Process Access via TrolleyExpress Exclusion

Status: test
Severity: high
Log source: category process_creation, product windows
Author: Florian Roth (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects a possible process memory dump that uses the white-listed Citrix TrolleyExpress.exe filename as a way to dump the lsass process memory

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1218.011` System Binary Proxy Execution: Rundll32
Credential Access	`T1003.001` OS Credential Dumping: LSASS Memory

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation

Rule body

title: Process Access via TrolleyExpress Exclusion
id: 4c0aaedc-154c-4427-ada0-d80ef9c9deb6
status: test
description: Detects a possible process memory dump that uses the white-listed Citrix TrolleyExpress.exe filename as a way to dump the lsass process memory
references:
    - https://twitter.com/_xpn_/status/1491557187168178176
    - https://www.youtube.com/watch?v=Ie831jF0bb0
author: Florian Roth (Nextron Systems)
date: 2022-02-10
modified: 2022-05-13
tags:
    - attack.stealth
    - attack.t1218.011
    - attack.credential-access
    - attack.t1003.001
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        CommandLine|contains:
      # We assume that the lsass.exe process has a process ID that's between 700 and 999 and the dumper uses just the PID as parameter
            - '\TrolleyExpress 7'
            - '\TrolleyExpress 8'
            - '\TrolleyExpress 9'
            - '\TrolleyExpress.exe 7'
            - '\TrolleyExpress.exe 8'
            - '\TrolleyExpress.exe 9'
      # Common dumpers
            - '\TrolleyExpress.exe -ma '
    renamed:
        Image|endswith: '\TrolleyExpress.exe'
    filter_renamed:
        OriginalFileName|contains: 'CtxInstall'
    filter_empty:
        OriginalFileName: null
    condition: selection or ( renamed and not 1 of filter* )
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

selection or ( renamed and not 1 of filter* )

Stage 1: `selection`

selection:
    CommandLine|contains:
        - '\TrolleyExpress 7'
        - '\TrolleyExpress 8'
        - '\TrolleyExpress 9'
        - '\TrolleyExpress.exe 7'
        - '\TrolleyExpress.exe 8'
        - '\TrolleyExpress.exe 9'
        - '\TrolleyExpress.exe -ma '

Stage 2: `renamed`

renamed:
    Image|endswith: '\TrolleyExpress.exe'

Stage 3: `not filter*`

filter_renamed:
    OriginalFileName|contains: 'CtxInstall'
filter_empty:
    OriginalFileName: null

Exclusions

The rule actively suppresses these predicates.

Field	Kind	Excluded values	Search
`OriginalFileName`	is_null	(no value, null check)	`excludes:OriginalFileName`
`OriginalFileName`	match	`CtxInstall`	`excludes:OriginalFileName` `field:"OriginalFileName" value:"CtxInstall"`

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`\TrolleyExpress 7` `\TrolleyExpress 8` `\TrolleyExpress 9` `\TrolleyExpress.exe -ma` `\TrolleyExpress.exe 7` `\TrolleyExpress.exe 8` `\TrolleyExpress.exe 9`	`field:"CommandLine" kind:match`
`Image`	ends_with	`\TrolleyExpress.exe`	`field:"Image" kind:ends_with value:"\TrolleyExpress.exe"`