-
Notifications
You must be signed in to change notification settings - Fork 11
/
viruses.html
1549 lines (1444 loc) · 75.6 KB
/
viruses.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE html>
<html lang="en"><head>
<script src="quarto_files/clipboard/clipboard.min.js"></script>
<script src="quarto_files/quarto-html/tabby.min.js"></script>
<script src="quarto_files/quarto-html/popper.min.js"></script>
<script src="quarto_files/quarto-html/tippy.umd.min.js"></script>
<link href="quarto_files/quarto-html/tippy.css" rel="stylesheet">
<link href="quarto_files/quarto-html/light-border.css" rel="stylesheet">
<link href="quarto_files/quarto-html/quarto-html.min.css" rel="stylesheet" data-mode="light">
<link href="quarto_files/quarto-html/quarto-syntax-highlighting-dark.css" rel="stylesheet" id="quarto-text-highlighting-styles"><meta charset="utf-8">
<meta name="generator" content="quarto-1.5.56">
<title>Viruses</title>
<meta name="apple-mobile-web-app-capable" content="yes">
<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no, minimal-ui">
<link rel="stylesheet" href="quarto_files/revealjs/dist/reset.css">
<link rel="stylesheet" href="quarto_files/revealjs/dist/reveal.css">
<style>
code{white-space: pre-wrap;}
span.smallcaps{font-variant: small-caps;}
div.columns{display: flex; gap: min(4vw, 1.5em);}
div.column{flex: auto; overflow-x: auto;}
div.hanging-indent{margin-left: 1.5em; text-indent: -1.5em;}
ul.task-list{list-style: none;}
ul.task-list li input[type="checkbox"] {
width: 0.8em;
margin: 0 0.8em 0.2em -1em; /* quarto-specific, see https://github.com/quarto-dev/quarto-cli/issues/4556 */
vertical-align: middle;
}
/* CSS for syntax highlighting */
pre > code.sourceCode { white-space: pre; position: relative; }
pre > code.sourceCode > span { line-height: 1.25; }
pre > code.sourceCode > span:empty { height: 1.2em; }
.sourceCode { overflow: visible; }
code.sourceCode > span { color: inherit; text-decoration: inherit; }
div.sourceCode { margin: 1em 0; }
pre.sourceCode { margin: 0; }
@media screen {
div.sourceCode { overflow: auto; }
}
@media print {
pre > code.sourceCode { white-space: pre-wrap; }
pre > code.sourceCode > span { display: inline-block; text-indent: -5em; padding-left: 5em; }
}
pre.numberSource code
{ counter-reset: source-line 0; }
pre.numberSource code > span
{ position: relative; left: -4em; counter-increment: source-line; }
pre.numberSource code > span > a:first-child::before
{ content: counter(source-line);
position: relative; left: -1em; text-align: right; vertical-align: baseline;
border: none; display: inline-block;
-webkit-touch-callout: none; -webkit-user-select: none;
-khtml-user-select: none; -moz-user-select: none;
-ms-user-select: none; user-select: none;
padding: 0 4px; width: 4em;
}
pre.numberSource { margin-left: 3em; padding-left: 4px; }
div.sourceCode
{ color: #f8f8f2; }
@media screen {
pre > code.sourceCode > span > a:first-child::before { text-decoration: underline; }
}
code span { color: #f8f8f2; } /* Normal */
code span.al { color: #f07178; background-color: #2a0f15; font-weight: bold; } /* Alert */
code span.an { color: #d4d0ab; } /* Annotation */
code span.at { color: #00e0e0; } /* Attribute */
code span.bn { color: #d4d0ab; } /* BaseN */
code span.bu { color: #abe338; } /* BuiltIn */
code span.cf { color: #ffa07a; font-weight: bold; } /* ControlFlow */
code span.ch { color: #abe338; } /* Char */
code span.cn { color: #ffd700; } /* Constant */
code span.co { color: #f8f8f2; font-style: italic; } /* Comment */
code span.cv { color: #ffd700; } /* CommentVar */
code span.do { color: #f8f8f2; } /* Documentation */
code span.dt { color: #ffa07a; } /* DataType */
code span.dv { color: #d4d0ab; } /* DecVal */
code span.er { color: #f07178; text-decoration: underline; } /* Error */
code span.ex { color: #00e0e0; font-weight: bold; } /* Extension */
code span.fl { color: #d4d0ab; } /* Float */
code span.fu { color: #ffa07a; } /* Function */
code span.im { color: #abe338; } /* Import */
code span.in { color: #d4d0ab; } /* Information */
code span.kw { color: #ffa07a; font-weight: bold; } /* Keyword */
code span.op { color: #ffa07a; } /* Operator */
code span.ot { color: #00e0e0; } /* Other */
code span.pp { color: #dcc6e0; } /* Preprocessor */
code span.re { color: #00e0e0; background-color: #f8f8f2; } /* RegionMarker */
code span.sc { color: #abe338; } /* SpecialChar */
