Banning .docm gives misleading error message

[Dino Edwards]

If you are trying to block office documents that will infect your PC with ransomware your approach will not work. The ransomware has been coming through with the old office document extensions. For example, the locky ransomware comes in with a .doc attachment. A more effective approach would be to block all old office extensions i.e. .xls, .doc etc.



From: amavis-users [mailto:amavis-users-bounces+dino.edwards=mydirectmail.net at amavis.org] On Behalf Of Kai Risku
Sent: Tuesday, April 5, 2016 2:54 AM
To: amavis-users at amavis.org
Subject: Banning .docm gives misleading error message

In order to guard against malicious macros, we have banned all macro-enabled Office document formats, i.e. added the following to $banned_filename_re:

# block macro-enabled office files
qr'.\.(xlsm|xltm|xlam|docm|dotm|pptm|potm|ppam|ppsm|sldm)$'i,

Since modern Office documents are technically zip-files, amavisd-new opens and processes the zip archive. For originating (outgoing) messages we bounce the banned emails so the poor sender can understand why his emails are not delivered, but in this case amavisd-new does not report the actual office document being banned but instead blames the first file inside the zip-archive. This results in very cryptic error messages, like:

                             Subject: BANNED contents from you (.txt,[Content_Types].xml)


Our content checker found

    banned name: .txt,[Content_Types].xml
                             in email presumable from you


It seems amavisd has a small “optimization” that skips banned checks for non-leaf nodes. I propose removing that so the actual office documents can be directly banned and correctly reported:

--- amavisd     5 Apr 2016 06:30:18 -0000       1.24
+++ amavisd     5 Apr 2016 06:30:29 -0000
@@ -9912,7 +9912,9 @@
       my(@path) = @{$part->path};
       next  if @path <= 1;
       shift(@path);  # ignore place-holder root node
-      next  if @{$part->children};  # ignore non-leaf nodes
+      # also process non-leaf nodes or we cannot block office documents
+      # without alert about wrong parts (blames the innocent zip member)
+      # next  if @{$part->children};  # ignore non-leaf nodes
       my(@descr_trad);  # a part path: list of predecessors of a message part
       my(@descr);  # same, but in form suitable for check on banned_namepath_re
       for my $p (@path) {



--
Kai.Risku at arrak.fi<mailto:Kai.Risku at arrak.fi>                     GSM +358-40-7678282
Oy Arrak Software Ab              http://www.arrak.fi

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