BlueSky Ransomware Artefacts

Status: test
Severity: high
Log source: product windows, service security
Author: j4son
Source: github.com/SigmaHQ/sigma

Detect access to files and shares with names and extensions used by BlueSky ransomware which could indicate a current or previous encryption attempt.

MITRE ATT&CK coverage

Tactic	Techniques
Impact	`T1486` Data Encrypted for Impact

Event coverage

Provider	Event	Title
Security-Auditing	Event ID 4656	A handle to an object was requested.
Security-Auditing	Event ID 4663	An attempt was made to access an object.
Security-Auditing	Event ID 5145	A network share object was checked to see whether client can be granted desired access.

Rule body yaml

title: BlueSky Ransomware Artefacts
id: eee8311f-a752-44f0-bf2f-6b007db16300
status: test
description: Detect access to files and shares with names and extensions used by BlueSky ransomware which could indicate a current or previous encryption attempt.
references:
    - https://unit42.paloaltonetworks.com/bluesky-ransomware/
author: j4son
date: 2023-05-23
tags:
    - attack.impact
    - attack.t1486
    - detection.emerging-threats
logsource:
    product: windows
    service: security
detection:
    selection_access_eid:
        EventID:
            - 4663
            - 4656
    selection_access_data:
        - ObjectName|endswith: '.bluesky'
        - ObjectName|contains: 'DECRYPT FILES BLUESKY'
    selection_share_eid:
        EventID: 5145
    selection_share_data:
        - RelativeTargetName|endswith: '.bluesky'
        - RelativeTargetName|contains: 'DECRYPT FILES BLUESKY'
    condition: all of selection_access_* or all of selection_share_*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

all of selection_access_* or all of selection_share_*

Stage 1: `selection_access_eid`

selection_access_eid:
    EventID:
        - 4663
        - 4656

Stage 2: `selection_access_data`

selection_access_data:
    - ObjectName|endswith: '.bluesky'
    - ObjectName|contains: 'DECRYPT FILES BLUESKY'

Stage 3: `selection_share_eid`

selection_share_eid:
    EventID: 5145

Stage 4: `selection_share_data`

selection_share_data:
    - RelativeTargetName|endswith: '.bluesky'
    - RelativeTargetName|contains: 'DECRYPT FILES BLUESKY'

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
`ObjectName`	ends_with	`.bluesky`
`ObjectName`	match	`DECRYPT FILES BLUESKY`
`RelativeTargetName`	ends_with	`.bluesky`
`RelativeTargetName`	match	`DECRYPT FILES BLUESKY`

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`/`	Focus search
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`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)

BlueSky Ransomware Artefacts

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_access_eid`

Stage 2: `selection_access_data`

Stage 3: `selection_share_eid`

Stage 4: `selection_share_data`

Indicators

Keyboard shortcuts

Search operators

BlueSky Ransomware Artefacts

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_access_eid

Stage 2: selection_access_data

Stage 3: selection_share_eid

Stage 4: selection_share_data

Indicators

Stage 0: `condition`

Stage 1: `selection_access_eid`

Stage 2: `selection_access_data`

Stage 3: `selection_share_eid`

Stage 4: `selection_share_data`