code span.ss { color: #abe338; } /* SpecialString */
code span.st { color: #abe338; } /* String */
code span.va { color: #00e0e0; } /* Variable */
code span.vs { color: #abe338; } /* VerbatimString */
code span.wa { color: #dcc6e0; } /* Warning */
</style>
<link rel="stylesheet" href="quarto_files/revealjs/dist/theme/quarto.css">
<link rel="stylesheet" href="ics.css">
<link href="quarto_files/revealjs/plugin/quarto-line-highlight/line-highlight.css" rel="stylesheet">
<link href="quarto_files/revealjs/plugin/reveal-menu/menu.css" rel="stylesheet">
<link href="quarto_files/revealjs/plugin/reveal-menu/quarto-menu.css" rel="stylesheet">
<link href="quarto_files/revealjs/plugin/reveal-chalkboard/font-awesome/css/all.css" rel="stylesheet">
<link href="quarto_files/revealjs/plugin/reveal-chalkboard/style.css" rel="stylesheet">
<link href="quarto_files/revealjs/plugin/quarto-support/footer.css" rel="stylesheet">
<style type="text/css">
.callout {
margin-top: 1em;
margin-bottom: 1em;
border-radius: .25rem;
}
.callout.callout-style-simple {
padding: 0em 0.5em;
border-left: solid #acacac .3rem;
border-right: solid 1px silver;
border-top: solid 1px silver;
border-bottom: solid 1px silver;
display: flex;
}
.callout.callout-style-default {
border-left: solid #acacac .3rem;
border-right: solid 1px silver;
border-top: solid 1px silver;
border-bottom: solid 1px silver;
}
.callout .callout-body-container {
flex-grow: 1;
}
.callout.callout-style-simple .callout-body {
font-size: 1rem;
font-weight: 400;
}
.callout.callout-style-default .callout-body {
font-size: 0.9rem;
font-weight: 400;
}
.callout.callout-titled.callout-style-simple .callout-body {
margin-top: 0.2em;
}
.callout:not(.callout-titled) .callout-body {
display: flex;
}
.callout:not(.no-icon).callout-titled.callout-style-simple .callout-content {
padding-left: 1.6em;
}
.callout.callout-titled .callout-header {
padding-top: 0.2em;
margin-bottom: -0.2em;
}
.callout.callout-titled .callout-title p {
margin-top: 0.5em;
margin-bottom: 0.5em;
}
.callout.callout-titled.callout-style-simple .callout-content p {
margin-top: 0;
}
.callout.callout-titled.callout-style-default .callout-content p {
margin-top: 0.7em;
}
.callout.callout-style-simple div.callout-title {
border-bottom: none;
font-size: .9rem;
font-weight: 600;
opacity: 75%;
}
.callout.callout-style-default div.callout-title {
border-bottom: none;
font-weight: 600;
opacity: 85%;
font-size: 0.9rem;
padding-left: 0.5em;
padding-right: 0.5em;
}
.callout.callout-style-default div.callout-content {
padding-left: 0.5em;
padding-right: 0.5em;
}
.callout.callout-style-simple .callout-icon::before {
height: 1rem;
width: 1rem;
display: inline-block;
content: "";
background-repeat: no-repeat;
background-size: 1rem 1rem;
}
.callout.callout-style-default .callout-icon::before {
height: 0.9rem;
width: 0.9rem;
display: inline-block;
content: "";
background-repeat: no-repeat;
background-size: 0.9rem 0.9rem;
}
.callout-title {
display: flex
}
.callout-icon::before {
margin-top: 1rem;
padding-right: .5rem;
}
.callout.no-icon::before {
display: none !important;
}
.callout.callout-titled .callout-body > .callout-content > :last-child {
padding-bottom: 0.5rem;
margin-bottom: 0;
}
.callout.callout-titled .callout-icon::before {
margin-top: .5rem;
padding-right: .5rem;
}
.callout:not(.callout-titled) .callout-icon::before {
margin-top: 1rem;
padding-right: .5rem;
}
/* Callout Types */
div.callout-note {
border-left-color: #4582ec !important;
}
div.callout-note .callout-icon::before {
background-image: url('data:image/png;base64,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');
}
div.callout-note.callout-style-default .callout-title {
background-color: #dae6fb
}
div.callout-important {
border-left-color: #d9534f !important;
}
div.callout-important .callout-icon::before {
background-image: url('data:image/png;base64,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');
}
div.callout-important.callout-style-default .callout-title {
background-color: #f7dddc
}
div.callout-warning {
border-left-color: #f0ad4e !important;
}
div.callout-warning .callout-icon::before {
background-image: url('data:image/png;base64,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');
}
div.callout-warning.callout-style-default .callout-title {
background-color: #fcefdc
}
div.callout-tip {
border-left-color: #02b875 !important;
}
