQL when possible, use parameters if needed
var users = context.Users
    .FromSqlRaw("SELECT * FROM Users WHERE Username = {0}", username)
    .ToList();

// Anti-forgery tokens for CSRF protection
[ValidateAntiForgeryToken]
public IActionResult UpdateProfile(ProfileModel model)
{
    // Process update
}

// Input validation with data annotations
public sealed class UserInput
{
    [Required]
    [StringLength(100, MinimumLength = 3)]
    [RegularExpression(@"^[a-zA-Z0-9_]+$")]
    public required string Username { get; init; }
}

Security Code Review Checklist

Input Handling

• All inputs validated on server side
• Allowlist validation used where possible
• Length limits enforced
• Type checking performed
• File uploads validated (type, size, content)

Output Encoding

• Context-appropriate encoding applied
• No raw user input in HTML/JS/SQL
• Content-Type headers set correctly
• X-Content-Type-Options: nosniff

Authentication

• Strong password hashing (Argon2id/bcrypt)
• Session tokens are random and unpredictable
• Session invalidation on logout
• Account lockout after failed attempts
• Credentials transmitted over HTTPS only

Authorization

• Access control on every request
• Deny by default policy
• Object-level authorization checks
• No direct object references exposed

Data Protection

• Sensitive data encrypted at rest
• TLS 1.2+ for data in transit
• No sensitive data in URLs or logs
• Proper key management

Error Handling

• Generic error messages to users
• Detailed errors logged securely
• No stack traces exposed
• Fail securely (deny on error)

Quick Decision Tree

What security concern are you addressing?

1. SQL/NoSQL injection → Use parameterized queries, ORMs
2. XSS (Cross-Site Scripting) → Context-aware output encoding, CSP
3. CSRF → Anti-forgery tokens, SameSite cookies
4. Authentication → See authentication-patterns skill
5. Authorization → See authorization-models skill
6. Cryptography → See cryptography skill
7. Secrets/Credentials → See secrets-management skill
8. API Security → See api-security skill

References

• OWASP Top 10 2025 Detailed Reference - Complete mitigations and examples
• CWE Top 25 Reference - Most dangerous software weaknesses
• Language-Specific Patterns - Per-language security guides

Related Skills

Version History

• v1.0.0 (2025-12-26): Initial release with OWASP Top 10 2025, core principles

Last Updated: 2025-12-26

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