Implement automatic certificate renewal (Option 2 - agent self-renewal)
AGENT SELF-RENEWAL: - Agent checks certificate expiry on startup - If < 30 days remaining, requests renewal from server - Server generates new certificate and updates database - Agent installs new cert, updates config, and restarts SERVER-SIDE (/api/endpoints/certificate/renew): - Requires authentication via existing (expiring) client certificate - Rate limited: 1 renewal per asset per 24 hours - Generates 2-year self-signed certificate with cryptography library - Computes SHA1 fingerprint (matches nginx $ssl_client_fingerprint) - Updates assets table with new fingerprint and expiration - Returns PFX/P12 certificate with random password to agent - Audit logs all renewal operations AGENT-SIDE (CertificateRenewal.cs): - CheckAndRenewIfNeeded() called on service startup before host starts - Finds current certificate by thumbprint in LocalMachine\My store - Checks expiry: if < 30 days, POST to /api/endpoints/certificate/renew - Authenticates renewal request with expiring certificate (mTLS) - Receives PFX + password in JSON response - Imports new certificate to LocalMachine\My store - Grants BUILTIN\Users read access to private key (for tray app) - Updates appsettings.json with new thumbprint - Exits with code 0 so service manager restarts agent with new cert CERTIFICATE GENERATION (certificate_renewal.py): - Uses cryptography library to generate RSA 2048-bit key pairs - Self-signed certificates with CN=hostname, 2-year validity - Extended key usage: CLIENT_AUTH - Subject Alternative Name: hostname as DNS name - Exports as password-protected PFX/P12 for Windows import - SHA1 fingerprint computed from DER encoding (nginx compatible) STARTUP FLOW: 1. Agent service starts (Program.cs RunHostAsync) 2. CertificateRenewal.CheckAndRenewIfNeeded() runs BEFORE host starts 3. If cert expires in < 30 days, requests renewal from server 4. If renewal succeeds, agent exits (Environment.Exit(0)) 5. Windows Service Manager automatically restarts the service 6. New process picks up updated thumbprint from appsettings.json 7. Agent continues with renewed certificate SECURITY: - Renewal requires valid (not expired) existing certificate - Rate limit prevents renewal loop abuse - Server validates cert not revoked before renewal - Audit logging tracks all renewal operations - PFX password is random, used once during import, never stored PERFORMANCE: - Renewal check adds ~100-200ms to service startup - Only triggers when expiry < 30 days (rare operation) - No impact on steady-state agent performance Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
This commit is contained in:
Proudlock
297
agent/ShopDbEndpointAgent.Core/CertificateRenewal.cs
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297
agent/ShopDbEndpointAgent.Core/CertificateRenewal.cs
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using System;
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using System.IO;
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using System.Net.Http;
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using System.Security.Cryptography.X509Certificates;
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using System.Text;
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using System.Text.Json;
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using System.Threading.Tasks;
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using Microsoft.Extensions.Logging;
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namespace ShopDbEndpointAgent.Core;
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/// <summary>
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/// Automatic client certificate renewal when expiring within 30 days.
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/// Agent checks cert expiry on startup, requests renewal from server, installs
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/// new cert, updates config, and schedules service restart.
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/// </summary>
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public class CertificateRenewal
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{
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private readonly ILogger<CertificateRenewal> logger;
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private readonly AgentConfig config;
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private const int RENEWAL_THRESHOLD_DAYS = 30;
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private const string RENEWAL_ENDPOINT = "/api/endpoints/certificate/renew";
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public CertificateRenewal(ILogger<CertificateRenewal> logger, AgentConfig config)
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{
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this.logger = logger;
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this.config = config;
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}
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/// <summary>
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/// Check if client certificate needs renewal (expires in < 30 days).
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/// Called on agent startup before first telemetry report.
