commit 95ba9053e20ad8d113af37b3f1f4cbfff1f6a8f1
Author: usa <usa@b2dd03c8-39d4-4d8f-98ff-823fe69b080e>
Date:   Wed Jul 7 10:38:10 2021 +0000

    Fix StartTLS stripping vulnerability
    
    Reported by Alexandr Savca in https://hackerone.com/reports/1178562
    
    Co-authored-by: Shugo Maeda <shugo@ruby-lang.org>
    
    
    git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67950 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 1c7e89ba14..91df89b79e 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -1213,12 +1213,14 @@ def get_tagged_response(tag, cmd)
       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
 
@@ -3714,6 +3716,10 @@ class BadResponseError < ResponseError
     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
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index 936f4e0f42..81928cb8fe 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -127,6 +127,24 @@ def test_starttls
         imap.disconnect
       end
     end
+
+    def test_starttls_stripping
+      starttls_stripping_test do |port|
+        imap = Net::IMAP.new("localhost", :port => port)
+        assert_raise(Net::IMAP::UnknownResponseError) do
+          imap.starttls(:ca_file => CA_FILE)
+        end
+        imap
+      end
+    end
+  end
+
+  def start_server
+    th = Thread.new do
+      yield
+    end
+    @threads << th
+    sleep 0.1 until th.stop?
   end
 
   def test_unexpected_eof
@@ -760,6 +760,27 @@ def starttls_test
     end
   end
 
+  def starttls_stripping_test
+    server = create_tcp_server
+    port = server.addr[1]
+    start_server do
+      sock = server.accept
+      begin
+        sock.print("* OK test server\r\n")
+        sock.gets
+        sock.print("RUBY0001 BUG unhandled command\r\n")
+      ensure
+        sock.close
+        server.close
+      end
+    end
+    begin
+      imap = yield(port)
+    ensure
+      imap.disconnect if imap && !imap.disconnected?
+    end
+  end
+
   def create_tcp_server
     return TCPServer.new(server_addr, 0)
   end