div.callout-tip .callout-icon::before {
background-image: url('data:image/png;base64,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');
}
div.callout-tip.callout-style-default .callout-title {
background-color: #ccf1e3
}
div.callout-caution {
border-left-color: #fd7e14 !important;
}
div.callout-caution .callout-icon::before {
background-image: url('data:image/png;base64,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');
}
div.callout-caution.callout-style-default .callout-title {
background-color: #ffe5d0
}
</style>
<style type="text/css">
.reveal div.sourceCode {
margin: 0;
overflow: auto;
}
.reveal div.hanging-indent {
margin-left: 1em;
text-indent: -1em;
}
.reveal .slide:not(.center) {
height: 100%;
}
.reveal .slide.scrollable {
overflow-y: auto;
}
.reveal .footnotes {
height: 100%;
overflow-y: auto;
}
.reveal .slide .absolute {
position: absolute;
display: block;
}
.reveal .footnotes ol {
counter-reset: ol;
list-style-type: none;
margin-left: 0;
}
.reveal .footnotes ol li:before {
counter-increment: ol;
content: counter(ol) ". ";
}
.reveal .footnotes ol li > p:first-child {
display: inline-block;
}
.reveal .slide ul,
.reveal .slide ol {
margin-bottom: 0.5em;
}
.reveal .slide ul li,
.reveal .slide ol li {
margin-top: 0.4em;
margin-bottom: 0.2em;
}
.reveal .slide ul[role="tablist"] li {
margin-bottom: 0;
}
.reveal .slide ul li > *:first-child,
.reveal .slide ol li > *:first-child {
margin-block-start: 0;
}
.reveal .slide ul li > *:last-child,
.reveal .slide ol li > *:last-child {
margin-block-end: 0;
}
.reveal .slide .columns:nth-child(3) {
margin-block-start: 0.8em;
}
.reveal blockquote {
box-shadow: none;
}
.reveal .tippy-content>* {
margin-top: 0.2em;
margin-bottom: 0.7em;
}
.reveal .tippy-content>*:last-child {
margin-bottom: 0.2em;
}
.reveal .slide > img.stretch.quarto-figure-center,
.reveal .slide > img.r-stretch.quarto-figure-center {
display: block;
margin-left: auto;
margin-right: auto;
}
.reveal .slide > img.stretch.quarto-figure-left,
.reveal .slide > img.r-stretch.quarto-figure-left {
display: block;
margin-left: 0;
margin-right: auto;
}
.reveal .slide > img.stretch.quarto-figure-right,
.reveal .slide > img.r-stretch.quarto-figure-right {
display: block;
margin-left: auto;
margin-right: 0;
}
</style>
</head>
<body class="quarto-dark">
<div class="reveal">
<div class="slides">
<section id="title-slide" class="quarto-title-block center">
<h1 class="title">CS 3710</h1>
<h2 class="subtitle">Introduction to Cybersecurity</h2>
<p class="titlep"> </p>
<div class="titlesmall"><p>
<a href="http://www.cs.virginia.edu/~asb">Aaron Bloomfield</a> ([email protected])<br>
<a href="http://github.com/aaronbloomfield/ccc">@github</a> | <a href="index.html">↑</a> | <a href="?print-pdf"><img class="print" width="20" src="../slides/images/print-icon.svg" style="top:0px;vertical-align:middle;background-color:transparent"></a>
</p></div>
<p class="titlep"> </p>
<h2 class="subtitle">Viruses</h2>
</section><section id="TOC">
<nav role="doc-toc">
<h2 id="toc-title">Contents</h2>
<ul>
<li><a href="#/introduction" id="/toc-introduction">Introduction</a></li>
<li><a href="#/file-formats" id="/toc-file-formats">File Formats</a></li>
<li><a href="#/virus-infections" id="/toc-virus-infections">Virus Infections</a></li>
<li><a href="#/basic-virus-stealth" id="/toc-basic-virus-stealth">Basic Virus Stealth</a></li>
<li><a href="#/advanced-virus-stealth" id="/toc-advanced-virus-stealth">Advanced Virus Stealth</a></li>
<li><a href="#/even-more-advanced-virus-stealth" id="/toc-even-more-advanced-virus-stealth">Even More Advanced Virus Stealth</a></li>
</ul>
</nav>
</section>
<section>
<section id="introduction" class="title-slide slide level1 center">
<h1>Introduction</h1>
</section>
<section id="computer-virus" class="slide level2">
<h2>Computer Virus</h2>
<ul>
<li>Malware that:
<ul>
<li>Alters an executable file and inserts its own code in there
<ul>
<li>Often a binary executable, but can be a script, batch file, etc.</li>
</ul></li>
<li>At some point during execution, control is transferred to the virus
<ul>
<li>Or at least the transfer of control is attempted…</li>
</ul></li>
<li>Often (but not always!) has a <em>payload</em></li>
</ul></li>
</ul>
</section>
<section id="why-windows-as-a-target" class="slide level2">
<h2>Why Windows as a target?</h2>
<ul>
<li>Most viruses affect Windows – but why?</li>
<li>Market share!
