Merge pull request #4506 from warwickmm/mono_ssh_avoid_ecdsa_algorithms

Avoid ECDSA algorithms when using SFTP with Mono

1 files changed, 39 insertions, 2 deletions

diff --git a/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs b/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs
index 61241914d..14bb5264a 100644
--- a/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs
+++ b/Duplicati/Library/Backend/SSHv2/SSHv2Backend.cs
@@ -28,6 +28,7 @@ using System;
 using System.Collections.Generic;

 using System.Globalization;

 using System.Linq;

+using System.Security.Cryptography;

 using System.Threading;

 using System.Threading.Tasks;

 

@@ -58,6 +59,28 @@ namespace Duplicati.Library.Backend
 

         private SftpClient m_con;

 

+        private static readonly bool supportsECDSA;

+

+        static SSHv2()

+        {

+            // SSH.NET relies on the System.Security.Cryptography.ECDsaCng class for

+            // ECDSA algorithms, which is not implemented in Mono (as of 6.12.0.144).

+            // This prevents clients from connecting if one of the ECDSA algorithms is

+            // chosen as the host key algorithm.  In this case, we will prevent the

+            // client from advertising support for ECDSA algorithms.

+            //

+            // See https://github.com/mono/mono/blob/mono-6.12.0.144/mcs/class/referencesource/System.Core/System/Security/Cryptography/ECDsaCng.cs.

+            try

+            {

+                ECDsaCng unused = new ECDsaCng();

+                SSHv2.supportsECDSA = true;

+            }

+            catch

+            {

+                SSHv2.supportsECDSA = false;

+            }

+        }

+

         public SSHv2()

         {

         }

@@ -273,7 +296,7 @@ namespace Duplicati.Library.Backend
 

             if (m_con != null && !m_con.IsConnected)

             {

-                m_con.Connect();

+                this.TryConnect(m_con);

                 return;

             }

 

@@ -328,11 +351,25 @@ namespace Duplicati.Library.Backend
             if (m_keepaliveinterval.Ticks != 0)

                 con.KeepAliveInterval = m_keepaliveinterval;

 

-            con.Connect();

+            this.TryConnect(con);

 

             m_con = con;

         }

 

+        private void TryConnect(SftpClient client)

+        {

+            if (!SSHv2.supportsECDSA)

+            {

+                List<string> ecdsaKeys = client.ConnectionInfo.HostKeyAlgorithms.Keys.Where(x => x.StartsWith("ecdsa")).ToList();

+                foreach (string key in ecdsaKeys)

+                {

+                    client.ConnectionInfo.HostKeyAlgorithms.Remove(key);

+                }

+            }

+

+            client.Connect();

+        }

+

         private void ChangeDirectory(string path)

         {

             if (string.IsNullOrEmpty(path))