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/// </summary>
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public async Task<bool> CheckAndRenewIfNeeded()
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{
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if (string.IsNullOrWhiteSpace(config.ClientCertThumbprint))
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{
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// No cert configured, skip renewal check
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return false;
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}
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// Find current certificate
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var cert = FindClientCertificate(config.ClientCertThumbprint);
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if (cert == null)
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{
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logger.LogWarning(
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"Certificate {Thumbprint} not found in LocalMachine\\My store. " +
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"Cannot check expiry or renew.",
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config.ClientCertThumbprint);
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return false;
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}
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// Check expiry
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var daysRemaining = (cert.NotAfter - DateTime.Now).TotalDays;
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logger.LogInformation(
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"Client certificate expires {ExpiryDate} ({DaysRemaining:F1} days remaining)",
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cert.NotAfter, daysRemaining);
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if (daysRemaining > RENEWAL_THRESHOLD_DAYS)
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{
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// Certificate still valid for > 30 days, no renewal needed
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return false;
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}
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// Certificate expiring soon or already expired, request renewal
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logger.LogWarning(
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"Certificate expires in {DaysRemaining:F1} days (threshold {Threshold} days). " +
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"Requesting renewal from server...",
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daysRemaining, RENEWAL_THRESHOLD_DAYS);
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try
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{
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await RenewCertificate(cert);
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return true; // Renewal succeeded, service should restart
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}
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catch (Exception ex)
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{
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logger.LogError(ex,
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"Certificate renewal failed. Agent will continue with existing cert " +
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"until next startup or manual intervention.");
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return false;
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}
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}
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private X509Certificate2? FindClientCertificate(string thumbprint)
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{
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using var store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
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store.Open(OpenFlags.ReadOnly);
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// Thumbprint lookup is case-insensitive, no separators
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var normalizedThumbprint = thumbprint.Replace(":", "").Replace(" ", "");
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var certs = store.Certificates.Find(
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X509FindType.FindByThumbprint,
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normalizedThumbprint,
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validOnly: false); // Include expired certs for renewal
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return certs.Count > 0 ? certs[0] : null;
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}
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private async Task RenewCertificate(X509Certificate2 oldCert)
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{
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// Step 1: Request new certificate from server
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var response = await RequestCertificateFromServer(oldCert);
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if (response == null)
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{
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throw new InvalidOperationException("Server returned null renewal response");
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}
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// Step 2: Import certificate to LocalMachine\My store
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var newCert = ImportPfxCertificate(response.Pfx, response.Password);
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// Step 3: Update appsettings.json with new thumbprint
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UpdateConfigFile(response.Thumbprint);
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// Step 4: Grant Users group access to private key (for tray app)
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GrantPrivateKeyAccess(newCert);
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logger.LogInformation(
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"Certificate renewal complete. New cert thumbprint: {Thumbprint}, " +
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"expires: {ExpiryDate}. Service restart required.",
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response.Thumbprint, response.ExpiresAt);
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}
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private async Task<RenewalResponse?> RequestCertificateFromServer(X509Certificate2 currentCert)
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{
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var handler = new HttpClientHandler();
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// Attach current (expiring) certificate to authenticate renewal request
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handler.ClientCertificates.Add(currentCert);
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// Accept server cert (TODO: validate via ServerCertThumbprint if configured)
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handler.ServerCertificateCustomValidationCallback = (sender, cert, chain, errors) =>
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{
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if (cert == null) return false;
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if (!string.IsNullOrWhiteSpace(config.ServerCertThumbprint))
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{
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var actualThumbprint = cert.GetCertHashString();
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var expectedThumbprint = config.ServerCertThumbprint.Replace(":", "").Replace(" ", "");
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if (!string.Equals(actualThumbprint, expectedThumbprint, StringComparison.OrdinalIgnoreCase))
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{
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logger.LogError(
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"Server certificate thumbprint mismatch during renewal. " +
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"Expected {Expected}, got {Actual}",
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expectedThumbprint, actualThumbprint);
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return false;
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}
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}
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return true;
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};
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using var http = new HttpClient(handler) { Timeout = TimeSpan.FromSeconds(30) };
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var url = config.BaseUrl.TrimEnd('/') + RENEWAL_ENDPOINT;
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logger.LogInformation("Requesting certificate renewal from {Url}", url);
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var httpResponse = await http.PostAsync(url, new StringContent("", Encoding.UTF8, "application/json"));
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if (!httpResponse.IsSuccessStatusCode)
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{
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var errorBody = await httpResponse.Content.ReadAsStringAsync();
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throw new HttpRequestException(
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$"Certificate renewal request failed: {httpResponse.StatusCode} - {errorBody}");
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}
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var json = await httpResponse.Content.ReadAsStringAsync();
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var envelope = JsonSerializer.Deserialize<RenewalResponseEnvelope>(json);
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if (envelope?.Data == null)
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{
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throw new InvalidOperationException("Server returned invalid renewal response");
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}
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return envelope.Data;
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}
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private X509Certificate2 ImportPfxCertificate(string pfxBase64, string password)
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{
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var pfxBytes = Convert.FromBase64String(pfxBase64);
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// Import to LocalMachine\My store
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var cert = new X509Certificate2(pfxBytes, password,
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X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet);
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using var store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
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store.Open(OpenFlags.ReadWrite);
