A Survey of XOR as a Digital Obfuscation Technique in a Corpus of Real Data

Abstract

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden The Department of the Navy Approved for public release; distribution is unlimited.The views expressed in this report are those of the author and do not reflect the official policy or position of the Department of Defense or the U.S. Government.To determine the usage of XOR and the need to adapt additional tools, we analyzed 2,411 drive images from devices acquired around the world for the use of bytewise XOR as an obfuscation technique. Using a modified version of the open source digital forensics tool bulk˙extractor, evidence of XOR obfuscation was found on 698 drive images, with a maximum of 21,031 XOR-obfuscated features on a single drive. XOR usage in our corpus was observed in files with timestamps between the years 1995 and 2009, but the majority use was found in unallocated space. On the corpus tested, XOR obfuscation was used to circumvent malware detection and reverse engineering, to hide information that was apparently being exfiltrated, and by malware detection tools for their quarantine directory and to distribute malware signatures. We conclude that XOR obfuscation is important to consider when performing malware investigations. To determine the usage of XOR and the need to adapt additional tools, we analyzed 2,411 drive images from devices acquired around the world for the use of bytewise XOR as an obfuscation technique. Using a modified version of the open source digital forensics tool bulk_extractor, evidence of XOR obfuscation was found on 698 drive images, with a maximum of 21,031 XOR-obfuscated features on a single drive. XOR usage in our corpus was observed in files with timestamps between the years 1995 and 2009, but the majority use was found in unallocated space. On the corpus tested, XOR obfuscation was used to circumvent malware detection and reverse engineering, to hide information that was apparently being exfiltrated, and by malware detection tools for their quarantine directory and to distribute malware signatures. We conclude that XOR obfuscation is important to consider when performing malware investigations.