Problem with parse json in strings

[Vladimir_Martyshin]

Hello!
I am trying to parse logs. The log have field "message".
When I have message: Connection lost after 18 seconds all ok.

But, when I get the log with message: CMD: exit. I get the following error in logstash:

object mapping for [input] tried to parse field [input] as object, but found a concrete value

How can I say to logstash ignore all special symbols, like ":" and recognize it as simple string?
I tried this " convert => { "[message]" => "string" but it didn't help. Any help someone?

[fadjar340 (Fadjar Tandabawana)]

Try this to change the CMD:

filter {
  mutate {
	gsub => [
          "message", "CMD:", "CMD -"
       ]
  }
.
.
.
.
}