Verify server ID before KILL QUERY to prevent cross-server cancellation

src/MySqlConnector/Core/ServerSession.cs

@@ -47,6 +47,7 @@ public ServerSession(ILogger logger, IConnectionPoolMetadata pool)
 	public int ActiveCommandId { get; private set; }
 	public int CancellationTimeout { get; private set; }
 	public int ConnectionId { get; set; }
+	public string? ServerHostname { get; set; }
 	public byte[]? AuthPluginData { get; set; }
 	public long CreatedTimestamp { get; }
 	public ConnectionPool? Pool { get; }
@@ -122,6 +123,24 @@ public void DoCancel(ICancellableCommand commandToCancel, MySqlCommand killComma
 				return;
 			}
 
+			// Verify server identity before executing KILL QUERY to prevent cancelling on the wrong server
+			var killSession = killCommand.Connection!.Session;
+			if (!string.IsNullOrEmpty(ServerHostname) && !string.IsNullOrEmpty(killSession.ServerHostname))
+			{
+				if (!string.Equals(ServerHostname, killSession.ServerHostname, StringComparison.Ordinal))
+				{
+					Log.IgnoringCancellationForDifferentServer(m_logger, Id, killSession.Id, ServerHostname, killSession.ServerHostname);
+					return;
+				}
+			}
+			else if (!string.IsNullOrEmpty(ServerHostname) || !string.IsNullOrEmpty(killSession.ServerHostname))
+			{
+				// One session has hostname, the other doesn't - this is a potential mismatch
+				Log.IgnoringCancellationForDifferentServer(m_logger, Id, killSession.Id, ServerHostname, killSession.ServerHostname);
+				return;
+			}
+			// If both sessions have no hostname, allow the operation for backward compatibility
+
 			// NOTE: This command is executed while holding the lock to prevent race conditions during asynchronous cancellation.
 			// For example, if the lock weren't held, the current command could finish and the other thread could set ActiveCommandId
 			// to zero, then start executing a new command. By the time this "KILL QUERY" command reached the server, the wrong
@@ -640,6 +659,9 @@ public async Task DisposeAsync(IOBehavior ioBehavior, CancellationToken cancella
 				ConnectionId = newConnectionId;
 			}
 
+			// Get server hostname for KILL QUERY verification
+			await GetServerHostnameAsync(ioBehavior, CancellationToken.None).ConfigureAwait(false);
+
 			m_payloadHandler.ByteHandler.RemainingTimeout = Constants.InfiniteTimeout;
 			return redirectionUrl;
 		}
@@ -1963,6 +1985,52 @@ private async Task GetRealServerDetailsAsync(IOBehavior ioBehavior, Cancellation
 		}
 	}
 
+	private async Task GetServerHostnameAsync(IOBehavior ioBehavior, CancellationToken cancellationToken)
+	{
+		Log.GettingServerHostname(m_logger, Id);
+		try
+		{
+			var payload = SupportsQueryAttributes ? s_selectHostnameWithAttributesPayload : s_selectHostnameNoAttributesPayload;
+			await SendAsync(payload, ioBehavior, cancellationToken).ConfigureAwait(false);
+
+			// column count: 1
+			_ = await ReceiveReplyAsync(ioBehavior, cancellationToken).ConfigureAwait(false);
+
+			// @@hostname column
+			_ = await ReceiveReplyAsync(ioBehavior, CancellationToken.None).ConfigureAwait(false);
+
+			if (!SupportsDeprecateEof)
+			{
+				payload = await ReceiveReplyAsync(ioBehavior, CancellationToken.None).ConfigureAwait(false);
+				_ = EofPayload.Create(payload.Span);
+			}
+
+			// first (and only) row
+			payload = await ReceiveReplyAsync(ioBehavior, CancellationToken.None).ConfigureAwait(false);
+
+			var reader = new ByteArrayReader(payload.Span);
+			var length = reader.ReadLengthEncodedIntegerOrNull();
+			var hostname = length > 0 ? Encoding.UTF8.GetString(reader.ReadByteString(length)) : null;
+
+			ServerHostname = hostname;
+
+			Log.RetrievedServerHostname(m_logger, Id, hostname);
+
+			// OK/EOF payload
+			payload = await ReceiveReplyAsync(ioBehavior, CancellationToken.None).ConfigureAwait(false);
+			if (OkPayload.IsOk(payload.Span, this))
+				OkPayload.Verify(payload.Span, this);
+			else
+				EofPayload.Create(payload.Span);
+		}
+		catch (MySqlException ex)
+		{
+			Log.FailedToGetServerHostname(m_logger, ex, Id);
+			// Set fallback value to ensure operation can continue
+			ServerHostname = null;
+		}
+	}
+
 	private void ShutdownSocket()
 	{
 		Log.ClosingStreamSocket(m_logger, Id);
@@ -2194,6 +2262,8 @@ protected override void OnStatementBegin(int index)
 	private static readonly PayloadData s_sleepWithAttributesPayload = QueryPayload.Create(true, "SELECT SLEEP(0) INTO @__MySqlConnector__Sleep;"u8);
 	private static readonly PayloadData s_selectConnectionIdVersionNoAttributesPayload = QueryPayload.Create(false, "SELECT CONNECTION_ID(), VERSION();"u8);
 	private static readonly PayloadData s_selectConnectionIdVersionWithAttributesPayload = QueryPayload.Create(true, "SELECT CONNECTION_ID(), VERSION();"u8);
+	private static readonly PayloadData s_selectHostnameNoAttributesPayload = QueryPayload.Create(false, "SELECT @@hostname;"u8);
+	private static readonly PayloadData s_selectHostnameWithAttributesPayload = QueryPayload.Create(true, "SELECT @@hostname;"u8);
 
