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Reinitialize server MAC HashAlgorithm between packets to prevent spurious MAC errors #1834

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Reinitialize server MAC HashAlgorithm between packets to prevent spurious MAC errors #1834

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Copilot Sep 1, 2026
7366401
Fix: reinitialize server MAC HashAlgorithm after each packet verifica…
Copilot Sep 1, 2026
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18 changes: 16 additions & 2 deletions src/Renci.SshNet/Session.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -1323,7 +1323,14 @@ private Message ReceiveMessage(Socket socket)

_ = _serverMac.TransformFinalBlock(Array.Empty<byte>(), 0, 0);

if (!CryptoAbstraction.FixedTimeEquals(_serverMac.Hash, _receiveBuffer.ActiveSpan.Slice(totalPacketLength - serverMacLength, serverMacLength)))
var macIsValid = CryptoAbstraction.FixedTimeEquals(_serverMac.Hash, _receiveBuffer.ActiveSpan.Slice(totalPacketLength - serverMacLength, serverMacLength));

// Not all HashAlgorithm implementations reset their internal state after
// TransformFinalBlock(), so we need to explicitly reinitialize the algorithm
// before it can be reused to compute the MAC of the next packet.
_serverMac.Initialize();

if (!macIsValid)
{
throw new SshConnectionException("MAC error", DisconnectReason.MacError);
}
Expand Down Expand Up @@ -1370,7 +1377,14 @@ private Message ReceiveMessage(Socket socket)

_ = _serverMac.TransformFinalBlock(Array.Empty<byte>(), 0, 0);

if (!CryptoAbstraction.FixedTimeEquals(_serverMac.Hash, _receiveBuffer.ActiveSpan.Slice(totalPacketLength - serverMacLength, serverMacLength)))
var macIsValid = CryptoAbstraction.FixedTimeEquals(_serverMac.Hash, _receiveBuffer.ActiveSpan.Slice(totalPacketLength - serverMacLength, serverMacLength));

// Not all HashAlgorithm implementations reset their internal state after
// TransformFinalBlock(), so we need to explicitly reinitialize the algorithm
// before it can be reused to compute the MAC of the next packet.
_serverMac.Initialize();

if (!macIsValid)
{
throw new SshConnectionException("MAC error", DisconnectReason.MacError);
}
Expand Down