AnalysisType: rule
Filename: azure_storage_blob_cpk_encryption_detected.py
RuleID: "Azure.MonitorActivity.Storage.Blob.CPKEncryptionDetected"
DisplayName: "Azure Storage Blob CPK Encryption Detected"
Enabled: true
LogTypes:
- Azure.MonitorActivity
Severity: High
Description: >
Detects when users attempt to access Azure Storage blobs that are encrypted with
Customer-Provided Keys (CPK) but fail because they don't have the encryption key.
This may indicate a ransomware operation that is using CPK encryption to hold data hostage,
as legitimate users cannot access their own encrypted blobs without the attacker's key.
Reports:
MITRE ATT&CK:
- TA0040:T1486 # Impact: Data Encrypted for Impact
- TA0040:T1490 # Impact: Inhibit System Recovery
Tags:
- Impact
- Data Encrypted for Impact
- Inhibit System Recovery
- Ransomware
Runbook: |
1. Query Azure MonitorActivity logs for all PutBlob operations on the affected resourceId in the 48 hours before the first error to identify when files were encrypted and from which callerIpAddress
2. Find all ListAccountSAS and ListKeys operations by any callerIpAddress or p_any_usernames in the 7 days before the first PutBlob operation to identify potential credential theft
3. Search for all other storage accounts accessed by the same callerIpAddress in the past 24 hours to determine the scope of the attack across the Azure environment
Reference: https://learn.microsoft.com/en-us/dotnet/api/azure.storage.blobs.models.bloberrorcode.blobusescustomerspecifiedencryption?view=azure-dotnet
SummaryAttributes:
- callerIpAddress
- resourceId
- p_any_usernames
Tests:
- Name: CPK Encrypted Blob Access Denied
ExpectedResult: true
Log:
{
"time": "2024-12-19T19:15:07.295Z",
"resourceId": "/subscriptions/12345678-1234-1234-1234-123456789abc/resourceGroups/myresourcegroup/providers/Microsoft.Storage/storageAccounts/mystorageaccount/blobServices/default",
"category": "StorageRead",
"operationName": "GetBlobMetadata",
"operationVersion": "2024-11-04",
"schemaVersion": "1.0",
"statusCode": 409,
"statusText": "BlobUsesCustomerSpecifiedEncryption",
"durationMs": 4,
"callerIpAddress": "1.1.1.1:57485",
"correlationId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
"identity": {
"type": "SAS",
"tokenHash": "key1(EXAMPLE_HASH)"
},
"location": "westus2",
"properties": {
"accountName": "mystorageaccount",
"userAgentHeader": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/7.7.7.7 Safari/537.36",
"serviceType": "blob",
"objectKey": "/mystorageaccount/corporate-files/documents/internal_doc_8.txt.ENCRYPTED",
"metricResponseType": "ClientOtherError",
"serverLatencyMs": 4,
"tlsVersion": "TLS 1.3"
},
"protocol": "HTTPS"
}
- Name: GetBlobProperties CPK Error
ExpectedResult: true
Log:
{
"time": "2024-12-19T19:15:07.304Z",
"resourceId": "/subscriptions/12345678-1234-1234-1234-123456789abc/resourceGroups/myresourcegroup/providers/Microsoft.Storage/storageAccounts/mystorageaccount/blobServices/default",
"category": "StorageRead",
"operationName": "GetBlobProperties",
"statusCode": 409,
"statusText": "BlobUsesCustomerSpecifiedEncryption",
"durationMs": 2,
"callerIpAddress": "1.1.1.1:57484",
"properties": {
"accountName": "mystorageaccount",
"userAgentHeader": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36",
"serviceType": "blob",
"objectKey": "/mystorageaccount/corporate-files/documents/sensitive_data.xlsx.ENCRYPTED",
"metricResponseType": "ClientOtherError"
}
}
- Name: Different Error Code
ExpectedResult: false
Log:
{
"time": "2024-12-19T19:15:08.173Z",
"resourceId": "/subscriptions/12345678-1234-1234-1234-123456789abc/resourceGroups/myresourcegroup/providers/Microsoft.Storage/storageAccounts/mystorageaccount/blobServices/default",
"category": "StorageRead",
"operationName": "GetBlobMetadata",
"statusCode": 403,
"statusText": "AuthorizationPermissionMismatch",
"callerIpAddress": "1.1.1.1:57484",
"properties": {
"accountName": "mystorageaccount",
"objectKey": "/mystorageaccount/test/documents/file.txt",
"metricResponseType": "AuthorizationError"
}
}
# ------ paired body: azure_storage_blob_cpk_encryption_detected.py ------
from panther_azureactivity_helpers import (
azure_activity_alert_context,
azure_resource_logs_failure,
extract_resource_name_from_id,
)
STORAGE_READ_CATEGORY = "STORAGEREAD"
CPK_ERROR_STATUS = "BLOBUSESCUSTOMERSPECIFIEDENCRYPTION"
def rule(event):
# Detect when users try to access CPK-encrypted blobs without the key
return (
event.get("category", "").upper() == STORAGE_READ_CATEGORY
and event.get("statusCode") == 409
and event.get("statusText", "").upper() == CPK_ERROR_STATUS
and azure_resource_logs_failure(event)
)
def title(event):
resource_id = event.get("resourceId", "<UNKNOWN_STORAGE_ACCOUNT>")
storage_account = extract_resource_name_from_id(
resource_id, "storageAccounts", default="<UNKNOWN_STORAGE_ACCOUNT>"
)
blob_path = event.deep_get("properties", "objectKey", default="<UNKNOWN_BLOB>")
return (
f"Access denied returned in storage account [{storage_account}] "
f"for CPK-encrypted blob [{blob_path}]"
)
def alert_context(event):
context = azure_activity_alert_context(event)
# Add blob-specific context
context["blob_path"] = event.deep_get("properties", "objectKey", default="<UNKNOWN>")
context["user_agent"] = event.deep_get("properties", "userAgentHeader", default="<UNKNOWN>")
context["status_code"] = event.get("statusCode", "<UNKNOWN>")
context["status_text"] = event.get("statusText", "<UNKNOWN>")
return context