<ul>
<li>As of September 2018, Windows has 82% of the market, Mac OS X 13%, Linux 2% (<a href="http://gs.statcounter.com/os-market-share/desktop/worldwide/">source</a>)
<ul>
<li>That is home computer share, and does not include, say, Amazon’s clusters</li>
<li>Android is 25% of all viruses, but that is not included above</li>
</ul></li>
</ul></li>
<li>Relative weak security, compared to Linux and Mac OS X</li>
<li>Main user often has admin access</li>
</ul>
</section>
<section id="virus-arms-race" class="slide level2">
<h2>Virus arms race</h2>
<ul>
<li>The loop:
<ul>
<li>Virus writers will develop a creative new way to “hide” from the anti-virus scanners</li>
<li>Anti-virus scanners will adapt, and be able to detect the new virus</li>
</ul></li>
<li>Rinse, lather, repeat</li>
<li>This played out especially in the 1980’s and 1990’s
<ul>
<li>Nowadays malware has moved on to other infection methods</li>
</ul></li>
</ul>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
</section></section>
<section>
<section id="file-formats" class="title-slide slide level1 center">
<h1>File Formats</h1>
</section>
<section id="windows-pe-file-format" class="slide level2">
<h2>Windows PE file format</h2>
<div class="right-float-img img-size-800" style="zoom:125%">
<p><img data-src="images/viruses/pe-file-format.webp"></p>
</div>
<ul>
<li>Parts:
<ul>
<li>DOS header</li>
<li>DOS stub</li>
<li>PE header
<ul>
<li>Where Windows<br>looks…</li>
</ul></li>
<li>PE sections
<ul>
<li>.data section</li>
<li>.rdata section</li>
<li>.text section</li>
</ul></li>
</ul></li>
</ul>
<p>Image from <a href="http://resources.infosecinstitute.com/2-malware-researchers-handbook-demystifying-pe-file/#gref">here</a></p>
</section>
<section id="linux-elf-files" class="slide level2">
<h2>Linux ELF Files</h2>
<div class="right-float-img img-size-600 noboxshadow">
<p><img data-src="https://upload.wikimedia.org/wikipedia/commons/thumb/7/77/Elf-layout--en.svg/693px-Elf-layout--en.svg.png"></p>
</div>
<ul>
<li>Similar in concept to PE files
<ul>
<li>No DOS header, though!</li>
</ul></li>
<li>Same common sections
<ul>
<li>.data, .text, .rodata, etc.</li>
</ul></li>
<li>Meant as a generic format
<ul>
<li>Runs on Linux, Playstation,<br>many UNIX OSes,<br>Android (sorta), etc.</li>
</ul></li>
<li>Image from <a href="https://en.wikipedia.org/wiki/Executable_and_Linkable_Format">Wikipedia</a></li>
</ul>
</section>
<section id="pe-sections" class="slide level2">
<h2>PE Sections</h2>
<ul>
<li>.text: Code</li>
<li>.data: Initialized data</li>
<li>.bss: Uninitialized data</li>
<li>.rdata: Const/read-only (and initialized) data</li>
<li>.edata: Export descriptors</li>
<li>.idata: Import descriptors</li>
<li>.reloc: Relocation table</li>
<li>.rsrc: Resources (icon, bitmap, dialog, …)</li>
</ul>
<p>(shamelessly copied from <a href="https://stackoverflow.com/questions/19012300/whats-the-difference-between-rdata-and-idata-segments">here</a>)</p>
</section>
<section id="linking-a-pe-file" class="slide level2">
<h2>Linking a PE file</h2>
<div class="right-float-img img-size-600 noboxshadow">
<p><img data-src="images/viruses/pe-linking.png"></p>
</div>
<ul>
<li>Compiler produces .o files</li>
<li>Each .o has its own sections</li>
<li>Linker combines into the PE</li>
<li>Linker: the last compiler stage</li>
</ul>
</section>
<section id="linking-a-pe-file-1" class="slide level2">
<h2>Linking a PE file</h2>
<ul>
<li>Why the dead spaces?</li>
<li>Alignment restrictions</li>
<li>Perhaps 128 byte boundaries</li>
<li>Some linkers make PE file align to page boundaries
<ul>
<li>Memory pages are much bigger (1 Kb to 4 Kb)</li>
<li>This simplifies the loader’s job</li>
<li>But makes PE file bigger on disk</li>
</ul></li>
</ul>
</section>
<section id="pe-file-expansion" class="slide level2">
<h2>PE file expansion</h2>
<img data-src="images/viruses/pe-expansion.png" class="r-stretch"></section>
<section id="pe-file-expansion-1" class="slide level2">
<h2>PE file expansion</h2>
<ul>
<li>Why the expansion?
<ul>
<li>Memory will have different alignment restrictions than the file on disk</li>
<li>Loader increases dead spaces to use page boundaries, while alignment is to a lesser size (e.g. 128 bytes) in the PE file on disk</li>
<li>Page sizes are 1 Kb to 4 Kb</li>
</ul></li>
<li>The OS can place <em>restrictions</em> on pages (read-only, no-execute, etc)</li>
<li>Like unpacking a suitcase…</li>
</ul>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
</section></section>
<section>
<section id="virus-infections" class="title-slide slide level1 center">
<h1>Virus Infections</h1>
</section>
<section id="file-infection-modalities" class="slide level2">
<h2>File infection modalities</h2>
<ul>
<li>Beginning of .text section with Destructive Overwrite</li>
<li>Random Location in .text section with Destructive Overwrite
<ul>
<li>Will not always execute!</li>
</ul></li>
<li>Appending Viruses</li>
<li>Multiple Techniques</li>
<li>Cavity Viruses (possibly fractionated)</li>
<li>Compressing Viruses</li>
<li>Entry-Point Obscuring (EPO) Viruses
<ul>
<li>Including IAT replacement</li>
</ul></li>
</ul>
</section>
<section id="the-tricky-jump" class="slide level2">
<h2>The “tricky jump”</h2>
<ul>
<li>The most common technique to transfer control to a virus</li>
<li>Recall that the <code>ret</code> assembly command pops an address from the stack and then jumps there</li>
</ul>
</section>
<section id="the-tricky-jump-1" class="slide level2">
<h2>The “tricky jump”</h2>
<ul>
<li>Insert virus code somewhere
<ul>
<li>At the end, in a cavity, etc.</li>
</ul></li>
<li>Find a suitable <code>ret</code> in some subroutine</li>
<li>Overwrite the last few instructions in that subroutine with:
<ul>
<li><code>push [virus code address]</code></li>
<li><code>ret</code></li>
</ul></li>
<li>Saved instructions are put into the virus code
<ul>
<li>At the end, prior to the ret</li>
</ul></li>
</ul>
</section>
<section id="detecting-a-tricky-jump" class="slide level2">
<h2>Detecting a tricky jump</h2>
<ul>
<li>This is very easy to find!
