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Fix race between SendMessage and server-initiated key re-exchange #1823

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Fix race between SendMessage and server-initiated key re-exchange #1823

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665f9b5
Fix race between SendMessage and server-initiated key re-exchange
radekvermirovsky Aug 12, 2026
a1cc0a8
Make ProFTPD rekey test pass on CI
radekvermirovsky Aug 12, 2026
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159 changes: 89 additions & 70 deletions src/Renci.SshNet/Session.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1045,17 +1045,41 @@ internal void WaitOnHandle(WaitHandle waitHandle, TimeSpan timeout)
/// <exception cref="InvalidOperationException">The size of the packet exceeds the maximum size defined by the protocol.</exception>
internal void SendMessage(Message message)
{
if (!_socket.IsConnected())
while (true)
{
throw new SshConnectionException("Client not connected.");
}
if (!_socket.IsConnected())
{
throw new SshConnectionException("Client not connected.");
}

if (!_keyExchangeCompletedWaitHandle.IsSet && message is not IKeyExchangedAllowed)
{
// Wait for key exchange to be completed
WaitOnHandle(_keyExchangeCompletedWaitHandle.WaitHandle);
if (!_keyExchangeCompletedWaitHandle.IsSet && message is not IKeyExchangedAllowed)
{
// Wait for key exchange to be completed
WaitOnHandle(_keyExchangeCompletedWaitHandle.WaitHandle);
}

// take a write lock to ensure the outbound packet sequence number is incremented
// atomically, and only after the packet has actually been sent
lock (_socketWriteLock)
{
if (!_keyExchangeCompletedWaitHandle.IsSet && message is not IKeyExchangedAllowed)
{
// A key re-exchange started between the check above and acquiring the
// write lock. Our SSH_MSG_KEXINIT may already have been sent, in which
// case sending this message now would violate RFC 4253 section 7.1 and
// cause the server to drop the connection. Go back to waiting for the
// key exchange to complete.
continue;
}

SendMessageWithinWriteLock(message);
return;
}
}
}

private void SendMessageWithinWriteLock(Message message)
{
if (_logger.IsEnabled(LogLevel.Trace))
{
_logger.LogTrace("[{SessionId}] Sending message {MessageName}({MessageNumber}) to server: '{Message}'.", SessionIdHex, message.MessageName, message.MessageNumber, message.ToString());
Expand All @@ -1074,82 +1098,77 @@ internal void SendMessage(Message message)
macLength = _clientMac.HashSize / 8;
}

// take a write lock to ensure the outbound packet sequence number is incremented
// atomically, and only after the packet has actually been sent
lock (_socketWriteLock)
{
var activeBufferLength = message.GetPacket(
ref _sendBuffer,
paddingMultiplier,
_clientCompression,
_clientEtm || _clientAead,
macLength);
var activeBufferLength = message.GetPacket(
ref _sendBuffer,
paddingMultiplier,
_clientCompression,
_clientEtm || _clientAead,
macLength);

// write outbound packet sequence to start of packet data
BinaryPrimitives.WriteUInt32BigEndian(_sendBuffer, _outboundPacketSequence);
// write outbound packet sequence to start of packet data
BinaryPrimitives.WriteUInt32BigEndian(_sendBuffer, _outboundPacketSequence);

if (_clientMac != null && !_clientEtm)
{
// non-ETM mac = MAC(key, sequence_number || unencrypted_packet)
if (_clientMac != null && !_clientEtm)
{
// non-ETM mac = MAC(key, sequence_number || unencrypted_packet)

var hashSuccess = _clientMac.TryComputeHash(
buffer: _sendBuffer,
offset: 0,
count: activeBufferLength - macLength,
destination: _sendBuffer.AsSpan(activeBufferLength - macLength),
bytesWritten: out var bytesWritten);
var hashSuccess = _clientMac.TryComputeHash(
buffer: _sendBuffer,
offset: 0,
count: activeBufferLength - macLength,
destination: _sendBuffer.AsSpan(activeBufferLength - macLength),
bytesWritten: out var bytesWritten);

Debug.Assert(hashSuccess && bytesWritten == macLength);
}
Debug.Assert(hashSuccess && bytesWritten == macLength);
}

if (_clientCipher != null)
{
_clientCipher.SetSequenceNumber(_outboundPacketSequence);
if (_clientCipher != null)
{
_clientCipher.SetSequenceNumber(_outboundPacketSequence);

