Refine Exchange IIS authentication detection

src/OCSentinelCli/ExchangeIisLogParser.cs

@@ -4,9 +4,13 @@ namespace OCSentinelCli;
 
 internal static class ExchangeIisLogParser
 {
+    private static readonly TimeSpan AuthenticationCompletionWindow = TimeSpan.FromMinutes(2);
+
     internal static IEnumerable<AttackEvent> ParseLines(IEnumerable<string> lines, DateTimeOffset since)
     {
         Dictionary<string, int>? fields = null;
+        var pendingFailures = new List<ExchangeIisObservation>();
+        var attacks = new List<AttackEvent>();
 
         foreach (string line in lines)
         {
@@ -18,51 +22,77 @@ internal static class ExchangeIisLogParser
                 continue;
             }
 
-            if (string.IsNullOrWhiteSpace(line) || line.StartsWith('#') || fields is null)
+            if (string.IsNullOrWhiteSpace(line) || line.StartsWith('#') || fields is null || !TryParseObservation(fields, line, since, out ExchangeIisObservation? observation))
             {
                 continue;
             }
 
+            if (observation is null)
+            {
+                continue;
+            }
+
+            FlushExpiredCandidates(pendingFailures, attacks, observation.Timestamp);
+
+            if (observation.IsSuccessfulAuthentication)
+            {
+                pendingFailures.RemoveAll(candidate => candidate.MatchesSuccessfulAuthentication(observation));
+            }
+            else if (observation.IsCredentialFailure)
+            {
+                pendingFailures.Add(observation);
+            }
+        }
+
+        attacks.AddRange(pendingFailures.Select(static candidate => candidate.ToAttackEvent()));
+        return attacks;
+    }
+
+    private static void FlushExpiredCandidates(List<ExchangeIisObservation> pendingFailures, List<AttackEvent> attacks, DateTimeOffset currentTimestamp)
+    {
+        DateTimeOffset cutoff = currentTimestamp - AuthenticationCompletionWindow;
+        foreach (ExchangeIisObservation candidate in pendingFailures.Where(candidate => candidate.Timestamp < cutoff).ToList())
+        {
+            attacks.Add(candidate.ToAttackEvent());
+            pendingFailures.Remove(candidate);
+        }
+    }
+
+    private static bool TryParseObservation(IReadOnlyDictionary<string, int> fields, string line, DateTimeOffset since, out ExchangeIisObservation? observation)
+    {
+        observation = null;
         string[] values = line.Split(' ', StringSplitOptions.RemoveEmptyEntries);
         if (!TryValue(fields, values, "date", out string date) || !TryValue(fields, values, "time", out string time)
             || !TryValue(fields, values, "c-ip", out string sourceIp) || !TryValue(fields, values, "cs-uri-stem", out string path)
-                || !TryValue(fields, values, "sc-status", out string statusText) || !int.TryParse(statusText, out int status))
+            || !TryValue(fields, values, "sc-status", out string statusText) || !int.TryParse(statusText, out int status)
+            || !TryClassify(path, out string service))
         {
-                continue;
+            return false;
         }
 
-            if (status is not 401 and not 403 || !TryClassify(path, out string service))
+        if ((status < 200 || status >= 400) && status is not 401 and not 403)
         {
-                continue;
+            return false;
         }
 
         if (!DateTime.TryParse($"{date} {time}", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal, out DateTime timestampUtc))
         {
-                continue;
+            return false;
         }
 
         DateTimeOffset timestamp = new(timestampUtc, TimeSpan.Zero);
         if (timestamp < since || string.IsNullOrWhiteSpace(sourceIp) || sourceIp == "-")
         {
-                continue;
+            return false;
         }
 
         int? destinationPort = TryValue(fields, values, "s-port", out string portText) && int.TryParse(portText, out int parsedPort) ? parsedPort : null;
         string username = TryValue(fields, values, "cs-username", out string loggedUser) && loggedUser != "-" ? loggedUser : "[not logged]";
+        string userAgent = TryValue(fields, values, "cs(User-Agent)", out string parsedUserAgent) && parsedUserAgent != "-" ? parsedUserAgent : string.Empty;
+        string substatus = TryValue(fields, values, "sc-substatus", out string parsedSubstatus) ? parsedSubstatus : string.Empty;
 