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store.Add(cert);
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logger.LogInformation(
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"Imported new certificate to LocalMachine\\My: {Subject}, thumbprint: {Thumbprint}",
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cert.Subject, cert.Thumbprint);
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return cert;
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}
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private void UpdateConfigFile(string newThumbprint)
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{
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// appsettings.json location: same directory as executable
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var configPath = Path.Combine(AppContext.BaseDirectory, "appsettings.json");
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if (!File.Exists(configPath))
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{
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throw new FileNotFoundException($"Configuration file not found: {configPath}");
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}
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// Read, parse, update, write back
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var json = File.ReadAllText(configPath);
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var doc = JsonDocument.Parse(json);
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using var stream = new MemoryStream();
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using var writer = new Utf8JsonWriter(stream, new JsonWriterOptions { Indented = true });
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writer.WriteStartObject();
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foreach (var prop in doc.RootElement.EnumerateObject())
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{
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if (prop.Name.Equals("clientcertthumbprint", StringComparison.OrdinalIgnoreCase))
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{
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writer.WriteString(prop.Name, newThumbprint);
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}
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else
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{
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prop.WriteTo(writer);
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}
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}
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writer.WriteEndObject();
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writer.Flush();
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var updatedJson = Encoding.UTF8.GetString(stream.ToArray());
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File.WriteAllText(configPath, updatedJson);
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logger.LogInformation("Updated {ConfigPath} with new certificate thumbprint", configPath);
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}
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private void GrantPrivateKeyAccess(X509Certificate2 cert)
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{
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// Grant BUILTIN\Users read access to private key so tray app can use it
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try
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{
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var rsaKey = cert.GetRSAPrivateKey();
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if (rsaKey == null)
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{
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logger.LogWarning("Certificate has no RSA private key, cannot set permissions");
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return;
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}
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// CNG key: use key name to find file
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if (rsaKey is System.Security.Cryptography.RSACng cngKey)
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{
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var keyPath = cngKey.Key.UniqueName;
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var searchPaths = new[]
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{
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Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData),
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@"Microsoft\Crypto\SystemKeys"),
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Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData),
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@"Microsoft\Crypto\Keys"),
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Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData),
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@"Microsoft\Crypto\RSA\MachineKeys")
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};
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foreach (var searchPath in searchPaths)
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{
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var keyFile = Path.Combine(searchPath, keyPath);
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if (File.Exists(keyFile))
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{
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var acl = File.GetAccessControl(keyFile);
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var rule = new System.Security.AccessControl.FileSystemAccessRule(
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"BUILTIN\\Users",
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System.Security.AccessControl.FileSystemRights.Read |
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System.Security.AccessControl.FileSystemRights.ReadAndExecute,
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System.Security.AccessControl.AccessControlType.Allow);
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acl.AddAccessRule(rule);
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File.SetAccessControl(keyFile, acl);
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logger.LogInformation("Granted Users group access to private key at {KeyFile}", keyFile);
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return;
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}
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}
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logger.LogWarning("Private key file not found in any search path");
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}
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}
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catch (Exception ex)
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{
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logger.LogWarning(ex, "Could not set private key permissions (tray app may not work)");
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}
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}
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// JSON response schema
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private class RenewalResponseEnvelope
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{
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public RenewalResponse? Data { get; set; }
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}
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private class RenewalResponse
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{
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public string Pfx { get; set; } = "";
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public string Password { get; set; } = "";
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public string Thumbprint { get; set; } = "";
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public DateTime ExpiresAt { get; set; }
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}
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}
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@@ -172,6 +172,35 @@ catch (Exception error)
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static async Task RunHostAsync(AgentConfig config, string agentVersion,
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string logDirectory, bool forceConsole)
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{
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// Certificate renewal check: if client cert expires within 30 days, request
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// renewal from server, install new cert, and exit so service manager restarts us.
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// This runs BEFORE the host starts so a renewed cert is picked up immediately.
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var renewalLogger = LoggerFactory.Create(logging =>
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ConfigureLogging(logging, logDirectory)).CreateLogger<CertificateRenewal>();
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var renewal = new CertificateRenewal(renewalLogger, config);
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try
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{
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var didRenew = await renewal.CheckAndRenewIfNeeded();
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if (didRenew)
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{
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renewalLogger.LogInformation(
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"Certificate renewed successfully. Exiting so service manager restarts " +
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"the agent with new certificate.");
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// Exit code 0 so service manager sees this as normal termination and
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// restarts the service (if configured as Automatic). The new process
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// will pick up the updated thumbprint from appsettings.json.
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Environment.Exit(0);
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}
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}
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catch (Exception ex)
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{
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renewalLogger.LogError(ex,
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"Certificate renewal check failed. Starting agent with existing certificate.");
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// Continue startup with existing (possibly expiring) certificate rather
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// than blocking service start. Next startup will retry renewal.
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}
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var builder = Host.CreateApplicationBuilder();
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if (!forceConsole)
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