Malicious PDF Detection is important! 129 Adobe Reader CVE's in 2015 - PowerPoint PPT Presentation

Extract Me If You Can: Abusing PDF Parsers in Malware Detectors Curtis Carmony, Mu Zhang, Xunchao Hu, Abhishek Vasisht Bhaskar, and Heng Yin Department of EECS, Syracuse University College of Engineering L.C.Smith and Computer Science

Malicious PDF Detection is important! • 129 Adobe Reader CVE's in 2015 • Up from 44 in 2014 • Existing detection techniques have limitations • Malicious PDF detection is difficult: • The PDF format is very complex and evolving • Adobe Reader with often process PDFs deviating from the specification in an attempt to “just work” 2

Existing Malicious PDF Detection Methods Technique Detectors Detection Parser Evasion Capability Requirement Techniques Signature-based AV Scanners Varies Low - Medium Malware Shafiq et al. Polymorphism PDF Malware Slayer Medium Medium Mimicry Attack Metadata & Structure -based PDFrate Reverse Mimicry Š rndi ć and Laskov Attack JavaScript-based Liu et al. Varies High MDScan PJScan 3

Parsing Matters • We need to actually look for malicious content • JavaScript based detection methods are most likely to detect modern threats, but have highest parser requirements • Successful malicious PDF detection depends on accurate and reliable parsing 4

Hypotheses • Significant parsing discrepancies between detectors and Adobe Reader likely exist • By improving the parser and removing these discrepancies existing detection methods can be improved 5

The Reference Extractor • To evaluate our hypotheses we need to know: • Which files Adobe Reader will actually open and those which it will not • Precisely the JS Adobe Reader executes • We can modify Adobe Reader to produce this information – “reference extractor” • Each reference extractor is specific to a version of Adobe Reader • We need a technique which is robust and repeatable • Mostly-automatic/low level of manual effort 6

Development of the Reference Extractor • Identify “tap points” – locations in Adobe Reader binary where we can extract information: • processing termination – indicates Adobe Reader has finished initial processing of file • processing error – indicates Adobe Reader has encountered an error during initial processing • JavaScript extraction – yields a reference to all executed JavaScript 7

Development of the Reference Extractor 8

Tap Point Identification • Processing Error/Processing Termination tap points: • Compare execution traces to identify basic-blocks executed precisely when the conditions for each tap point are met • JavaScript extraction tap point: • Group memory accesses into contiguous memory operations • Look for JavaScript which we know was executed • Based on existing technique (Dolan-Gavitt et al. ’13) • Full details are in paper 9

Reference Extractor Deployment 10

Data Set • Collected 163,306 PDF’s from VT, no restrictions • Ran them through two reference extractors and four open source tools • 5,267 were identified as containing JavaScript by any single tool • 1,453 of the samples we consider malicious with 15 or more VT detections 11

Differential Analysis Results Version 9.5.0 Reference Extractor libpfjs jsunpack-n Origami PDFiD Total 4397 4625 5053 4508 4398 Matches - 3940 4247 3863 3721 Invalid (ben./mal.) - 7(7/0) 26(10/16) 23(0/23) - Zero (ben./mal.) - 450(20/430) 124(113/11) 511(76/435) 676(253/423) Inconclusive - 356 500 318 677 Version 11.0.08 Reference Extractor libpfjs jsunpack-n Origami PDFiD Total 4704 4625 5053 4508 4398 Matches - 4269 4537 4167 3904 Invalid (ben./mal.) - 0(0/0) 16(0/16) 23(0/23) - Zero (ben./mal.) - 435(6/429) 151(140/11) 514(80/434) 800(377/423) Inconclusive - 356 500 318 494 12

Failings and Limitations Affected Extractors libpdfjs jsunpack-n Origami û û ü Comment in trailer û ü ü Comment in dictionary û ü û Trailing whitespace in stream data Security handler revision 5 hex encoded encryption Implementation bugs û ü û data parsing Security handler revision 3, 4 encryption key û ü û computation û ü û Hexadecimal string literal in encoded objects û ü ü Use of orphaned encryption objects Design Errors Security handler revision 5 encryption key û ü û computation without encrypted metadata ü û û No XFA support Omissions ü û û No security handler revision 5 support ü û û No security handler revision 6 support No cross-reference table and invalid object Ambiguities û û ü keywords 19

Malicious PDF Detection is important! 129 Adobe Reader CVE's in 2015 - PowerPoint PPT Presentation

Extract Me If You Can: Abusing PDF Parsers in Malware Detectors Curtis Carmony, Mu Zhang, Xunchao Hu, Abhishek Vasisht Bhaskar, and Heng Yin Department of EECS, Syracuse University College of Engineering L.C.Smith and Computer Science Malicious

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Malicious PDF Detection is important! 129 Adobe Reader CVE's in 2015 - PowerPoint PPT Presentation

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