 	private readonly ILogger m_logger;
 #if NET9_0_OR_GREATER

src/MySqlConnector/Logging/EventIds.cs

@@ -86,6 +86,9 @@ internal static class EventIds
 	public const int CertificateErrorUnixSocket = 2158;
 	public const int CertificateErrorNoPassword = 2159;
 	public const int CertificateErrorValidThumbprint = 2160;
+	public const int GettingServerHostname = 2161;
+	public const int RetrievedServerHostname = 2162;
+	public const int FailedToGetServerHostname = 2163;
 
 	// Command execution events, 2200-2299
 	public const int CannotExecuteNewCommandInState = 2200;
@@ -108,6 +111,7 @@ internal static class EventIds
 	public const int IgnoringCancellationForInactiveCommand = 2306;
 	public const int CancelingCommand = 2307;
 	public const int SendingSleepToClearPendingCancellation = 2308;
+	public const int IgnoringCancellationForDifferentServer = 2309;
 
 	// Cached procedure events, 2400-2499
 	public const int GettingCachedProcedure = 2400;

src/MySqlConnector/Logging/Log.cs

@@ -189,6 +189,18 @@ internal static partial class Log
 	[LoggerMessage(EventIds.FailedToGetConnectionId, LogLevel.Information, "Session {SessionId} failed to get CONNECTION_ID(), VERSION()")]
 	public static partial void FailedToGetConnectionId(ILogger logger, Exception exception, string sessionId);
 
+	[LoggerMessage(EventIds.GettingServerHostname, LogLevel.Debug, "Session {SessionId} getting server hostname")]
+	public static partial void GettingServerHostname(ILogger logger, string sessionId);
+
+	[LoggerMessage(EventIds.RetrievedServerHostname, LogLevel.Debug, "Session {SessionId} retrieved server hostname: {ServerHostname}")]
+	public static partial void RetrievedServerHostname(ILogger logger, string sessionId, string? serverHostname);
+
+	[LoggerMessage(EventIds.FailedToGetServerHostname, LogLevel.Information, "Session {SessionId} failed to get server hostname")]
+	public static partial void FailedToGetServerHostname(ILogger logger, Exception exception, string sessionId);
+
+	[LoggerMessage(EventIds.IgnoringCancellationForDifferentServer, LogLevel.Warning, "Session {SessionId} ignoring cancellation from session {KillSessionId}: server hostname mismatch (this hostname={ServerHostname}, kill hostname={KillServerHostname})")]
+	public static partial void IgnoringCancellationForDifferentServer(ILogger logger, string sessionId, string killSessionId, string? serverHostname, string? killServerHostname);
+
 	[LoggerMessage(EventIds.ClosingStreamSocket, LogLevel.Debug, "Session {SessionId} closing stream/socket")]
 	public static partial void ClosingStreamSocket(ILogger logger, string sessionId);
 

tests/IntegrationTests/ServerIdentificationTests.cs

@@ -0,0 +1,53 @@
+using System.Diagnostics;
+
+namespace IntegrationTests;
+
+public class ServerIdentificationTests : IClassFixture<DatabaseFixture>, IDisposable
+{
+	public ServerIdentificationTests(DatabaseFixture database)
+	{
+		m_database = database;
+	}
+
+	public void Dispose()
+	{
+	}
+
+	[SkippableFact(ServerFeatures.Timeout)]
+	public void CancelCommand_WithServerVerification()
+	{
+		// This test verifies that cancellation still works with server verification
+		using var connection = new MySqlConnection(AppConfig.ConnectionString);
+		connection.Open();
+
+		using var cmd = new MySqlCommand("SELECT SLEEP(5)", connection);
+		var task = Task.Run(async () =>
+		{
+			await Task.Delay(TimeSpan.FromSeconds(0.5));
+			cmd.Cancel();
+		});
+
+		var stopwatch = Stopwatch.StartNew();
+		TestUtilities.AssertExecuteScalarReturnsOneOrIsCanceled(cmd);
+		Assert.InRange(stopwatch.ElapsedMilliseconds, 250, 2500);
+
+#pragma warning disable xUnit1031 // Do not use blocking task operations in test method
+		task.Wait(); // shouldn't throw
+#pragma warning restore xUnit1031 // Do not use blocking task operations in test method
+	}
+
+	[SkippableFact(ServerFeatures.KnownCertificateAuthority)]
+	public void ServerHasServerHostname()
+	{
+		using var connection = new MySqlConnection(AppConfig.ConnectionString);
+		connection.Open();
+
+		// Test that we can query server hostname
+		using var cmd = new MySqlCommand("SELECT @@hostname", connection);
+		var hostname = cmd.ExecuteScalar();
+
+		// Hostname might be null on some server configurations, but the query should succeed
+	}
+
+	private readonly DatabaseFixture m_database;
+}