Correlate Exchange IIS failures with security accounts

config/ocsentinel-settings.example.json

@@ -1,5 +1,6 @@
 {
   "warningEventThreshold": 10,
+  "maxReportedEvents": 1000,
   "criticalEventThreshold": 30,
   "warningUniqueIpThreshold": 5,
   "criticalUniqueIpThreshold": 12,

release/beta/version.json

@@ -1,8 +1,8 @@
 {
   "channel": "beta",
-  "version": "1.5.0-beta.5",
-  "publishedAtUtc": "2026-07-31T23:44:23.3751992Z",
-  "artifactUrl": "https://gitea.officecom.cloud/officecom/oc-sentinel/releases/download/v1.5.0-beta.5/OCSentinelClient-win-x64.zip",
-  "sha256": "d1ac7e9cc9e0da5ea9cbff5e2be5795a5e691c89bbe329df265254c1b4898dbf",
+  "version": "1.5.0-beta.7",
+  "publishedAtUtc": "2026-08-02T23:20:59.4454466Z",
+  "artifactUrl": "https://gitea.officecom.cloud/officecom/oc-sentinel/releases/download/v1.5.0-beta.7/OCSentinelClient-win-x64.zip",
+  "sha256": "b40da61499d0d9b5f95922f84aebbb3b6de6bf8a2ba2196873ecf2fab1789c0d",
   "minUpdaterVersion": "1.0.0"
 }

src/OCSentinelCli/AttackScanner.cs

@@ -42,6 +42,7 @@ internal sealed class AttackScanner
         ScanExchangeIisLogons(attacks, errors, since, configuration);
         ScanIisFtpLogs(attacks, errors, since, configuration);
         ScanFileZillaLogs(attacks, errors, since, configuration);
+        attacks = ExchangeIisAccountCorrelator.Enrich(attacks);
         RansomwareBetaSummary ransomwareBeta = RansomwareBetaDetector.Scan(configuration, errors);
 
         if (configuration.ExcludedIps.Count > 0)
@@ -53,6 +54,14 @@ internal sealed class AttackScanner
 
         attacks.Sort(static (left, right) => left.Timestamp.CompareTo(right.Timestamp));
 
+        int totalEventCount = attacks.Count;
+        int maxReportedEvents = Math.Clamp(configuration.MaxReportedEvents, 100, 5000);
+        List<AttackEvent> reportedEvents = attacks
+            .OrderByDescending(static attack => attack.Timestamp)
+            .Take(maxReportedEvents)
+            .OrderBy(static attack => attack.Timestamp)
+            .ToList();
+
         List<AggregatedAttack> topSources = attacks
             .GroupBy(static attack => attack.SourceIp)
             .Select(group => AggregatedAttack.FromGroup(group))
@@ -81,7 +90,9 @@ internal sealed class AttackScanner
             GeneratedAtUtc = generatedAtUtc,
             ClientVersion = BuildMetadata.Version,
             LookbackDays = options.LookbackDays,
-            TotalEvents = attacks.Count,
+            TotalEvents = totalEventCount,
+            ReportedEventCount = reportedEvents.Count,
+            EventsTruncated = reportedEvents.Count < totalEventCount,
             UniqueIpCount = uniqueIpCount,
             AlertState = correlationAssessment.FinalAlertState,
             AlertReason = correlationAssessment.CorrelationReason == "No CVE correlation applied." ? baseAlertReason : correlationAssessment.CorrelationReason,
@@ -95,7 +106,7 @@ internal sealed class AttackScanner
                 FinishedAtUtc = generatedAtUtc,
                 UploadAttempted = false
             },
-            Events = attacks,
+            Events = reportedEvents,
             TopSources = topSources,
             Errors = errors
         };
@@ -171,6 +182,7 @@ internal sealed class AttackScanner
                 Target = "Windows login",
                 Username = ReadProperty(eventRecord, 5, "[unknown]"),
                 Source = "Security",
+                Service = ReadProperty(eventRecord, 10) == "3" ? "Network" : string.Empty,
                 InstanceId = 4625
             });
         });

src/OCSentinelCli/Configuration.cs

@@ -6,6 +6,8 @@ internal sealed record ScannerConfiguration
 {
     public int WarningEventThreshold { get; init; } = 10;
 
+    public int MaxReportedEvents { get; init; } = 1000;
+
     public int CriticalEventThreshold { get; init; } = 30;
 
     public int WarningUniqueIpThreshold { get; init; } = 5;

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);
+    }
+}

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;
             }
 
-            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))
+            if (observation is null)
             {
                 continue;
             }
 
