fix: basic mitigation for scp's CVE-2020-15778

This CVE will not be fixed by scp authors, and as far as The Bastion
is concerned, this can't be achieved by anybody that doesn't already
have shell access to the remote server in addition to the scp rights,
but let's still block it for good measure.

bin/plugin/open/scp

@@ -12,6 +12,12 @@ use OVH::Result;
 use OVH::Bastion;
 use OVH::Bastion::Plugin qw( :DEFAULT );
 
+# stdout is used by scp, so ensure we output everything through stderr
+local $ENV{'FORCE_STDERR'} = 1;
+
+# don't output fancy stuff, this can get digested by scp and we get garbage output
+local $ENV{'PLUGIN_QUIET'} = 1;
+
 my ($scpCmd);
 my $remainingOptions = OVH::Bastion::Plugin::begin(
     argv    => \@ARGV,
@@ -97,7 +103,6 @@ if (not $host) {
 }
 
 if (not $ip) {
-
     # use STDERR because stdout is eaten by scp itself
     print STDERR "\nscp: Sorry, couldn't resolve the host you specified ('$host'), aborting.\n";
 
@@ -117,9 +122,8 @@ my $decoded = $scpCmd;
 $decoded =~ s/(?<!#)#(?!#)/ /g;
 $decoded =~ s/##/#/g;
 if ($decoded !~ /^(?:scp )(?:.*)-([tf]) (?:.+)$/) {
-
     # security
-    die "not scp ($decoded)";
+    osh_exit 'ERR_SECURITY_VIOLATION', "scp command format unrecognized";
 }
 my $userToCheck = $1 eq 't' ? '!scpupload' : '!scpdownload';    ## no critic (CaptureWithoutTest) ## false positive
 
@@ -194,22 +198,16 @@ foreach my $keyfile (keys %keys) {
 
 if (not $atleastonekey) {
     osh_exit('KO_ACCESS_DENIED',
-        "Sorry, you seem to have access through ssh and through scp but by different and distinct means (distinct keys). The intersection between your rights for ssh and for scp needs to be at least one."
-    );
+        "Sorry, you seem to have access through ssh and through scp but by different and distinct means (distinct keys)."
+          . " The intersection between your rights for ssh and for scp needs to be at least one.");
 }
 
-push @cmd, "--", $ip, $decoded;
-
-=cut attempt to be more secure than even standard scp, but don't bother ...
-my ($additionalParams,$remoteFile) = ($2,$3);
-
-push @cmd, 'scp';
-if (defined $additionalParams)
-{
-    push @cmd, split(/ /, $additionalParams);
+# basic mitigation for CVE-2020-15778
+if ($decoded =~ m{[\`\$\;><\|\&]}) {
+    osh_exit('ERR_SECURITY_VIOLATION', "Invalid characters detected, bailing out");
 }
-push @cmd, '-t', "\Q$remoteFile\E";
-=cut
+
+push @cmd, "--", $ip, $decoded;
 
 print STDERR ">>> Hello $self, transferring your file through the bastion "
   . ($userToCheck eq '!scpupload' ? 'to' : 'from')
@@ -218,8 +216,7 @@ print STDERR ">>> Hello $self, transferring your file through the bastion "
 #print STDERR join('^', @cmd)."\n";
 $fnret = OVH::Bastion::execute(cmd => \@cmd, expects_stdin => 1, is_binary => 1);
 if ($fnret->err ne 'OK') {
-    print STDERR ">>> Error launching transfer: " . $fnret->msg . "\n";
-    exit OVH::Bastion::EXIT_PLUGIN_ERROR;
+    osh_exit 'ERR_TRANSFER_FAILED', "Error launching transfer: $fnret";
 }
 print STDERR ">>> Done, "
   . $fnret->value->{'bytesnb'}{'stdin'}
@@ -229,5 +226,7 @@ print STDERR ">>> Done, "
 if ($fnret->value->{'sysret'} != 0) {
     print STDERR ">>> On bastion side, scp exited with return code " . $fnret->value->{'sysret'} . ".\n";
 }
+
+# don't use osh_exit() to avoid getting a footer
 exit OVH::Bastion::EXIT_OK;