Dotnet Secrets Management

dotnet-secrets-management

Cloud-agnostic secrets management for .NET applications. Covers the full lifecycle: user secrets for local development, environment variables for production, IConfiguration binding patterns, secret rotation, and managed identity as a production best practice. Includes anti-patterns to avoid (secrets in source, appsettings.json, hardcoded connection strings).

Out of scope: Cloud-provider-specific vault services (Azure Key Vault, AWS Secrets Manager, GCP Secret Manager) -- those are covered by cloud-specific epics. Authentication/authorization implementation (OAuth, Identity) -- see [skill:dotnet-api-security] and [skill:dotnet-blazor-auth]. Cryptographic algorithm selection -- see [skill:dotnet-cryptography]. General Options pattern and configuration sources -- see [skill:dotnet-csharp-configuration].

Cross-references: [skill:dotnet-security-owasp] for OWASP A02 (Cryptographic Failures) and deprecated pattern warnings, [skill:dotnet-csharp-configuration] for Options pattern and configuration source precedence.

Secrets Lifecycle

Principle: Secrets must never exist in the source repository or in any file committed to version control. Each environment tier uses the appropriate mechanism for its trust boundary.

User Secrets (Local Development)

User secrets store sensitive configuration outside the project directory in the user profile, preventing accidental commits.

Setup

# Initialize user secrets for a project (creates UserSecretsId in csproj)
dotnet user-secrets init

# Set individual secrets
dotnet user-secrets set "ConnectionStrings:DefaultDb" "Server=localhost;Database=myapp;User=sa;Password=dev123"
dotnet user-secrets set "Smtp:ApiKey" "SG.dev-key-here"
dotnet user-secrets set "Jwt:SigningKey" "dev-signing-key-min-32-chars-long!!"

# List current secrets
dotnet user-secrets list

# Remove a secret
dotnet user-secrets remove "Smtp:ApiKey"

# Clear all secrets
dotnet user-secrets clear

How It Works

User secrets are stored at:

• Windows: %APPDATA%\Microsoft\UserSecrets\<UserSecretsId>\secrets.json
• macOS/Linux: ~/.microsoft/usersecrets/<UserSecretsId>/secrets.json

The secrets.json file is plain JSON with the same structure as appsettings.json:

{
  "ConnectionStrings": {
    "DefaultDb": "Server=localhost;Database=myapp;User=sa;Password=dev123"
  },
  "Smtp": {
    "ApiKey": "SG.dev-key-here"
  },
  "Jwt": {
    "SigningKey": "dev-signing-key-min-32-chars-long!!"
  }
}

Loading in Code

User secrets are loaded automatically by WebApplication.CreateBuilder and Host.CreateDefaultBuilder when DOTNET_ENVIRONMENT or ASPNETCORE_ENVIRONMENT is Development:

var builder = WebApplication.CreateBuilder(args);

// User secrets are already loaded. Access them via IConfiguration:
var connectionString = builder.Configuration.GetConnectionString("DefaultDb");

For non-web hosts (console apps, worker services):

var builder = Host.CreateApplicationBuilder(args);
// User secrets are loaded automatically in Development environment.
// For explicit control:
if (builder.Environment.IsDevelopment())
{
    builder.Configuration.AddUserSecrets<Program>();
}

Gotcha: User secrets are not encrypted -- they are just stored outside the repo. They are appropriate for development only, never for production.

Environment Variables (Production)

Environment variables are the standard mechanism for injecting secrets into production applications without touching the filesystem.

Configuration Precedence

In the default ASP.NET Core configuration stack, environment variables override file-based sources (last wins):

1. appsettings.json
2. appsettings.{Environment}.json
3. User secrets (Development only)
4. Environment variables (overrides all above)
5. Command-line arguments

Mapping Convention

.NET maps environment variables to configuration keys using __ (double underscore) as the section separator:

# These environment variables map to configuration sections:
export ConnectionStrings__DefaultDb="Server=prod-db;Database=myapp;..."
export Smtp__ApiKey="SG.production-key"
export Jwt__SigningKey="production-signing-key-256-bits"

# With a prefix (recommended to avoid collisions):
export MYAPP_ConnectionStrings__DefaultDb="Server=prod-db;..."

// Load prefixed environment variables
builder.Configuration.AddEnvironmentVariables(prefix: "MYAPP_");

// Access the same way as any configuration source:
var smtpKey = builder.Configuration["Smtp:ApiKey"];

Container Environments

# docker-compose.yml -- inject secrets via environment
services:
  api:
    image: myapp:latest
    environment:
      - ConnectionStrings__DefaultDb=Server=db;Database=myapp;User=sa;Password=${DB_PASSWORD}
      - Smtp__ApiKey=${SMTP_API_KEY}
    env_file:
      - .env  # NOT committed to source control

# Dockerfile -- do NOT bake secrets into images
# Use environment variables at runtime instead
ENV ASPNETCORE_URLS=http://+:8080
# NEVER: ENV ConnectionStrings__DefaultDb="Server=..."