-            yield return new AttackEvent
+        observation = new ExchangeIisObservation(timestamp, sourceIp, path, service, destinationPort, username, userAgent, status, substatus);
-            {
+        return true;
-                Timestamp = timestamp.ToLocalTime(),
-                SourceIp = sourceIp,
-                Target = $"Exchange {service} login",
-                Username = username,
-                Source = "IIS W3C",
-                Service = service,
-                DestinationPort = destinationPort,
-                Endpoint = path,
-                InstanceId = status
-            };
-        }
     }
 
     private static bool TryValue(IReadOnlyDictionary<string, int> fields, IReadOnlyList<string> values, string field, out string value)
@@ -94,4 +124,45 @@ internal static class ExchangeIisLogParser
         };
         return service.Length > 0;
     }
+
+    private sealed record ExchangeIisObservation(
+        DateTimeOffset Timestamp,
+        string SourceIp,
+        string Endpoint,
+        string Service,
+        int? DestinationPort,
+        string Username,
+        string UserAgent,
+        int Status,
+        string Substatus)
+    {
+        public bool IsSuccessfulAuthentication => Status is >= 200 and < 400;
+
+        // IIS 401.0 and 401.2 commonly occur during normal authentication negotiation or server configuration checks.
+        public bool IsCredentialFailure => Status == 403 || (Status == 401 && (string.IsNullOrWhiteSpace(Substatus) || Substatus == "1"));
+
+        public bool MatchesSuccessfulAuthentication(ExchangeIisObservation success)
+        {
+            return success.IsSuccessfulAuthentication
+                && success.Timestamp >= Timestamp
+                && success.Timestamp - Timestamp <= AuthenticationCompletionWindow
+                && string.Equals(success.SourceIp, SourceIp, StringComparison.OrdinalIgnoreCase)
+                && string.Equals(success.Endpoint, Endpoint, StringComparison.OrdinalIgnoreCase)
+                && success.DestinationPort == DestinationPort
+                && string.Equals(success.UserAgent, UserAgent, StringComparison.OrdinalIgnoreCase);
+        }
+
+        public AttackEvent ToAttackEvent() => new()
+        {
+            Timestamp = Timestamp.ToLocalTime(),
+            SourceIp = SourceIp,
+            Target = $"Exchange {Service} login",
+            Username = Username,
+            Source = "IIS W3C",
+            Service = Service,
+            DestinationPort = DestinationPort,
+            Endpoint = Endpoint,
+            InstanceId = Status
+        };
+    }
 }

tests/OCSentinelCli.Tests/ExchangeIisLogParserTests.cs

@@ -39,4 +39,29 @@ public sealed class ExchangeIisLogParserTests
         Assert.Equal(444, attack.DestinationPort);
         Assert.Equal("user@example.test", attack.Username);
     }
+
+    [Fact]
+    public void IgnoresNormalIisAuthenticationHandshake()
+    {
+        string[] lines =
+        [
+            "#Fields: date time cs-uri-stem cs-username c-ip s-port cs(User-Agent) sc-status sc-substatus",
+            "2026-08-02 04:15:00 /mapi/emsmdb/ - 198.51.100.8 443 Outlook 401 1",
+            "2026-08-02 04:15:01 /mapi/emsmdb/ user@example.test 198.51.100.8 443 Outlook 200 0"
+        ];
+
+        Assert.Empty(ExchangeIisLogParser.ParseLines(lines, new DateTimeOffset(2026, 8, 2, 4, 0, 0, TimeSpan.Zero)));
+    }
+
+    [Fact]
+    public void IgnoresNonCredentialIis401Substatus()
+    {
+        string[] lines =
+        [
+            "#Fields: date time cs-uri-stem cs-username c-ip s-port sc-status sc-substatus",
+            "2026-08-02 04:15:00 /ews/Exchange.asmx - 198.51.100.8 443 401 0"
+        ];
+
+        Assert.Empty(ExchangeIisLogParser.ParseLines(lines, new DateTimeOffset(2026, 8, 2, 4, 0, 0, TimeSpan.Zero)));
+    }
 }