CVE-2015-2320 (C#, published 2018-01-08T19:29Z)
Description: value: The TLS stack in Mono before 3.12.1 allows remote attackers to have unspecified impact via vectors related to client-side SSLv2 fallback.
Severity: HIGH (CVSS v3 base score: 9.8)
Weakness: CWE-295 \u2014 Improper Certificate Validation
The product does not validate, or incorrectly validates, a certificate.
Repository: https://github.com/mono/mono
Files changed:
mcs/class/Mono.Security/Mono.Security.Protocol.Tls/RecordProtocol.cs
Commit message: Remove the client-side SSLv2 fallback.

    There's almost no SSLv3 web site left so a v2 fallback is only extra
    code we do not need to carry forward.


Vulnerable code:
			switch (contentType)
			{
				case 0x80:
					return this.ReadClientHelloV2(record);

				default:
					if (!Enum.IsDefined(typeof(ContentType), (ContentType)contentType))
					{
						throw new TlsException(AlertDescription.DecodeError);
					}
					return this.ReadStandardRecordBuffer(record);
			}
		}

		private byte[] ReadClientHelloV2 (Stream record)
		{
			int msgLength = record.ReadByte ();
			// process further only if the whole record is available
			if (record.CanSeek && (msgLength + 1 > record.Length)) 
			{
				return null;
			}

			byte[] message = new byte[msgLength];
			record.Read (message, 0, msgLength);

			int msgType		= message [0];
			if (msgType != 1)
			{
				throw new TlsException(AlertDescription.DecodeError);
			}
			int protocol = (message [1] << 8 | message [2]);
			int cipherSpecLength = (message [3] << 8 | message [4]);
			int sessionIdLength = (message [5] << 8 | message [6]);
			int challengeLength = (message [7] << 8 | message [8]);
			int length = (challengeLength > 32) ? 32 : challengeLength;

			// Read CipherSpecs
			byte[] cipherSpecV2 = new byte[cipherSpecLength];
			Buffer.BlockCopy (message, 9, cipherSpecV2, 0, cipherSpecLength);

			// Read session ID
			byte[] sessionId = new byte[sessionIdLength];
			Buffer.BlockCopy (message, 9 + cipherSpecLength, sessionId, 0, sessionIdLength);

			// Read challenge ID
			byte[] challenge = new byte[challengeLength];
			Buffer.BlockCopy (message, 9 + cipherSpecLength + sessionIdLength, challenge, 0, challengeLength);
		
			if (challengeLength < 16 || cipherSpecLength == 0 || (cipherSpecLength % 3) != 0)
			// Updated the Session ID
			if (sessionId.Length > 0)
			{
				this.context.SessionId = sessionId;
			}

			// Update the protocol version
			this.Context.ChangeProtocol((short)protocol);

			// Select the Cipher suite
			this.ProcessCipherSpecV2Buffer(this.Context.SecurityProtocol, cipherSpecV2);

			// Updated the Client Random
			this.context.ClientRandom = new byte [32]; // Always 32
			// 1. if challenge is bigger than 32 bytes only use the last 32 bytes
			// 2. right justify (0) challenge in ClientRandom if less than 32
			Buffer.BlockCopy (challenge, challenge.Length - length, this.context.ClientRandom, 32 - length, length);

			// Set 
			this.context.LastHandshakeMsg = HandshakeType.ClientHello;
			this.context.ProtocolNegotiated = true;

			return message;
		}

		private byte[] ReadStandardRecordBuffer (Stream record)
		{

		#region CipherSpecV2 processing

		private void ProcessCipherSpecV2Buffer (SecurityProtocolType protocol, byte[] buffer)
		{
			TlsStream codes = new TlsStream(buffer);

			string prefix = (protocol == SecurityProtocolType.Ssl3) ? "SSL_" : "TLS_";

			while (codes.Position < codes.Length)
			{
				byte check = codes.ReadByte();

				if (check == 0)
				{
					// SSL/TLS cipher spec
					short code = codes.ReadInt16();	
					int index = this.Context.SupportedCiphers.IndexOf(code);
					if (index != -1)
					{
						this.Context.Negotiating.Cipher = this.Context.SupportedCiphers[index];
						break;
					}
				}
				else
				{
					byte[] tmp = new byte[2];
					codes.Read(tmp, 0, tmp.Length);

					int tmpCode = ((check & 0xff) << 16) | ((tmp[0] & 0xff) << 8) | (tmp[1] & 0xff);
					CipherSuite cipher = this.MapV2CipherCode(prefix, tmpCode);

					if (cipher != null)
					{
						this.Context.Negotiating.Cipher = cipher;
						break;
					}
				}
			}

			if (this.Context.Negotiating == null)
			{
				throw new TlsException(AlertDescription.InsuficientSecurity, "Insuficient Security");
			}
		}

		private CipherSuite MapV2CipherCode(string prefix, int code)
		{
			try
			{
				switch (code)
				{
					case 65664:
						// TLS_RC4_128_WITH_MD5
						return this.Context.SupportedCiphers[prefix + "RSA_WITH_RC4_128_MD5"];
					
					case 131200:
						// TLS_RC4_128_EXPORT40_WITH_MD5
						return this.Context.SupportedCiphers[prefix + "RSA_EXPORT_WITH_RC4_40_MD5"];
					
					case 196736:
						// TLS_RC2_CBC_128_CBC_WITH_MD5
						return this.Context.SupportedCiphers[prefix + "RSA_EXPORT_WITH_RC2_CBC_40_MD5"];
					
					case 262272:
						// TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
						return this.Context.SupportedCiphers[prefix + "RSA_EXPORT_WITH_RC2_CBC_40_MD5"];
					
					case 327808:
						// TLS_IDEA_128_CBC_WITH_MD5
						return null;
					
					case 393280:
						// TLS_DES_64_CBC_WITH_MD5
						return null;

					case 458944:
						// TLS_DES_192_EDE3_CBC_WITH_MD5
						return null;

					default:
						return null;
				}
			}
			catch
			{
				return null;
			}
		}

		#endregion

Fixed code:
			if (!Enum.IsDefined(typeof(ContentType), (ContentType)contentType))