Fix StartTLS stripping vulnerability

This fixes CVE-2021-32066.
Reported by Alexandr Savca in <https://hackerone.com/reports/1178562>.

1 files changed, 7 insertions, 1 deletions

diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 505b4c8950..d45304f289 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -1218,12 +1218,14 @@ module Net
       end
       resp = @tagged_responses.delete(tag)
       case resp.name
+      when /\A(?:OK)\z/ni
+        return resp
       when /\A(?:NO)\z/ni
         raise NoResponseError, resp
       when /\A(?:BAD)\z/ni
         raise BadResponseError, resp
       else
-        return resp
+        raise UnknownResponseError, resp
       end
     end
 
@@ -3719,6 +3721,10 @@ module Net
     class ByeResponseError < ResponseError
     end
 
+    # Error raised upon an unknown response from the server.
+    class UnknownResponseError < ResponseError
+    end
+
     RESPONSE_ERRORS = Hash.new(ResponseError)
     RESPONSE_ERRORS["NO"] = NoResponseError
     RESPONSE_ERRORS["BAD"] = BadResponseError