Default SecurityProtocol in .NET 4.5

Some of the those leaving comments on other answers have noted that setting System.Net.ServicePointManager.SecurityProtocol to specific values means that your app won't be able to take advantage of future TLS versions that may become the default values in future updates to .NET. Instead of specifying a fixed list of protocols, do the following:

For .NET 4.7 or later, do not set System.Net.ServicePointManager.SecurityProtocol. The default value (SecurityProtocolType.SystemDefault) will allow the operating system to use whatever versions it knows and has been configured for, including any new versions that may not have existed at the time the app was created.

For earlier versions of .NET Framework, you can instead turn on or off protocols you know and care about, leaving any others as they are.

To turn on TLS 1.1 and 1.2 without affecting other protocols:

System.Net.ServicePointManager.SecurityProtocol |= 
    SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

Notice the use of |= to turn on these flags without turning others off.

To turn off SSL3 without affecting other protocols:

System.Net.ServicePointManager.SecurityProtocol &= ~SecurityProtocolType.Ssl3;

The default System.Net.ServicePointManager.SecurityProtocol in both .NET 4.0/4.5 is SecurityProtocolType.Tls|SecurityProtocolType.Ssl3.

.NET 4.0 supports up to TLS 1.0 while .NET 4.5 supports up to TLS 1.2

However, an application targeting .NET 4.0 can still support up to TLS 1.2 if .NET 4.5 is installed in the same environment. .NET 4.5 installs on top of .NET 4.0, replacing System.dll.

I've verified this by observing the correct security protocol set in traffic with fiddler4 and by manually setting the enumerated values in a .NET 4.0 project:

ServicePointManager.SecurityProtocol = (SecurityProtocolType)192 |
(SecurityProtocolType)768 | (SecurityProtocolType)3072;

Reference:

namespace System.Net
{
    [System.Flags]
    public enum SecurityProtocolType
    {
       Ssl3 = 48,
       Tls = 192,
       Tls11 = 768,
       Tls12 = 3072,
    }
}

If you attempt the hack on an environment with ONLY .NET 4.0 installed, you will get the exception:

Unhandled Exception: System.NotSupportedException: The requested security protocol is not supported. at System.Net.ServicePointManager.set_SecurityProtocol(SecurityProtocolType v alue)

However, I wouldn't recommend this "hack" since a future patch, etc. may break it.*

Therefore, I've decided the best route to remove support for SSLv3 is to:

1. Upgrade all applications to .NET 4.5

2. Add the following to boostrapping code to override the default and future proof it:

   System.Net.ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls | SecurityProtocolType.Tls11 | SecurityProtocolType.Tls12;

*Someone correct me if this hack is wrong, but initial tests I see it works