Azure Storage Blob CPK Encryption Detected

Severity: high
Log types: Azure.MonitorActivity
Tags: Impact, Data Encrypted for Impact, Inhibit System Recovery, Ransomware
Reference: https://learn.microsoft.com/en-us/dotnet/api/azure.storage.blobs.models.bloberrorcode.blobusescustomerspecifiedencryption?view=azure-dotnet
Source: github.com/panther-labs/panther-analysis

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.

MITRE ATT&CK coverage

Tactic	Techniques
Impact	`T1486` Data Encrypted for Impact, `T1490` Inhibit System Recovery

Rule body yaml

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"
        }
      }

Detection logic

Condition

category eq "STORAGEREAD"
statusCode eq "409"
statusText eq "BLOBUSESCUSTOMERSPECIFIEDENCRYPTION"
properties.metricResponseType ne "Success"

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`category`	eq	`STORAGEREAD`
`properties.metricResponseType`	ne	`Success`
`statusCode`	eq	`409`
`statusText`	eq	`BLOBUSESCUSTOMERSPECIFIEDENCRYPTION`

Output fields

Fields the rule emits when it matches. Chronicle authors list these in the outcome block; they appear on the detection and $risk_score drives alerting. Sentinel / Defender XDR rules build them up through project / summarize / extend stages. Sentinel maps these into alert fields via entityMappings and customDetails; Defender XDR custom detections surface them as alert fields directly.

Field	Source
`objectKey`	`properties.objectKey`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)