A simple ACMEv2 client for Windows (for use with Let's Encrypt et al.)


This plugin uploads the validation challenge to a (secure) FTP server.


Before allowing the ACME server to validate, the program will attempt to request the validation file itself and note the result of that request in the log. A side effect of this is that it forces the application to start in case it’s application pool or equivalent went to sleep, warming up the caches etc. This reduces the chance of time-outs during validation. This used to be optional behaviour controlled by the --warmup switch, but that is no longer in use.


Optionally this plugin can place a web.config next to the validation file, to help IIS properly serve the response. There are many reasons why IIS can fail to properly server the file. Some of them can be fixed this way.

In interactive mode the program will ask you if you want to do this. In unattended mode you can request it with the parameter --manualtargetisiis

Changing the template

The web.config that will be copied lives in the root of the program directory with the name web_config.xml. You can modify it to fit your needs, e.g. for MVC sites you might need:

            <mimeMap fileExtension = ".*" mimeType="text/json" />
            <clear />
            <add name="StaticFile" 
			responseBufferLimit="4194304" />

Or to disable URL Rewriting you might need to add this (in the beginning, right after <clear />).

<rule name="LetsEncrypt Rule" stopProcessing="true">
    <match url="^\.well-known.*$" />
    <action type="None" />

Microsoft TLS vs. GnuTLS

If you experience connection issues with Unix FTPS servers, using the GnuTLS library instead of Microsofts native TLS might solve the problem. This page by the FluentFTP project explains the reasons behind and limitations of this method.

Using this requires:

  • A change in settings.config: Validation.Ftp.UseGnuTls = true
  • The pluggable x64 release of win-acme (it is not available for x86 or ARM due to limitiations of the upstream package, and also doesn’t work on the trimmed build)
  • Download and extract the additonal artifact gnutls.v{build}


--validation ftp --webroot ftps://x/ --username admin --password ******