<ul>
<li>Just look for a <code>push</code> followed immediately by a <code>ret</code>
<ul>
<li>No compiler would ever compile assembly that does that</li>
</ul></li>
</ul></li>
<li>Virus writers got creative:
<ul>
<li>Added a bunch of nops between (<code>add eax, 0</code>, etc.)</li>
</ul></li>
<li>Anti-virus researchers figured out how to detect those</li>
<li>So better stealth was needed…</li>
</ul>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
</section></section>
<section>
<section id="basic-virus-stealth" class="title-slide slide level1 center">
<h1>Basic Virus Stealth</h1>
</section>
<section id="viruses-must-be-stealthy" class="slide level2">
<h2>Viruses must be stealthy</h2>
<ul>
<li>Otherwise you immediately know something is wrong, and you fix it, removing the virus</li>
<li>Thus, the virus must:
<ul>
<li>Quickly execute its payload (and possible other infections)</li>
<li>Jump back to the “expected” behavior when done</li>
</ul></li>
</ul>
</section>
<section id="entry-point-obscuring-epo-viruses" class="slide level2">
<h2>Entry-Point Obscuring (EPO) Viruses</h2>
<ul>
<li>An EPO virus obscures its own entry point by finding a call instruction in the targeted PE file and “hijacking” the call so that the virus code is called instead</li>
<li>Thus, we find a <code>call</code> opcode and replace the target with the virus address</li>
<li>A good EPO virus will save all registers, and then restore them on the way out</li>
</ul>
</section>
<section id="detecting-call-hijacking-viruses" class="slide level2">
<h2>Detecting Call-Hijacking Viruses</h2>
<ul>
<li>Check if any of the <code>call</code> targets are outside the .text section of code</li>
<li>The .reloc section is examined by modern anti-virus software to see if it looks like a legitimate .reloc section
<ul>
<li>Code patterns such as saving state, tricky jumps, etc., can be detected in the .reloc section</li>
</ul></li>
</ul>
</section>
<section id="calling-library-functions" class="slide level2">
<h2>Calling library functions</h2>
<ul>
<li>When your binary code calls a library function
<ul>
<li>such as <code>printf()</code>, or <code><<</code> on <code>cout</code>, etc.</li>
</ul></li>
<li>The program consults the Import Address Table (in Windows)
<ul>
<li>aka the .dynsym section in Linux</li>
<li>aka the LC_DYLD_INFO section in Mac OS X</li>
</ul></li>
<li>And looks up the address (in the libc library) for <code>printf()</code></li>
<li>The <em>loader</em> patches the address for <code>printf()</code> (and other methods) into the IAT when the program starts up</li>
</ul>
</section>
<section id="epo-viruses-iat-replacement" class="slide level2">
<h2>EPO Viruses: IAT Replacement</h2>
<ul>
<li>A virus can change the address in the IAT to that of the virus code
<ul>
<li>It saves the “real” address and jumps there when done</li>
</ul></li>
<li>Several function pointers can be saved in the virus body, then replaced with pointers to the virus code</li>
<li>After the virus code is memory-resident, it can restore the IAT in memory so that the API is preserved and stealth is maintained</li>
</ul>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
</section></section>
<section>
<section id="advanced-virus-stealth" class="title-slide slide level1 center">
<h1>Advanced Virus Stealth</h1>
</section>
<section id="anti-disassembly-techniques" class="slide level2">
<h2>Anti-disassembly techniques</h2>
<ul>
<li>Prevent a disassembly (i.e., printout) to hinder analysis</li>
<li>Examples:
<ul>
<li>Encrypt the virus (machine) code (next slide)</li>
<li>Obfuscated computation (next next slide)</li>
<li>Using checksums (next next next slide)</li>
<li>Compressed code (like encrypted viruses)</li>
</ul></li>
</ul>
</section>
<section id="example-cascade-virus" class="slide level2">
<h2>Example: Cascade Virus</h2>
<ul>
<li>The simple decryptor of Cascade, circa 1990:</li>
</ul>
<div class="sourceCode" id="cb1"><pre class="sourceCode numberSource asm number-lines code-with-copy"><code class="sourceCode fasm"><span id="cb1-1"><a href=""></a> <span class="bu">lea</span> <span class="kw">si</span><span class="op">,</span>Start <span class="co">; start of encrypted code </span></span>
<span id="cb1-2"><a href=""></a> <span class="co">; (computed by virus)</span></span>
<span id="cb1-3"><a href=""></a> <span class="bu">mov</span> <span class="kw">sp</span><span class="op">,</span><span class="bn">0682h</span> <span class="co">; length of encrypted code (1666 bytes)</span></span>
<span id="cb1-4"><a href=""></a><span class="fu">Decrypt:</span></span>
<span id="cb1-5"><a href=""></a> <span class="bu">xor</span> <span class="op">[</span><span class="kw">si</span><span class="op">],</span><span class="kw">si</span> <span class="co">; xor code with its address</span></span>
<span id="cb1-6"><a href=""></a> <span class="bu">xor</span> <span class="op">[</span><span class="kw">si</span><span class="op">],</span><span class="kw">sp</span> <span class="co">; xor code with its inverse index</span></span>
<span id="cb1-7"><a href=""></a> <span class="bu">inc</span> <span class="kw">si</span> <span class="co">; increment address pointer</span></span>
