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Add minimum rename count to avoid false positives #56

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merged 2 commits into from
Feb 16, 2025
Merged

Add minimum rename count to avoid false positives #56

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539ad2e
add file rename count
erikgruetter Feb 13, 2025
a01f902
add Timestamp
erikgruetter Feb 13, 2025
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50 changes: 35 additions & 15 deletions Defender For Endpoint/Ransomware/RansomwareDoubleExtention.md
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,7 @@
#### Description
Detects possible ransomware file changes by adding a custom extension to the encrypted files, such as ".docx.encrypted" or ".pdf.ezz". This is a technique that is used by multiple Ransomware groups, they do not change the currenct extention, but they add a new one to the current file.

A false positive could be a administrator that changes a lot of files.
A false positive could be a administrator that changes a lot of files. To avoid false positive by users, a minimum file rename count of 10 is implemented.

#### Risk
Ransomware is being deployed in your environment.
Expand Down Expand Up @@ -41,15 +41,25 @@ DeviceFileEvents
// Remove duplicate file extensions to limit false positives (e.g. .pdf.pdf or .docx.docx)
| extend DuplicateExtensionCheck = split(NewFileExtension, ".")
| where tostring(DuplicateExtensionCheck[1]) != tostring(DuplicateExtensionCheck[2])
// Group by device and process to count renamed files
| summarize
FileCount = count(),
RenamedFiles = make_list(FileName),
Timestamp = arg_max(Timestamp, *)
by DeviceName, InitiatingProcessAccountName
// Filter for more than 10 files renamed
| where FileCount > 10
// Display results
| project-reorder
Timestamp,
PreviousFileExtension,
PreviousFileName,
NewFileExtension,
FileName,
DeviceName,
InitiatingProcessAccountName
Timestamp,
FileCount,
DeviceName,
InitiatingProcessAccountName,
RenamedFiles,
PreviousFileExtension,
PreviousFileName,
NewFileExtension,
FileName
```

## Sentinel
Expand All @@ -70,15 +80,25 @@ DeviceFileEvents
// Remove duplicate file extensions to limit false positives (e.g. .pdf.pdf or .docx.docx)
| extend DuplicateExtensionCheck = split(NewFileExtension, ".")
| where tostring(DuplicateExtensionCheck[1]) != tostring(DuplicateExtensionCheck[2])
// Group by device and process to count renamed files
| summarize
FileCount = count(),
RenamedFiles = make_list(FileName),
Timestamp = arg_max(Timestamp, *)
by DeviceName, InitiatingProcessAccountName
// Filter for more than 10 files renamed
| where FileCount > 10
// Display results
| project-reorder
TimeGenerated,
PreviousFileExtension,
PreviousFileName,
NewFileExtension,
FileName,
DeviceName,
InitiatingProcessAccountName
Timestamp,
FileCount,
DeviceName,
InitiatingProcessAccountName,
RenamedFiles,
PreviousFileExtension,
PreviousFileName,
NewFileExtension,
FileName
```


Expand Down