Monitoring for Physical Data Exfiltration with MDE advanced hunting

By Gianni Castaldi

Microsoft Defender for Endpoint collects a lot of information. One of the items is the mounting of external drives. The idea for this query started with the counting of files written to external storage. Unfortunately, this query wasn’t very actionable.

I’ve started using the function series_decompose_anomalies, which I think is a great way to create a more actionable query. So I began by doing anomaly detection on MB copied per day, but this wasn’t very clear. I created another part in the script to count file types, unique SHA hashes, and creating a set of all the files the user copied in 24 hours.

When I discussed the query with one of our auditors, he was very excited because this gives him a way to audit physical data exfiltration. It’s also a great addition to the cloud monitoring of Microsoft Cloud App Security.

The query consists of 9 steps:

  • Create arrays of file extensions
  • List all files copied to external storage
  • Create a column with the file extensions
  • Create a set of copied files and do several counts
  • List all files copied to external storage for the second time
  • Create a series for the anomaly detection based on copied size, by the user, system and per day
  • Do the anomaly detection and only display the items with an anomaly detected
  • Join the file information with the detected anomalies
  • And filter on MB copied or number of document files

To customize this script to your environment, you could change the file extension arrays or change the filtering at the end of the query.

// Set the amount of days to monitor
let StartTime = ago(7d);
// Create lists to categorize files based on their extension
let CertificateFileExtensions = dynamic([".crt",".cer",".ca-bundle",".p7b",".p7c",".p7s",".pem",".key",".keystore",".jks",".p12",".pfx",".pem"]);
let CompressedFileExtensions = dynamic([".7z",".arj",".deb",".pkg",".rar",".rpm",".gz",".z",".zip",".001",".002",".003",".004",".005",".006",".007",".008",".009",".010",".011",".012",".013",".014",".015",".016",".017",".018",".019",".020",".021",".022",".023",".024",".025",".026",".027",".028",".029",".030",".031",".032",".033",".034",".035",".036",".037",".038",".039",".040",".041",".042",".043",".044",".045",".046",".047",".048",".049",".050",".051",".052",".053",".054",".055",".056",".057",".058",".059",".060",".061",".062",".063",".064",".065",".066",".067",".068",".069",".070",".071",".072",".073",".074",".075",".076",".077",".078",".079",".080",".081",".082",".083",".084",".085",".086",".087",".088",".089",".090",".091",".092",".093",".094",".095",".096",".097",".098",".099"]);
let DatabaseFileExtensions = dynamic([".$ER",".3DB",".4MP",".ACAD",".ACCDB",".ACCDT",".ADE",".ADP",".APX",".AWDB",".BIB",".BTR",".CDB",".CLG",".CMA",".CRP",".CWDB",".DB",".DB2",".DB3",".DBF",".DBS",".DBW",".DBX",".DCX",".DF1",".DF2",".DF3",".DF4",".DNL",".DSD",".DTF",".FDB",".FP5",".FP7",".FW2",".FW3",".FW4",".GDB",".IND",".INX",".IPD",".ITDB",".JOD",".KDB",".LACCDB",".LDB",".LK",".MDB",".MDE",".MDF",".MDN",".MN4",".MODB",".MPD",".NCB",".NDB",".NDF",".NDX",".NS2",".NS3",".NS4",".NS5",".NSF",".NTF",".OD1",".OD2",".OD3",".OD4-9",".ODB",".OECL",".OIF",".OV",".PDB",".PDT",".PHD",".PHO",".PX",".RFP",".RPD",".RSD",".SD2",".SDB",".SQL",".SQLITE",".SSD",".SVY",".SWD",".SWDB",".TDB",".THM",".USR",".WD2",".WDB",".XG0",".XG1",".XG2",".XG3",".XVU",".ZBD"]);
let DocumentFileExtensions = dynamic([".ods",".xls",".xlsm",".xltm",".xlsx",".doc",".docx",".odt",".pdf",".ppt",".pptx",".rtf",".tex",".txt",".wpd",".csv",".dot",".dotm",".docm",".vsd",".vsdx"]);
let EmailFileExtensions = dynamic([".email",".eml",".emlx",".msg",".oft",".ost",".pst",".vcf"]);
let ExecutableFileExtensions = dynamic([".apk",".bat",".bin",".cgi",".ps1",".pl",".cmd",".com",".exe",".gadget",".hta",".jar",".msi",".msp",".mst",".py",".wsf",".vbs"]);
let ImageFileExtensions = dynamic([".ai",".bmp",".gif",".ico",".jpeg",".jpg",".png",".ps",".psd",".svg",".stl",".tif",".tiff"]);
let SystemFileExtensions = dynamic([".bak",".cab",".cfg",".config",".cpl",".cur",".dll",".dmp",".drv",".efi",".reg",".icns",".ico",".inf",".ini",".nls",".lnk",".not_dll",".msi",".stz",".not_exe",".winmd",".sys",".tmp",".mui"]);
let WebFileExtensions = dynamic([".asp",".aspx",".cfm",".css",".crdownload",".htm",".html",".js",".jsp",".part",".php",".ocx",".rss",".swf",".xhtml"]);
