libinterimap: abort on PREAUTH greeting received on plaintext connections

Set "STARTTLS = NO" to ignore.  This is similar to CVE-2020-12398 and
CVE-2020-14093.

1 files changed, 10 insertions, 1 deletions

diff --git a/lib/Net/IMAP/InterIMAP.pm b/lib/Net/IMAP/InterIMAP.pm
index f0dd2df..b01e1a9 100644
--- a/lib/Net/IMAP/InterIMAP.pm
+++ b/lib/Net/IMAP/InterIMAP.pm
@@ -464,6 +464,7 @@ sub new($%) {
             $self->logger('S: xxx ', $IMAP_text);
             $self->{debug} = $dbg;
         }
+        $self->{_STATE} = 'AUTH';
 
         unless ($IMAP_text =~ /\A\Q$IMAP_cond\E \[CAPABILITY /) {
             # refresh the CAPABILITY list since the previous one had only pre-login capabilities
@@ -471,7 +472,15 @@ sub new($%) {
             $self->capabilities();
         }
     }
-    $self->{_STATE} = 'AUTH';
+    elsif ($IMAP_cond eq 'PREAUTH') {
+        if ($self->{type} eq 'imap' and $self->{STARTTLS} != 0) {
+            $self->fail("PREAUTH greeting on plaintext connection? MiTM in action? Aborting, set \"STARTTLS = NO\" to ignore.");
+        }
+        $self->{_STATE} = 'AUTH';
+    }
+    else {
+        $self->panic();
+    }
 
     # Don't send the COMPRESS command before STARTTLS or AUTH, as per RFC 4978
     if ($self->{compress} // 1 and