// Not encrypting the sequence number (it is not part of the packet),
// nor the packet length for ETM.
var offset = _clientEtm ? 8 : 4;
// Not encrypting the sequence number (it is not part of the packet),
// nor the packet length for ETM.
var offset = _clientEtm ? 8 : 4;

var numberOfBytesEncrypted = _clientCipher.Encrypt(
input: _sendBuffer,
offset,
length: activeBufferLength - offset - macLength,
output: _sendBuffer,
outputOffset: offset);
var numberOfBytesEncrypted = _clientCipher.Encrypt(
input: _sendBuffer,
offset,
length: activeBufferLength - offset - macLength,
output: _sendBuffer,
outputOffset: offset);

Debug.Assert(numberOfBytesEncrypted == activeBufferLength - offset - macLength + (_clientAead ? macLength : 0));
}
Debug.Assert(numberOfBytesEncrypted == activeBufferLength - offset - macLength + (_clientAead ? macLength : 0));
}

if (_clientMac != null && _clientEtm)
{
// ETM mac = MAC(key, sequence_number || packet_length || encrypted_packet)
if (_clientMac != null && _clientEtm)
{
// ETM mac = MAC(key, sequence_number || packet_length || encrypted_packet)

var hashSuccess = _clientMac.TryComputeHash(
buffer: _sendBuffer,
offset: 0,
count: activeBufferLength - macLength,
destination: _sendBuffer.AsSpan(activeBufferLength - macLength),
bytesWritten: out var bytesWritten);
var hashSuccess = _clientMac.TryComputeHash(
buffer: _sendBuffer,
offset: 0,
count: activeBufferLength - macLength,
destination: _sendBuffer.AsSpan(activeBufferLength - macLength),
bytesWritten: out var bytesWritten);

Debug.Assert(hashSuccess && bytesWritten == macLength);
}
Debug.Assert(hashSuccess && bytesWritten == macLength);
}

SendPacket(_sendBuffer, 4, activeBufferLength - 4);
SendPacket(_sendBuffer, 4, activeBufferLength - 4);

if (_isStrictKex && message is NewKeysMessage)
{
_outboundPacketSequence = 0;
}
else
{
// increment the packet sequence number only after we're sure the packet has
// been sent; even though it's only used for the MAC, it needs to be incremented
// for each package sent.
//
// the server will use it to verify the data integrity, and as such the order in
// which messages are sent must follow the outbound packet sequence number
_outboundPacketSequence++;
}
if (_isStrictKex && message is NewKeysMessage)
{
_outboundPacketSequence = 0;
}
else
{
// increment the packet sequence number only after we're sure the packet has
// been sent; even though it's only used for the MAC, it needs to be incremented
// for each package sent.
//
// the server will use it to verify the data integrity, and as such the order in
// which messages are sent must follow the outbound packet sequence number
_outboundPacketSequence++;
}
}

Expand Down
1 change: 1 addition & 0 deletions test/Renci.SshNet.IntegrationTests/.dockerignore
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Original file line number Diff line number Diff line change
@@ -1,3 +1,4 @@
*
!server/*
!user/*
!proftpd/*
45 changes: 45 additions & 0 deletions test/Renci.SshNet.IntegrationTests/Logging/TextWriterLogger.cs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,45 @@
#nullable enable

using Microsoft.Extensions.Logging;

namespace Renci.SshNet.IntegrationTests.Logging
{
internal class TextWriterLogger(TextWriter writer, string categoryName) : ILogger
{
public IDisposable? BeginScope<TState>(TState state)
where TState : notnull
{
return null;
}

public bool IsEnabled(LogLevel logLevel)
{
return true;
}

public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception, Func<TState, Exception?, string> formatter)
{
StringBuilder sb = new StringBuilder();
sb.Append(logLevel);
sb.Append(": ");
sb.Append(categoryName);
sb.Append(": ");

string message = formatter(state, exception);
sb.Append(message);

if (exception != null)
{
sb.Append(": ");
sb.Append(exception);
}

string line = sb.ToString();

lock (writer)
{
writer.WriteLine(line);
}
}
}
}
18 changes: 18 additions & 0 deletions test/Renci.SshNet.IntegrationTests/Logging/TextWriterLoggerProvider.cs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,18 @@
#nullable enable

using Microsoft.Extensions.Logging;

namespace Renci.SshNet.IntegrationTests.Logging
{
internal class TextWriterLoggerProvider(TextWriter writer) : ILoggerProvider
{
public ILogger CreateLogger(string categoryName)
{
return new TextWriterLogger(writer, categoryName);
}

public void Dispose()
{
}
}
}
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