Correlate Exchange IIS failures with security accounts

src/OCSentinelCli/ExchangeIisAccountCorrelator.cs

@@ -0,0 +1,53 @@
+namespace OCSentinelCli;
+
+internal static class ExchangeIisAccountCorrelator
+{
+    private static readonly TimeSpan CorrelationWindow = TimeSpan.FromMinutes(2);
+
+    internal static List<AttackEvent> Enrich(IReadOnlyList<AttackEvent> attacks)
+    {
+        List<AttackEvent> securityFailures = attacks
+            .Where(IsNetworkSecurityFailure)
+            .Where(HasRecordedAccount)
+            .ToList();
+
+        return attacks.Select(attack => IsUnresolvedExchangeIisFailure(attack)
+            ? EnrichFromSecurityFailure(attack, securityFailures)
+            : attack).ToList();
+    }
+
+    private static AttackEvent EnrichFromSecurityFailure(AttackEvent iisFailure, IReadOnlyList<AttackEvent> securityFailures)
+    {
+        List<string> accounts = securityFailures
+            .Where(failure => string.Equals(failure.SourceIp, iisFailure.SourceIp, StringComparison.OrdinalIgnoreCase))
+            .Where(failure => (failure.Timestamp - iisFailure.Timestamp).Duration() <= CorrelationWindow)
+            .Select(failure => failure.Username)
+            .Distinct(StringComparer.OrdinalIgnoreCase)
+            .ToList();
+
+        // A shared NAT address can produce concurrent failures; do not guess between accounts.
+        return accounts.Count == 1 ? iisFailure with { Username = accounts[0] } : iisFailure;
+    }
+
+    private static bool IsUnresolvedExchangeIisFailure(AttackEvent attack)
+    {
+        return string.Equals(attack.Source, "IIS W3C", StringComparison.OrdinalIgnoreCase)
+            && (string.Equals(attack.Username, "[not logged]", StringComparison.OrdinalIgnoreCase)
+                || string.Equals(attack.Username, "[unknown]", StringComparison.OrdinalIgnoreCase));
+    }
+
+    private static bool IsNetworkSecurityFailure(AttackEvent attack)
+    {
+        return attack.InstanceId == 4625
+            && string.Equals(attack.Source, "Security", StringComparison.OrdinalIgnoreCase)
+            && string.Equals(attack.Service, "Network", StringComparison.OrdinalIgnoreCase)
+            && string.Equals(attack.Target, "Windows login", StringComparison.OrdinalIgnoreCase);
+    }
+
+    private static bool HasRecordedAccount(AttackEvent attack)
+    {
+        return !string.IsNullOrWhiteSpace(attack.Username)
+            && !string.Equals(attack.Username, "[unknown]", StringComparison.OrdinalIgnoreCase)
+            && !string.Equals(attack.Username, "[not logged]", StringComparison.OrdinalIgnoreCase);
+    }
+}