Gotcha: Environment variables are visible to all processes under the same user. In multi-tenant container environments, use container-level isolation (Kubernetes secrets, Docker secrets) rather than host-level env vars.

IConfiguration Binding Patterns

Bind secrets to strongly typed options classes for compile-time safety and validation.

public sealed class JwtOptions
{
    public const string SectionName = "Jwt";

    [Required, MinLength(32)]
    public string SigningKey { get; set; } = "";

    /// <summary>
    /// Previous signing key retained during rotation window.
    /// Set this when rotating keys so tokens signed with the old key
    /// remain valid until they expire. Remove after rotation completes.
    /// </summary>
    public string? PreviousSigningKey { get; set; }

    [Required]
    public string Issuer { get; set; } = "";

    [Required]
    public string Audience { get; set; } = "";

    [Range(1, 1440)]
    public int ExpirationMinutes { get; set; } = 60;
}

// Registration with validation
builder.Services
    .AddOptions<JwtOptions>()
    .BindConfiguration(JwtOptions.SectionName)
    .ValidateDataAnnotations()
    .ValidateOnStart(); // Fail fast if secrets are missing

// Inject and use
public sealed class TokenService(IOptions<JwtOptions> jwtOptions)
{
    private readonly JwtOptions _jwt = jwtOptions.Value;

    public string GenerateToken(string userId)
    {
        var key = new SymmetricSecurityKey(
            Encoding.UTF8.GetBytes(_jwt.SigningKey));
        var credentials = new SigningCredentials(key, SecurityAlgorithms.HmacSha256);

        var token = new JwtSecurityToken(
            issuer: _jwt.Issuer,
            audience: _jwt.Audience,
            claims: [new Claim(ClaimTypes.NameIdentifier, userId)],
            expires: DateTime.UtcNow.AddMinutes(_jwt.ExpirationMinutes),
            signingCredentials: credentials);

        return new JwtSecurityTokenHandler().WriteToken(token);
    }
}

Options classes must use { get; set; } (not { get; init; }) because the configuration binder and PostConfigure need to mutate properties after construction. Use data annotation attributes ([Required], [MinLength]) for validation.

Gotcha: ValidateOnStart() catches missing secrets at application startup rather than at first use. Always use it for secrets-bearing options to fail fast with a clear error message.

Secret Rotation

Design applications to handle secret rotation without downtime.

Rotation-Friendly Patterns

// Use IOptionsMonitor<T> for secrets that may change at runtime
public sealed class EmailService(IOptionsMonitor<SmtpOptions> smtpOptions, ILogger<EmailService> logger)
{
    public async Task SendAsync(string to, string subject, string body)
    {
        // CurrentValue reads the latest configuration on every call
        var options = smtpOptions.CurrentValue;
        logger.LogDebug("Using SMTP host {Host}", options.Host);

        // ... send email using current options ...
    }
}

// Audit-log configuration changes via a hosted service.
// IHostedService is always activated by the host, so the subscription is guaranteed.
public sealed class SmtpOptionsChangeLogger(
    IOptionsMonitor<SmtpOptions> monitor,
    ILogger<SmtpOptionsChangeLogger> logger) : IHostedService, IDisposable
{
    private IDisposable? _subscription;

    public Task StartAsync(CancellationToken cancellationToken)
    {
        _subscription = monitor.OnChange(options =>
        {
            logger.LogInformation("SMTP configuration reloaded at {Time}", DateTime.UtcNow);
        });
        return Task.CompletedTask;
    }

    public Task StopAsync(CancellationToken cancellationToken) => Task.CompletedTask;

    public void Dispose() => _subscription?.Dispose();
}

// Registration:
builder.Services.AddHostedService<SmtpOptionsChangeLogger>();

// Dual-key validation for zero-downtime rotation
// Accept both old and new signing keys during rotation window
public sealed class DualKeyTokenValidator(IOptionsMonitor<JwtOptions> optionsMonitor)
{
    public TokenValidationParameters GetParameters()
    {
        // Read CurrentValue on every call so rotated keys are picked up
        // without restarting the application
        var options = optionsMonitor.CurrentValue;

        var keys = new List<SecurityKey>
        {
            new SymmetricSecurityKey(Encoding.UTF8.GetBytes(options.SigningKey))
        };

        if (!string.IsNullOrEmpty(options.PreviousSigningKey))
        {
            keys.Add(new SymmetricSecurityKey(
                Encoding.UTF8.GetBytes(options.PreviousSigningKey)));
        }

        return new TokenValidationParameters
        {
            ValidateIssuer = true,
            ValidIssuer = options.Issuer,
            ValidateAudience = true,
            ValidAudience = options.Audience,
            ValidateLifetime = true,
            IssuerSigningKeys = keys
        };
    }
}

Rotation Checklist

1. Deploy the new secret alongside the old one (dual-key window)
2. Update the application to accept both old and new secrets
3. Roll the deployment so all instances use the new secret for signing/encrypting
4. After all clients have rotated, remove the old secret
5. Audit-log every rotation event

Managed Identity (Production Best Practice)

Managed identity eliminates secrets entirely for cloud-hosted applications by using the platform's identity system to authenticate to services.

