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Security researchers , beginning with Mark Russinovich in October 2005 , have described the program as functionally identical to a rootkit : a computer program used by computer hackers to conceal unauthorised activities on a computer system .
 Security, Mark Russinovich
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Security, Mark Russinovich