Azure Storage Immutability Policy Deleted

Severity: high
Log types: Azure.MonitorActivity
Tags: Defense Evasion, Impair Defenses, Inhibit System Recovery, Data Destruction, Ransomware, Storm-0501, WORM
Reference: https://www.microsoft.com/en-us/security/blog/2025/08/27/storm-0501s-evolving-techniques-lead-to-cloud-based-ransomware/
Source: github.com/panther-labs/panther-analysis

Detects deletion of Azure Storage immutability policies that provide WORM protection. Storm-0501 ransomware operators delete these policies before encrypting data, as WORM-protected blobs cannot be modified even by administrators. Critical pre-ransomware indicator.

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1562.001` Impair Defenses: Disable or Modify Tools
Impact	`T1485` Data Destruction, `T1490` Inhibit System Recovery

Rule body yaml

AnalysisType: rule
Filename: azure_storage_immutability_policy_deleted.py
RuleID: "Azure.MonitorActivity.Storage.ImmutabilityPolicyDeleted"
DisplayName: "Azure Storage Immutability Policy Deleted"
Enabled: true
LogTypes:
  - Azure.MonitorActivity
Severity: High
Description: >
  Detects deletion of Azure Storage immutability policies that provide WORM protection.
  Storm-0501 ransomware operators delete these policies before encrypting data, as
  WORM-protected blobs cannot be modified even by administrators. Critical pre-ransomware indicator.
Reports:
  MITRE ATT&CK:
    - TA0005:T1562 # Defense Evasion: Impair Defenses
    - TA0005:T1562.001 # Defense Evasion: Disable or Modify Tools
    - TA0040:T1490 # Impact: Inhibit System Recovery
    - TA0040:T1485 # Impact: Data Destruction
Reference: https://www.microsoft.com/en-us/security/blog/2025/08/27/storm-0501s-evolving-techniques-lead-to-cloud-based-ransomware/
Tags:
  - Defense Evasion
  - Impair Defenses
  - Inhibit System Recovery
  - Data Destruction
  - Ransomware
  - Storm-0501
  - WORM
Runbook: |
  1. Query Azure Monitor Activity logs for all immutability policy deletions by the callerIpAddress and caller identity in the past 6 hours to calculate the total number of containers affected
  2. Search for resource lock deletions on the same storage account by the same caller in the 2 hours before this policy deletion to identify the Storm-0501 attack pattern
  3. Find all destructive operations on the affected storage account (DeleteBlob, PutBlob with CPK encryption, storage account deletion) in the 2 hours after policy deletion to assess if data destruction has begun
  4. Check if the storage account has been accessed from external IPs or if bulk GetBlob operations occurred after policy deletion to identify potential data exfiltration
  5. Review Azure.Audit logs for authentication events from the callerIpAddress in the 48 hours before to identify credential compromise indicators (unusual locations, failed MFA, privilege escalations)
  6. Search for other immutability policy or backup-related alerts triggered by the same callerIpAddress across all storage accounts in the past 7 days to determine attack scope
SummaryAttributes:
  - resourceId
  - callerIpAddress
  - correlationId
Tests:
  - Name: Immutability Policy Deleted Successfully
    ExpectedResult: true
    Log:
      {
        "time": "2025-01-27T14:45:00.0000000Z",
        "resourceId": "/subscriptions/12345678-1234-1234-1234-123456789abc/resourceGroups/data-rg/providers/Microsoft.Storage/storageAccounts/criticaldata001/blobServices/default/containers/backups/immutabilityPolicies/default",
        "operationName": "MICROSOFT.STORAGE/STORAGEACCOUNTS/BLOBSERVICES/CONTAINERS/IMMUTABILITYPOLICIES/DELETE",
        "operationVersion": "2021-09-01",
        "category": "Administrative",
        "resultType": "Success",
        "resultSignature": "200",
        "callerIpAddress": "203.0.113.42",
        "correlationId": "a1b2c3d4-e5f6-7890-abcd-ef1234567890",