Concept: Instead of storing a connection string with a password, the application authenticates to the database/service using its platform-assigned identity. No secret to manage, rotate, or leak.

// Example: passwordless connection to SQL Server using DefaultAzureCredential
// This pattern works across Azure, and similar patterns exist for AWS and GCP
var connectionString = "Server=myserver.database.windows.net;Database=mydb;Authentication=Active Directory Default";
builder.Services.AddDbContext<AppDbContext>(options =>
    options.UseSqlServer(connectionString));
// No password in the connection string -- identity is resolved from the environment

When to use managed identity:

• Production and staging environments hosted on cloud platforms
• Any service-to-service communication where the platform supports identity federation
• Database connections, message queue access, storage access

When you still need secrets:

• Third-party APIs that only support API keys
• Legacy systems without identity federation
• Local development (use user secrets as a fallback)

Anti-Patterns

Secrets in Source Control

// NEVER: hardcoded secrets in source code
private const string ApiKey = "sk-live-abc123def456";           // WRONG
private const string ConnectionString = "Server=prod;Password=secret"; // WRONG

Fix: Use user secrets (dev) or environment variables (production). See sections above.

Secrets in appsettings.json

// NEVER: real credentials in appsettings.json (committed to repo)
{
  "ConnectionStrings": {
    "DefaultDb": "Server=prod-db;Password=RealPassword123!"
  }
}

Fix: appsettings.json should contain only non-sensitive defaults. Use placeholder values that fail visibly:

{
  "ConnectionStrings": {
    "DefaultDb": "Server=localhost;Database=myapp;Integrated Security=true"
  },
  "Smtp": {
    "ApiKey": "REPLACE_VIA_ENV_OR_USER_SECRETS"
  }
}

Hardcoded Connection Strings

// NEVER: connection strings directly in code
var connection = new SqlConnection("Server=prod-db;Database=myapp;User=sa;Password=P@ssw0rd!");

Fix: Always resolve connection strings from IConfiguration:

// Correct: resolve from configuration
public sealed class OrderRepository(IConfiguration configuration)
{
    private readonly string _connectionString =
        configuration.GetConnectionString("DefaultDb")
        ?? throw new InvalidOperationException("ConnectionStrings:DefaultDb is not configured");
}

// Better: use Options pattern with validation
public sealed class OrderRepository(IOptions<DatabaseOptions> options)
{
    private readonly string _connectionString = options.Value.ConnectionString;
}

Logging Secrets

// NEVER: log secret values
logger.LogInformation("Using API key: {ApiKey}", apiKey);       // WRONG
logger.LogDebug("Connection string: {Conn}", connectionString); // WRONG

Fix: Log that a secret was loaded, not its value:

logger.LogInformation("API key configured: {IsConfigured}", !string.IsNullOrEmpty(apiKey));
logger.LogInformation("Database connection configured for {Server}", new SqlConnectionStringBuilder(connectionString).DataSource);

Agent Gotchas

1. Do not generate code with hardcoded secrets -- always use IConfiguration or IOptions<T> to resolve secrets. Even in examples, use placeholder values.
2. Do not put real secrets in appsettings.json -- it is committed to source control. Use user secrets for development, environment variables for production.
3. Do not use { get; init; } on Options classes -- the configuration binder requires mutable setters. Use { get; set; } with data annotation validation instead.
4. Do not skip ValidateOnStart() -- without it, missing secrets cause runtime failures at first use rather than a clear startup error.
5. Do not log secret values -- log whether a secret is configured (IsConfigured: true/false) or metadata (server name from connection string), never the value.
6. Do not use IOptions<T> for secrets that rotate -- use IOptionsMonitor<T> for runtime-reloadable secrets so rotation does not require a restart.
7. Do not bake secrets into Docker images -- use environment variables or mounted secrets at container runtime.

Prerequisites

• .NET 8.0+ (LTS baseline)
• Microsoft.Extensions.Configuration.UserSecrets (included in ASP.NET Core SDK; add manually for console apps)
• Microsoft.Extensions.Options.DataAnnotations for ValidateDataAnnotations()

References

• Safe Storage of App Secrets in Development
• Configuration in ASP.NET Core
• Options Pattern in ASP.NET Core
• ASP.NET Core Security
• Secure Coding Guidelines for .NET
• Use Managed Identities