let KnownFileExtensions = array_concat(CertificateFileExtensions, CompressedFileExtensions, DatabaseFileExtensions, DocumentFileExtensions, EmailFileExtensions ,ImageFileExtensions, ExecutableFileExtensions, SystemFileExtensions, WebFileExtensions);
// Part 1: Create a list of files writen to external storage per date and device
let ListAllFiles = (
// Get the external storage mounts from StartTime
DeviceEvents
| where ActionType == "UsbDriveMount" and Timestamp > StartTime
| extend AF=parse_json(AdditionalFields)
| extend USBDeviceName = strcat(tostring(AF.Manufacturer), " ", tostring(AF.ProductName)), DriveLetter=tostring(AF.DriveLetter)
| project USBMountTime = Timestamp, DeviceId, DeviceName, DriveLetter, USBDeviceName
| join (
// List all file creates to drive letters from StartTime
DeviceFileEvents
| where ActionType == "FileCreated" and Timestamp > StartTime
| parse FolderPath with DriveLetter '\\' * 
| extend DriveLetter = tostring(DriveLetter) 
)
// join external mounts and file creates
on DeviceId, DriveLetter
// create a column for the file extensions
| extend DotCount = countof(FileName,".")
| extend FileExtension = strcat(".", split(FileName,".",DotCount)[0])
// Create an array of the copied Files and their information
| summarize FileSet = make_set(FolderPath), CopiedFilesMB = (sum(FileSize)/1000000), TotalFileCount = count(FileExtension), CerticateFileCount = count(FileExtension in~ (CertificateFileExtensions)), CompressedFileCount = count(FileExtension in~ (CompressedFileExtensions)), DatabaseFileCount = count(FileExtension in~ (DatabaseFileExtensions)), DocumentFileCount = count(FileExtension in~ (DocumentFileExtensions)), EmailFileCount = count(FileExtension in~ (EmailFileExtensions)), SystemFileCount = count(FileExtension in~ (SystemFileExtensions)), ExecutableFileCount = count(FileExtension in~ (ExecutableFileExtensions)), WebFileCount = count(FileExtension in~ (WebFileExtensions)), OtherFileCount = count(FileExtension !in~(KnownFileExtensions)), UniqueSHA1Hash = dcount(SHA1), UnknownFileExtensions = make_set_if(FileExtension, FileExtension !in~(KnownFileExtensions)) by bin(Timestamp, 1d), DeviceId, DeviceName ,InitiatingProcessAccountName, USBDeviceName
);
// Part 2: Create the anomaly detection of USB writes based on MB written
// Get the external storage mounts from StartTime
DeviceEvents
| where ActionType == "UsbDriveMount" and Timestamp > StartTime
| extend AF=parse_json(AdditionalFields)
| extend DriveLetter=tostring(AF.DriveLetter)
| project USBMountTime = Timestamp, DeviceId, DriveLetter
| join (
// List all file creates to drive letters from StartTime
DeviceFileEvents
| where ActionType == "FileCreated" and Timestamp > StartTime
| parse FolderPath with DriveLetter '\\' * 
| extend DriveLetter = tostring(DriveLetter) 
)
// join external mounts and file creates
on DeviceId, DriveLetter
// Make a series with the FileSizes per day on Device ID and InitiatingProcessAccountName
| make-series CopiedFilesMB = (sum(FileSize)/1000000) on Timestamp in range(startofday(StartTime),now(), 1d) by DeviceId, InitiatingProcessAccountName
// Use series_decompose_anomalies to do anomaly detection
| extend (AnomaliesDetected, AnomaliesScore, AnomaliesBaseline) = series_decompose_anomalies(CopiedFilesMB, 1.5, -1, 'linefit')
// Use mv-expand expand all results to single lines
| mv-expand CopiedFilesMB to typeof(double), Timestamp to typeof(datetime), AnomaliesDetected to typeof(double), AnomaliesScore to typeof(double), AnomaliesBaseline to typeof(long)
// Only show items where anomalies are detected
| where AnomaliesDetected == 1
// join the Anomalies and the information of part 1
| join ListAllFiles on DeviceId, InitiatingProcessAccountName, Timestamp
// project-away the duplicate columns
| project-away *1
// Apply filtering
| where CopiedFilesMB >= 100 or DocumentFileCount > 10
Alternative Text

By Gianni Castaldi

MVP | NinjaCat | Researching and Engineering Cyber Security @ Wortell

By continuing to use the site, you agree to the use of cookies. more information

The cookie settings on this website are set to "allow cookies" to give you the best browsing experience possible. If you continue to use this website without changing your cookie settings or you click "Accept" below then you are consenting to this.

Close