        "level": "Informational",
        "location": "eastus",
        "tenantId": "87654321-4321-4321-4321-111111111111",
        "identity": {
          "claims": {
            "name": "attacker@example.com",
            "http://schemas.xmlsoap.org/ws/2005/05/identity/claims/upn": "attacker@example.com"
          }
        }
      }
  - Name: Immutability Policy Deleted Case Insensitive
    ExpectedResult: true
    Log:
      {
        "time": "2025-01-27T15:20:00.0000000Z",
        "resourceId": "/subscriptions/aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee/resourceGroups/compliance-rg/providers/Microsoft.Storage/storageAccounts/compliancedata/blobServices/default/containers/audit-logs/immutabilityPolicies/default",
        "operationName": "microsoft.storage/storageaccounts/blobservices/containers/immutabilitypolicies/delete",
        "category": "Administrative",
        "resultType": "Succeeded",
        "callerIpAddress": "198.51.100.50",
        "correlationId": "b2c3d4e5-f6a7-8901-bcde-f23456789012",
        "level": "Information",
        "location": "westus2",
        "tenantId": "11111111-1111-1111-1111-111111111111"
      }
  - Name: Multiple Containers Immutability Deleted - Ransomware Pattern
    ExpectedResult: true
    Log:
      {
        "time": "2025-01-27T16:00:00.0000000Z",
        "resourceId": "/subscriptions/12345678-1234-1234-1234-123456789abc/resourceGroups/prod-rg/providers/Microsoft.Storage/storageAccounts/proddata/blobServices/default/containers/financial-records/immutabilityPolicies/default",
        "operationName": "Microsoft.Storage/storageAccounts/blobServices/containers/immutabilityPolicies/delete",
        "category": "Administrative",
        "resultType": "Success",
        "callerIpAddress": "192.0.2.100",
        "correlationId": "c3d4e5f6-a7b8-9012-cdef-345678901234",
        "tenantId": "22222222-2222-2222-2222-222222222222"
      }
  - Name: Failed Immutability Policy Deletion
    ExpectedResult: false
    Log:
      {
        "time": "2025-01-27T17:00:00.0000000Z",
        "resourceId": "/subscriptions/12345678-1234-1234-1234-123456789abc/resourceGroups/locked-rg/providers/Microsoft.Storage/storageAccounts/protecteddata/blobServices/default/containers/legal-hold/immutabilityPolicies/default",
        "operationName": "MICROSOFT.STORAGE/STORAGEACCOUNTS/BLOBSERVICES/CONTAINERS/IMMUTABILITYPOLICIES/DELETE",
        "category": "Administrative",
        "resultType": "Failed",
        "resultSignature": "409",
        "callerIpAddress": "198.18.0.50",
        "correlationId": "d4e5f6a7-b8c9-0123-def0-456789012345",
        "tenantId": "33333333-3333-3333-3333-333333333333"
      }
  - Name: Different Operation - Immutability Policy Extended
    ExpectedResult: false
    Log:
      {
        "time": "2025-01-27T18:00:00.0000000Z",
        "resourceId": "/subscriptions/12345678-1234-1234-1234-123456789abc/resourceGroups/data-rg/providers/Microsoft.Storage/storageAccounts/storageacct/blobServices/default/containers/archives/immutabilityPolicies/default",
        "operationName": "MICROSOFT.STORAGE/STORAGEACCOUNTS/BLOBSERVICES/CONTAINERS/IMMUTABILITYPOLICIES/EXTEND",
        "category": "Administrative",
        "resultType": "Success",
        "callerIpAddress": "203.0.113.10",
        "correlationId": "e5f6a7b8-c9d0-1234-ef01-567890123456",
        "tenantId": "44444444-4444-4444-4444-444444444444"
      }

Detection logic

Condition

operationName eq "MICROSOFT.STORAGE/STORAGEACCOUNTS/BLOBSERVICES/CONTAINERS/IMMUTABILITYPOLICIES/DELETE"
resultType in ["Success", "Succeeded"]

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
`operationName`	eq	`MICROSOFT.STORAGE/STORAGEACCOUNTS/BLOBSERVICES/CONTAINERS/IMMUTABILITYPOLICIES/DELETE`
`resultType`	in	`Succeeded` `Success`

`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)