<span id="cb1-8"><a href=""></a> <span class="bu">dec</span> <span class="kw">sp</span> <span class="co">; decrement byte counter</span></span>
<span id="cb1-9"><a href=""></a> <span class="cf">jnz</span> Decrypt <span class="co">; loop if more bytes to decrypt</span></span>
<span id="cb1-10"><a href=""></a><span class="fu">Start:</span> <span class="co">; virus code body</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<ul>
<li>Note that one of the <code>xor</code> commands is xor’ing it with the address, which will change with each infection
<ul>
<li>Thus, pattern matching won’t find the code!</li>
<li>But pattern matching can find the decryptor…</li>
</ul></li>
</ul>
</section>
<section id="obfuscated-computation" class="slide level2">
<h2>Obfuscated Computation</h2>
<ul>
<li>Example from Szor text, p. 223 (section 6.2.3):</li>
<li>Straightforward code to write 256 bytes into a file:</li>
</ul>
<div class="sourceCode" id="cb2"><pre class="sourceCode numberSource asm number-lines code-with-copy"><code class="sourceCode fasm"><span id="cb2-1"><a href=""></a><span class="bu">mov</span> <span class="kw">cx</span><span class="op">,</span> <span class="bn">100h</span> <span class="co">; 100h = 256 bytes to write</span></span>
<span id="cb2-2"><a href=""></a><span class="bu">mov</span> <span class="kw">ah</span><span class="op">,</span> <span class="bn">40h</span> <span class="co">; 40h = DOS function number</span></span>
<span id="cb2-3"><a href=""></a><span class="bu">int</span> <span class="bn">21h</span> <span class="co">; Invoke DOS handler</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<ul>
<li>Convoluted code to do the same thing:</li>
</ul>
<div class="sourceCode" id="cb3"><pre class="sourceCode numberSource asm number-lines code-with-copy"><code class="sourceCode fasm"><span id="cb3-1"><a href=""></a><span class="bu">mov</span> <span class="kw">cx</span><span class="op">,</span> <span class="dv">003</span><span class="er">Fh</span> <span class="co">; cx = 003fh</span></span>
<span id="cb3-2"><a href=""></a><span class="bu">inc</span> <span class="kw">cx</span> <span class="co">; cx = 0040h</span></span>
<span id="cb3-3"><a href=""></a><span class="bu">xchg</span> <span class="kw">ch</span><span class="op">,</span> <span class="kw">cl</span> <span class="co">; swap ch, cl (cx = 4000h)</span></span>
<span id="cb3-4"><a href=""></a><span class="bu">xchg</span> <span class="kw">ax</span><span class="op">,</span> <span class="kw">cx</span> <span class="co">; swap ax, cx (ax = 4000h)</span></span>
<span id="cb3-5"><a href=""></a><span class="bu">mov</span> <span class="kw">cx</span><span class="op">,</span> <span class="bn">0100h</span> <span class="co">; cx = 100h</span></span>
<span id="cb3-6"><a href=""></a><span class="bu">int</span> <span class="bn">21h</span> <span class="co">; Invoke DOS handler </span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</section>
<section id="checksums" class="slide level2">
<h2>Checksums</h2>
<ul>
<li>Instead of:</li>
</ul>
<div class="sourceCode" id="cb4"><pre class="sourceCode numberSource c number-lines code-with-copy"><code class="sourceCode c"><span id="cb4-1"><a href=""></a><span class="cf">for</span> <span class="op">(</span>each prototype in DLL export table<span class="op">)</span> </span>
<span id="cb4-2"><a href=""></a> <span class="cf">if</span> <span class="op">(</span><span class="dv">0</span> <span class="op">==</span> strcmp<span class="op">(</span>name<span class="op">,</span><span class="st">"GetFileHandle(int)"</span><span class="op">))</span></span>
<span id="cb4-3"><a href=""></a> infect<span class="op">(</span>current export table address<span class="op">);</span></span>
<span id="cb4-4"><a href=""></a>endfor</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
<p>which has the subroutine name clearly visible, use:</p>
<div class="sourceCode" id="cb5"><pre class="sourceCode numberSource c number-lines code-with-copy"><code class="sourceCode c"><span id="cb5-1"><a href=""></a><span class="dt">int</span> ConstantName <span class="op">=</span> <span class="bn">0x89f7e5b2</span><span class="op">;</span> <span class="co">// Computed by virus writer</span></span>
<span id="cb5-2"><a href=""></a><span class="cf">for</span> <span class="op">(</span>each prototype in DLL export table<span class="op">)</span></span>
<span id="cb5-3"><a href=""></a> <span class="cf">if</span> <span class="op">(</span>checksum<span class="op">(</span>name<span class="op">)</span> <span class="op">==</span> ConstantName<span class="op">)</span></span>
<span id="cb5-4"><a href=""></a> infect<span class="op">(</span>current export table address<span class="op">);</span></span>
<span id="cb5-5"><a href=""></a>endfor</span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</section>
<section id="anti-debugging-techniques" class="slide level2">
<h2>Anti-debugging techniques</h2>
<ul>
<li>Prevent tracing the code in a debugger</li>
<li>Techniques:
<ul>
<li>Hook interrupts 1 and 3 (next slide)</li>
<li>Pre-compute a checksum of code, so if it’s modified (by a debugger placing an interrupt there), it can choose not to execute</li>
<li>Detect debugger changes to the stack (next next slide)</li>
