Potentially Suspicious Desktop Background Change Using Reg.EXE

Status: test
Severity: medium
Log source: product windows, category process_creation
Author: Stephen Lincoln @slincoln-aiq (AttackIQ)
Source: github.com/SigmaHQ/sigma

Detects the execution of "reg.exe" to alter registry keys that would replace the user's desktop background. This is a common technique used by malware to change the desktop background to a ransom note or other image.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1112` Modify Registry
Defense Impairment	`T1112` Modify Registry
Impact	`T1491.001` Defacement: Internal Defacement

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation

Rule body yaml

title: Potentially Suspicious Desktop Background Change Using Reg.EXE
id: 8cbc9475-8d05-4e27-9c32-df960716c701
related:
    - id: 85b88e05-dadc-430b-8a9e-53ff1cd30aae
      type: similar
status: test
description: |
    Detects the execution of "reg.exe" to alter registry keys that would replace the user's desktop background.
    This is a common technique used by malware to change the desktop background to a ransom note or other image.
references:
    - https://www.attackiq.com/2023/09/20/emulating-rhysida/
    - https://research.checkpoint.com/2023/the-rhysida-ransomware-activity-analysis-and-ties-to-vice-society/
    - https://www.trendmicro.com/en_us/research/23/h/an-overview-of-the-new-rhysida-ransomware.html
    - https://www.virustotal.com/gui/file/a864282fea5a536510ae86c77ce46f7827687783628e4f2ceb5bf2c41b8cd3c6/behavior
    - https://admx.help/?Category=Windows_10_2016&Policy=Microsoft.Policies.WindowsDesktop::Wallpaper
    - https://admx.help/?Category=Windows_10_2016&Policy=Microsoft.Policies.ControlPanelDisplay::CPL_Personalization_NoDesktopBackgroundUI
author: Stephen Lincoln @slincoln-aiq (AttackIQ)
date: 2023-12-21
tags:
    - attack.persistence
    - attack.impact
    - attack.defense-impairment
    - attack.t1112
    - attack.t1491.001
logsource:
    product: windows
    category: process_creation
detection:
    # TODO: Improve this to also focus on variation using PowerShell and other CLI tools
    selection_reg_img:
        - Image|endswith: '\reg.exe'
        - OriginalFileName: 'reg.exe'
    selection_reg_flag:
        CommandLine|contains: 'add'
    selection_keys:
        CommandLine|contains:
            - 'Control Panel\Desktop'
            - 'CurrentVersion\Policies\ActiveDesktop'
            - 'CurrentVersion\Policies\System'
    selection_cli_reg_1:
        CommandLine|contains|all:
            - '/v NoChangingWallpaper'
            - '/d 1' # Prevent changing desktop background
    selection_cli_reg_2:
        CommandLine|contains|all:
            - '/v Wallpaper'
            - '/t REG_SZ'
    selection_cli_reg_3:
        CommandLine|contains|all:
            - '/v WallpaperStyle'
            - '/d 2' # Stretch
    condition: all of selection_reg_* and selection_keys and 1 of selection_cli_reg_*
falsepositives:
    - Administrative scripts that change the desktop background to a company logo or other image.
level: medium

Stages and Predicates

Stage 0: `condition`

all of selection_reg_* and selection_keys and 1 of selection_cli_reg_*

Stage 1: `selection_reg_img`

selection_reg_img:
    - Image|endswith: '\reg.exe'
    - OriginalFileName: 'reg.exe'

Stage 2: `selection_reg_flag`

selection_reg_flag:
    CommandLine|contains: 'add'

Stage 3: `selection_keys`

selection_keys:
    CommandLine|contains:
        - 'Control Panel\Desktop'
        - 'CurrentVersion\Policies\ActiveDesktop'
        - 'CurrentVersion\Policies\System'

Stage 4: `selection_cli_reg_1`

selection_cli_reg_1:
    CommandLine|contains|all:
        - '/v NoChangingWallpaper'
        - '/d 1'

Stage 5: `selection_cli_reg_2`

selection_cli_reg_2:
    CommandLine|contains|all:
        - '/v Wallpaper'
        - '/t REG_SZ'

Stage 6: `selection_cli_reg_3`

selection_cli_reg_3:
    CommandLine|contains|all:
        - '/v WallpaperStyle'
        - '/d 2'

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
`CommandLine`	match	`/d 1` corpus 2 (sigma 2) `/d 2` `/t REG_SZ` corpus 2 (sigma 2) `/v NoChangingWallpaper` `/v Wallpaper` `/v WallpaperStyle` `Control Panel\Desktop` `CurrentVersion\Policies\ActiveDesktop` `CurrentVersion\Policies\System` `add` corpus 34 (sigma 26, splunk 4, chronicle 2, kusto 2)
`Image`	ends_with	`\reg.exe` corpus 58 (sigma 58)
`OriginalFileName`	eq	`reg.exe` corpus 42 (sigma 32, splunk 8, elastic 2)

`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)

Potentially Suspicious Desktop Background Change Using Reg.EXE

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_reg_img`

Stage 2: `selection_reg_flag`

Stage 3: `selection_keys`

Stage 4: `selection_cli_reg_1`

Stage 5: `selection_cli_reg_2`

Stage 6: `selection_cli_reg_3`

Indicators

Keyboard shortcuts

Search operators

Potentially Suspicious Desktop Background Change Using Reg.EXE

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_reg_img

Stage 2: selection_reg_flag

Stage 3: selection_keys

Stage 4: selection_cli_reg_1

Stage 5: selection_cli_reg_2

Stage 6: selection_cli_reg_3

Indicators

Stage 0: `condition`

Stage 1: `selection_reg_img`

Stage 2: `selection_reg_flag`

Stage 3: `selection_keys`

Stage 4: `selection_cli_reg_1`

Stage 5: `selection_cli_reg_2`

Stage 6: `selection_cli_reg_3`