[Exim] auth check ok?

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Hi

I just wanted to verify that the authenticators i am planning on using are
secure. They are simply supposed to check a plain file for
username/passwords:

fixed_plain:
  driver = plaintext
  public_name = PLAIN
  server_prompts = :
  server_condition = "${if eq\
                      {${lookup{$2}lsearch{/usr/exim/auth}}$value}fail}}\
                      {$3} {yes}{no}}"
  server_set_id = $2

login:
  driver = plaintext
  public_name = LOGIN
  server_prompts = Username:: : Password::
  server_condition = "${if eq\
                      {${lookup{$1}lsearch{/usr/exim/auth}{$value}fail}}\
                      {$2} {yes}{no}}"
  server_set_id = $1

they appear to work in use, but are they secure?

thanks
Simon
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