<li>Use the stack pointer as a counter for a decryption routine</li>
</ul></li>
</ul>
</section>
<section id="hooking-interrupts" class="slide level2">
<h2>Hooking interrupts</h2>
<ul>
<li>There are interrupts for single-stepping (INT 1) and breakpoints (INT 3)</li>
<li>A debugger will “register” (aka “hook”) the interrupt
<ul>
<li>This means telling the CPU to execute a particular address of code when that interrupt is called</li>
<li>It will then <em>overwrite</em> the instruction to break at with <code>INT 3</code>
<ul>
<li>Once in the handler, the original instruction is restored, and the debugger pauses at a break point</li>
</ul></li>
</ul></li>
</ul>
</section>
<section id="hooking-interrupts-contd" class="slide level2">
<h2>Hooking interrupts, cont’d</h2>
<ul>
<li>So a virus can put its code as an <em>interrupt handler</em> for either, and then call the interrupt directly
<ul>
<li>The virus won’t work under a debugger, then!</li>
</ul></li>
</ul>
</section>
<section id="detecting-stack-changes" class="slide level2">
<h2>Detecting Stack Changes</h2>
<ul>
<li>Without single-step debug state changes, a location on the stack will remain unchanged until an instruction changes it
<ul>
<li>But will be changed by the debugger after every instruction during single-step debug</li>
</ul></li>
<li>To detect this:</li>
</ul>
<div class="sourceCode" id="cb6"><pre class="sourceCode numberSource asm number-lines code-with-copy"><code class="sourceCode fasm"><span id="cb6-1"><a href=""></a><span class="bu">mov</span> <span class="kw">rbp</span><span class="op">,</span><span class="kw">rsp</span> <span class="co">; rbp gets current stack pointer</span></span>
<span id="cb6-2"><a href=""></a><span class="bu">push</span> <span class="kw">rax</span></span>
<span id="cb6-3"><a href=""></a><span class="bu">pop</span> <span class="kw">rax</span> <span class="co">; old pushed value still at [rbp-8]</span></span>
<span id="cb6-4"><a href=""></a> <span class="co">; which is beyond current stack</span></span>
<span id="cb6-5"><a href=""></a><span class="bu">cmp</span> <span class="dt">qword</span> <span class="dt">ptr</span> <span class="op">[</span><span class="kw">rbp</span><span class="op">-</span><span class="dv">8</span><span class="op">],</span><span class="kw">rax</span> <span class="co">; equal if no debugger</span></span>
<span id="cb6-6"><a href=""></a><span class="cf">jne</span> DEBUG <span class="co">; debugger detected! Go abort!</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</section>
<section id="anti-emulation-techniques" class="slide level2">
<h2>Anti-emulation techniques</h2>
<ul>
<li>Prevent emulation, either in a full virtual environment (such as VirtualBox) or just a regular emulator</li>
<li>Techniques:
<ul>
<li>Attempt to throw exceptions
<ul>
<li>Emulators have a hard time determining exactly when an exception will occur</li>
</ul></li>
<li>Use long idle loops
<ul>
<li>Takes longer in emulators</li>
</ul></li>
<li>Be a time or logic bomb
<ul>
<li>Won’t always work, then!</li>
</ul></li>
</ul></li>
</ul>
</section>
<section id="anti-heuristic-techniques" class="slide level2">
<h2>Anti-heuristic techniques</h2>
<ul>
<li>A heuristic is an algorithm for determining if there is a virus
<ul>
<li>Something that can find <em>if</em> a virus exists (by a tricky jump, say), if not necessarily the exact virus</li>
</ul></li>
<li>Techniques:
<ul>
<li>EPO viruses</li>
<li>Re-arrange PE files so the virus is not “appended” anymore</li>
</ul></li>
</ul>
</section>
<section id="anti-goat-techniques" class="slide level2">
<h2>Anti-goat techniques</h2>
<ul>
<li>A goat file (from the concept of sacrificial goats) is a dummy file of known content whose infection will signal the presence of a virus
<ul>
<li>Scattered around the disk in various file types that are prone to infection, e.g. .exe, .vbs, etc.</li>
</ul></li>
<li>Viruses examine files for goat file characteristics:
<ul>
<li>Lots of no-ops and do-nothing instructions</li>
<li>Clusters of files with sequential numbers in their names, e.g. abcd0001.vbs, abcd0002.vbs, etc.</li>
</ul></li>
</ul>
</section>
<section id="retroviruses" class="slide level2">
<h2>Retroviruses</h2>
<ul>
<li>Malware that attacks the defenses
<ul>
<li>In biology, that’s the immune system, such as what HIV does</li>
<li>For computers, that’s the anti-virus software</li>
</ul></li>
<li>Techniques:
<ul>
<li>Direct attack on the anti-virus software (rename the executable, kill the process, etc.)</li>
<li>Affect the integrity database</li>
<li>Deter anti-virus use with a “warning”</li>
</ul></li>
</ul>
<!-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -->
</section></section>
<section>
<section id="even-more-advanced-virus-stealth" class="title-slide slide level1 center">
<h1>Even More Advanced Virus Stealth</h1>
</section>
<section id="encrypted-viruses" class="slide level2">
<h2>Encrypted viruses</h2>
<ul>
<li>Encrypted viruses started appearing in the late 1980’s</li>
<li>One could not pattern match on the virus body…
<ul>
<li>… since part of the encryption was the <em>address</em> in the executable, which would vary with each different file infected</li>
