How it works
NOTE: This plugin is based on the OcrPlugin written by Maarten de Boer and was extended and improved.
This plugin checks for specific keywords in image/gif or image/jpeg attachments, using gocr (an optical character recognition program).
This plugin can be used to detect spam that puts all the real spam content in an attached image. The mail itself only random text and random html, without any URL's or identifiable information.
Additionally to the normal OcrPlugin, it can do approximate matches on words, so errors in recognition or attempts to obfuscate the text inside the image will not cause the detection to fail. Another improvement was to move the wordlist into the configuration file so it can be easily extended.
Requirements
You will need giftopnm and jpegtopnm (netpbm) and gocr installed.
Additionally, you will need the perl module
String::Approx
.
Installation
Save the two files below in your local configuration directory. Open FuzzyOcr.cf and extend the wordlist as you wish.
The scoring is dynamic, more word matches lead to a higher score. The scoring is done in FuzzyOcr.pm in line 74 (see ToDo) and is basically a fixed score for the first $countreq matches (default 4) + 1 Point for every additional match. This can be adjusted easily as you wish.
The variable $countreq can be adjusted via the configuration file parameter focr_counts_required and indicates the number of matches that need to be found before any score will be triggered.
The variable $treshold is similarly adjusted with the configuration file parameter focr_treshold. This is a float value between 0 and 1 and indicates the maximum relative edit distance between the wordlist word and the obfuscated version (less means the words need to be more similar, 0 means identical). The default of 0.3 normally does not need any change. Note that this module also matches substrings (see example).
Example of work
Lets say you have defined focr_word investor in your configuration. Now you receive an image which, after converted and recognized gives you:
ATTENTION ALL IN\lESTORS AND DAY TRADERS
Then the plugin will find the word investor. It would even succeed if the text was ATTENTION ALL STUPUDIN\lESTORSHAHA or INVSTORSZ etc.
Generally, the plugin follows these rules:
- The case is not relevant
- All special characters or numbers are stripped before any matching is done
- Your wordlist word will be found even if it is inside another word (submatching)
- The distance is calculated from the amount of character additions, deletions and substitutions, that need to be done.
Remarks
- The words checked for are specific for some spam I received a lot of recently.
gocrcan take up quite a bit of resources, so be careful. But it is only executed for messages that contain gif or jpeg attachments.
ToDo
- Make the score adjustable in the configuration.
- Avoid usage of tmp files for gocr, redirect output directly back to the script
– Author: Christian Holler, decoder_at_own-hero_dot_net
Code
FuzzyOcr.cf
loadplugin FuzzyOcr FuzzyOcr.pm body FUZZY_OCR eval:check_fuzzy_ocr() describe FUZZY_OCR Mail contains an image with common spam text inside # Here we defined the words to scan for focr_word stock focr_word investor focr_word international focr_word company focr_word money focr_word million focr_word thousand focr_word buy focr_word price focr_word trade focr_word banking focr_word service focr_word kunde focr_word volksbank focr_word sparkasse focr_word software focr_word viagra focr_word cialis focr_word levitra # These parameters can be used to change other detection settings # Normally these don't need to be changed. # #focr_treshold 0.3 #focr_counts_required 2
FuzzyOcr.pm
# FuzzyOcr plugin, version 1
# written by Christian Holler decoder_at_own-hero_dot_net
package FuzzyOcr;
use strict;
use Mail::SpamAssassin;
use Mail::SpamAssassin::Util;
use Mail::SpamAssassin::Plugin;
use String::Approx 'adistr';
our @ISA = qw (Mail::SpamAssassin::Plugin);
our @words = ( );
our $cnt = 0;
# Default values
our $treshold = "0.3";
our $countreq = 2;
# constructor: register the eval rule
sub new {
my ( $class, $mailsa ) = @_;
$class = ref($class) || $class;
my $self = $class->SUPER::new($mailsa);
bless( $self, $class );
$self->register_eval_rule("check_fuzzy_ocr");
return $self;
}
sub parse_config {
my ($self, $opts) = @_;
if ($opts->{key} eq "focr_word") {
push(@words, $opts->{value});
} elsif ($opts->{key} eq "focr_treshold") {
$treshold = $opts->{value};
} elsif ($opts->{key} eq "focr_counts_required") {
$countreq = $opts->{value};
}
}
sub check_fuzzy_ocr {
my ( $self, $pms ) = @_;
$cnt = 0;
foreach my $p ( $pms->{msg}->find_parts("image") ) {
my ( $ctype, $boundary, $charset, $name ) =
Mail::SpamAssassin::Util::parse_content_type(
$p->get_header('content-type') );
if (($ctype eq "image/gif") || ($ctype eq "image/jpeg")) {
if ($ctype eq "image/gif") {
open OCR, "|/usr/bin/giftopnm - |/usr/bin/gocr -i - > /tmp/spamassassin.focr.$$";
} else {
open OCR, "|/usr/bin/jpegtopnm - |/usr/bin/gocr -i - > /tmp/spamassassin.focr.$$";
}
foreach $p ( $p->decode() ) {
print OCR $p;
}
close OCR;
open OCR, "/tmp/spamassassin.focr.$$";
while (<OCR>) {
s/[^a-zA-Z ]//g;
$_ = lc;
my $w;
foreach $w (@words) {
$w = lc $w;
my $matched = adistr($w, $_);
if (abs($matched) < $treshold) {
$cnt++;
}
}
}
close OCR;
unlink "/tmp/spamassassin.focr.$$";
}
}
if ($cnt >= $countreq) {
my $score = 4 + ($cnt - $countreq);
$pms->_handle_hit("FUZZY_OCR", $score, "BODY: ", $pms->{conf}->{descriptions}->{FUZZY_OCR}." ($cnt word occurrences found)");
}
return 0;
}
1;