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[Rule Tuning] Fixes FPs related to a process.args_count bug #4971

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Merged
merged 1 commit into from
Aug 13, 2025
Merged

[Rule Tuning] Fixes FPs related to a process.args_count bug #4971

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7 changes: 5 additions & 2 deletions rules/windows/defense_evasion_masquerading_werfault.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@
creation_date = "2020/08/24"
integration = ["endpoint", "windows"]
maturity = "production"
updated_date = "2025/05/05"
updated_date = "2025/08/12"

[transform]
[[transform.osquery]]
Expand Down Expand Up @@ -120,7 +120,10 @@ type = "eql"

query = '''
sequence by host.id, process.entity_id with maxspan = 5s
[process where host.os.type == "windows" and event.type:"start" and process.name : ("wermgr.exe", "WerFault.exe") and process.args_count == 1]
[process where host.os.type == "windows" and event.type:"start" and process.name : ("wermgr.exe", "WerFault.exe") and
(process.args_count == 1 and
/* Excludes bug where a missing closing quote sets args_count to 1 despite extra args */
not process.command_line regex~ """\".*\.exe[^\"].*""")]
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FP example
image
[network where host.os.type == "windows" and process.name : ("wermgr.exe", "WerFault.exe") and network.protocol != "dns" and
network.direction : ("outgoing", "egress") and destination.ip !="::1" and destination.ip !="127.0.0.1"
]
Expand Down
6 changes: 4 additions & 2 deletions rules/windows/defense_evasion_rundll32_no_arguments.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@
creation_date = "2020/09/02"
integration = ["endpoint", "windows"]
maturity = "production"
updated_date = "2025/03/20"
updated_date = "2025/08/12"

[transform]
[[transform.osquery]]
Expand Down Expand Up @@ -116,7 +116,9 @@ query = '''
sequence with maxspan=1h
[process where host.os.type == "windows" and event.type == "start" and
(process.name : "rundll32.exe" or process.pe.original_file_name == "RUNDLL32.EXE") and
process.args_count == 1
(process.args_count == 1 and
/* Excludes bug where a missing closing quote sets args_count to 1 despite extra args */
not process.command_line regex~ """\".*\.exe[^\"].*""")
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Example FP
image
] by process.entity_id
[process where host.os.type == "windows" and event.type == "start" and process.parent.name : "rundll32.exe"
] by process.parent.entity_id
Expand Down
10 changes: 8 additions & 2 deletions rules/windows/defense_evasion_unusual_dir_ads.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@
creation_date = "2020/12/04"
integration = ["endpoint", "windows", "m365_defender", "sentinel_one_cloud_funnel", "crowdstrike"]
maturity = "production"
updated_date = "2025/03/20"
updated_date = "2025/08/12"

[rule]
author = ["Elastic"]
Expand Down Expand Up @@ -79,7 +79,13 @@ type = "eql"

query = '''
process where host.os.type == "windows" and event.type == "start" and
process.args : "?:\\*:*" and process.args_count == 1
process.args : "?:\\*:*" and
(
process.args_count == 1 and

/* Excludes bug where a missing closing quote sets args_count to 1 despite extra args */
not process.command_line regex~ """\".*\.exe[^\"].*"""
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Example FP
image
)
'''


Expand Down
10 changes: 8 additions & 2 deletions rules/windows/defense_evasion_unusual_network_connection_via_rundll32.toml
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@
creation_date = "2020/02/18"
integration = ["endpoint", "windows"]
maturity = "production"
updated_date = "2025/03/20"
updated_date = "2025/08/12"

[rule]
author = ["Elastic"]
Expand Down Expand Up @@ -73,7 +73,13 @@ type = "eql"

query = '''
sequence by host.id, process.entity_id with maxspan=1m
[process where host.os.type == "windows" and event.type == "start" and process.name : "rundll32.exe" and process.args_count == 1]
[process where host.os.type == "windows" and event.type == "start" and process.name : "rundll32.exe" and
(
process.args_count == 1 and

/* Excludes bug where a missing closing quote sets args_count to 1 despite extra args */
not process.command_line regex~ """\".*\.exe[^\"].*"""
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Example FP
image
)]
[network where host.os.type == "windows" and process.name : "rundll32.exe" and
not cidrmatch(destination.ip, "10.0.0.0/8", "127.0.0.0/8", "169.254.0.0/16", "172.16.0.0/12", "192.0.0.0/24",
"192.0.0.0/29", "192.0.0.8/32", "192.0.0.9/32", "192.0.0.10/32", "192.0.0.170/32", "192.0.0.171/32",
Expand Down
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