</ul></li>
<li>So one had to mutate the <em>decryptor</em></li>
<li>This was all to prevent pattern matching detection</li>
</ul>
</section>
<section id="encryptor-mutation-types" class="slide level2">
<h2>Encryptor mutation types</h2>
<ul>
<li>Three types:
<ul>
<li>Oligomorphic have a pre-set number of different encryptors (say, a few dozen)</li>
<li>Polymorphic can permute the encryptor into millions or billions of variations</li>
<li>Metamorphic mutate the virus <em>body</em> into countless variations
<ul>
<li>Thus, no need for encryption!</li>
</ul></li>
</ul></li>
</ul>
</section>
<section id="assembly-code-permutation" class="slide level2">
<h2>Assembly code permutation</h2>
<ul>
<li>Add nop instructions: <code>nop</code>; <code>add rax, 0</code>; <code>shr rax, 0</code>; etc.</li>
<li>Instructions that modify registers that are not used by the code</li>
<li>Instructions that have a cumulative zero net effect: <code>inc rax</code> followed by <code>sub rax, 1</code></li>
<li>Re-ordering of instructions</li>
<li>Register replacement</li>
</ul>
</section>
<section id="oligomorphic-viruses" class="slide level2">
<h2>Oligomorphic viruses</h2>
<ul>
<li>Memorial was a Windows 95 oligomorphic virus that generated 96 different decryptors, choosing one at replication time
<ul>
<li>Detecting 96 different patterns is an impractical solution for virus scanners that must deal with thousands of viruses; pattern database size explosion would result</li>
</ul></li>
<li>It inserted junk instructions at various points in the decryptor code</li>
</ul>
</section>
<section id="polymorphic-viruses" class="slide level2">
<h2>Polymorphic viruses</h2>
<ul>
<li>First one was in 1990: <a href="https://en.wikipedia.org/wiki/1260_%28computer_virus%29">V2PX</a> or 1260 (because it was only 1260 bytes!)
<ul>
<li>Created by an anti-virus researcher as a proof-of-concept
<ul>
<li>Thus, it had no payload</li>
</ul></li>
</ul></li>
<li>Instructions were in three groups; within each group the instructions could have any order</li>
<li>A limited number of junk instructions could be inserted</li>
</ul>
</section>
<section id="the-1260-virus-decryptor" class="slide level2">
<h2>The 1260 Virus Decryptor</h2>
<ul>
<li>One instance of a decryptor:</li>
</ul>
<div class="sourceCode" id="cb7"><pre class="sourceCode numberSource asm number-lines code-with-copy"><code class="sourceCode fasm"><span id="cb7-1"><a href=""></a><span class="co">; Group 1: Prolog instructions</span></span>
<span id="cb7-2"><a href=""></a> <span class="bu">mov</span> <span class="kw">ax</span><span class="op">,</span><span class="dv">0</span><span class="er">E9Bh</span> <span class="co">; set key 1</span></span>
<span id="cb7-3"><a href=""></a> <span class="bu">mov</span> <span class="kw">di</span><span class="op">,</span><span class="dv">012</span><span class="er">Ah</span> <span class="co">; offset of virus Start</span></span>
<span id="cb7-4"><a href=""></a> <span class="bu">mov</span> <span class="kw">cx</span><span class="op">,</span><span class="bn">0571h</span> <span class="co">; byte count, used as key 2</span></span>
<span id="cb7-5"><a href=""></a><span class="co">; Group 2: Decryption instructions</span></span>
<span id="cb7-6"><a href=""></a><span class="fu">Decrypt:</span></span>
<span id="cb7-7"><a href=""></a> <span class="bu">xor</span> <span class="op">[</span><span class="kw">di</span><span class="op">],</span><span class="kw">cx</span> <span class="co">; decrypt first 16-bit word with key 2</span></span>
<span id="cb7-8"><a href=""></a> <span class="bu">xor</span> <span class="op">[</span><span class="kw">di</span><span class="op">],</span><span class="kw">ax</span> <span class="co">; decrypt first 16-bit word with key 1</span></span>
<span id="cb7-9"><a href=""></a><span class="co">; Group 3: Decryption instructions</span></span>
<span id="cb7-10"><a href=""></a> <span class="bu">inc</span> <span class="kw">di</span> <span class="co">; move on to next byte</span></span>
<span id="cb7-11"><a href=""></a> <span class="bu">inc</span> <span class="kw">ax</span> <span class="co">; slide key 1</span></span>
<span id="cb7-12"><a href=""></a><span class="co">; loop instruction (not part of Group 3)</span></span>
<span id="cb7-13"><a href=""></a> <span class="cf">loop</span> Decrypt <span class="co">; slide key 2 and loop back if not zero</span></span>
<span id="cb7-14"><a href=""></a><span class="co">; Random padding up to 39 bytes</span></span>
<span id="cb7-15"><a href=""></a><span class="fu">Start:</span> <span class="co">; encrypted virus body starts here</span></span></code><button title="Copy to Clipboard" class="code-copy-button"><i class="bi"></i></button></pre></div>
</section>
<section id="permutations" class="slide level2">
<h2>1260 Permutations</h2>
<ul>
<li>Group sizes were 3, 2, 2: total of 24 orderings of instructions</li>
<li>Could choose around a hundred thousand junk instructions</li>
<li>A limit of about 15 total junk instructions could be placed in 11 locations: several thousand possibilities</li>
<li>This yields around a billion variants
<ul>
<li>And in only 1260 bytes of (machine) code!</li>
</ul></li>
</ul>