-            if (status is not 401 and not 403 || !TryClassify(path, out string service))
+            FlushExpiredCandidates(pendingFailures, attacks, observation.Timestamp);
+
+            if (observation.IsSuccessfulAuthentication)
             {
-                continue;
+                pendingFailures.RemoveAll(candidate => candidate.MatchesSuccessfulAuthentication(observation));
             }
-
-            if (!DateTime.TryParse($"{date} {time}", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal, out DateTime timestampUtc))
+            else if (observation.IsCredentialFailure)
             {
-                continue;
+                pendingFailures.Add(observation);
             }
-
-            DateTimeOffset timestamp = new(timestampUtc, TimeSpan.Zero);
-            if (timestamp < since || string.IsNullOrWhiteSpace(sourceIp) || sourceIp == "-")
-            {
-                continue;
-            }
-
-            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]";
-
-            yield return new AttackEvent
-            {
-                Timestamp = timestamp.ToLocalTime(),
-                SourceIp = sourceIp,
-                Target = $"Exchange {service} login",
-                Username = username,
-                Source = "IIS W3C",
-                Service = service,
-                DestinationPort = destinationPort,
-                Endpoint = path,
-                InstanceId = status
-            };
         }
+
+        attacks.AddRange(pendingFailures.Select(static candidate => candidate.ToAttackEvent()));
+        return attacks;
+    }
+
+    private static void FlushExpiredCandidates(List<ExchangeIisObservation> pendingFailures, List<AttackEvent> attacks, DateTimeOffset currentTimestamp)
+    {
+        DateTimeOffset cutoff = currentTimestamp - AuthenticationCompletionWindow;
+        foreach (ExchangeIisObservation candidate in pendingFailures.Where(candidate => candidate.Timestamp < cutoff).ToList())
+        {
+            attacks.Add(candidate.ToAttackEvent());
+            pendingFailures.Remove(candidate);
+        }
+    }
+
+    private static bool TryParseObservation(IReadOnlyDictionary<string, int> fields, string line, DateTimeOffset since, out ExchangeIisObservation? observation)
+    {
+        observation = null;
+        string[] values = line.Split(' ', StringSplitOptions.RemoveEmptyEntries);
+        if (!TryValue(fields, values, "date", out string date) || !TryValue(fields, values, "time", out string time)
+            || !TryValue(fields, values, "c-ip", out string sourceIp) || !TryValue(fields, values, "cs-uri-stem", out string path)
+            || !TryValue(fields, values, "sc-status", out string statusText) || !int.TryParse(statusText, out int status)
+            || !TryClassify(path, out string service))
+        {
+            return false;
+        }
+
+        if ((status < 200 || status >= 400) && status is not 401 and not 403)
+        {
+            return false;
+        }
+
+        if (!DateTime.TryParse($"{date} {time}", CultureInfo.InvariantCulture, DateTimeStyles.AssumeUniversal | DateTimeStyles.AdjustToUniversal, out DateTime timestampUtc))
+        {
+            return false;
+        }
+
+        DateTimeOffset timestamp = new(timestampUtc, TimeSpan.Zero);
+        if (timestamp < since || string.IsNullOrWhiteSpace(sourceIp) || sourceIp == "-")
+        {
+            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;
+
+        observation = new ExchangeIisObservation(timestamp, sourceIp, path, service, destinationPort, username, userAgent, status, substatus);
+        return true;
     }
 
     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
+        };
+    }
 }

src/OCSentinelCli/Models.cs

@@ -107,6 +107,10 @@ internal sealed record ScanResult
 
     public int TotalEvents { get; init; }
 
+    public int ReportedEventCount { get; init; }
+
+    public bool EventsTruncated { get; init; }
+
     public int UniqueIpCount { get; init; }
 
     public string AlertState { get; init; } = "ok";

src/OCSentinelCli/OCSentinelCli.csproj

@@ -9,10 +9,10 @@
     <RootNamespace>OCSentinelCli</RootNamespace>
     <Product>OfficeCom Sentinel</Product>
     <Company>OfficeCom</Company>
-    <Version>1.5.0-beta.6</Version>
+    <Version>1.5.0-beta.7</Version>
     <AssemblyVersion>1.5.0.0</AssemblyVersion>
     <FileVersion>1.5.0.0</FileVersion>
-    <InformationalVersion>1.5.0-beta.6</InformationalVersion>
+    <InformationalVersion>1.5.0-beta.7</InformationalVersion>
   </PropertyGroup>
 
   <ItemGroup>

tests/OCSentinelCli.Tests/ExchangeIisAccountCorrelatorTests.cs

@@ -0,0 +1,58 @@
+using OCSentinelCli;
+using Xunit;
+
+namespace OCSentinelCli.Tests;
+
+public sealed class ExchangeIisAccountCorrelatorTests
+{
+    [Fact]
+    public void EnrichesAnIisFailureFromOneMatchingSecurityFailure()
+    {
+        DateTimeOffset timestamp = new(2026, 8, 5, 4, 0, 0, TimeSpan.Zero);
+        List<AttackEvent> events =
+        [
+            IisFailure(timestamp),
+            SecurityFailure(timestamp.AddSeconds(20), "user@example.test")
+        ];
+
+        AttackEvent enriched = Assert.Single(ExchangeIisAccountCorrelator.Enrich(events), attack => attack.Source == "IIS W3C");
+
+        Assert.Equal("user@example.test", enriched.Username);
+    }
+
+    [Fact]
+    public void KeepsIisAccountUnresolvedWhenMultipleAccountsMatch()
+    {
+        DateTimeOffset timestamp = new(2026, 8, 5, 4, 0, 0, TimeSpan.Zero);
+        List<AttackEvent> events =
+        [
+            IisFailure(timestamp),
+            SecurityFailure(timestamp.AddSeconds(20), "first@example.test"),
+            SecurityFailure(timestamp.AddSeconds(30), "second@example.test")
+        ];
+
+        AttackEvent unresolved = Assert.Single(ExchangeIisAccountCorrelator.Enrich(events), attack => attack.Source == "IIS W3C");
+
+        Assert.Equal("[not logged]", unresolved.Username);
+    }
+
+    private static AttackEvent IisFailure(DateTimeOffset timestamp) => new()
+    {
+        Timestamp = timestamp,
+        SourceIp = "198.51.100.8",
+        Username = "[not logged]",
+        Source = "IIS W3C",
+        Target = "Exchange ActiveSync login"
+    };
+
+    private static AttackEvent SecurityFailure(DateTimeOffset timestamp, string username) => new()
+    {
+        Timestamp = timestamp,
+        SourceIp = "198.51.100.8",
+        Username = username,
+        Source = "Security",
+        Service = "Network",
+        Target = "Windows login",
+        InstanceId = 4625
+    };
+}

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)));
+    }
 }