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SEKOIA_Koi_Netstealer

Detects NET ofbuscated Stealer used loaded by KoiLoader

MalwareKoiloaderSEKOIA.IO CommunityDetection Rule License 1.1Compiles with YARA-X
Author
Sekoia.io
Date
2024-03-20
Modified
2024-12-19
Score / Quality
75 / 80

What this rule detects

Detects NET ofbuscated Stealer used loaded by KoiLoader

Associated malware family: Koiloader.

Targets: Windows PE.

It defines 5 patterns (5 text, 0 hex, 0 regex).

Summary generated from the rule's metadata and condition.

Rule source

koi-netstealer.yarDetection Rule License 1.1
rule SEKOIA_Koi_Netstealer : FILE
{
	meta:
		description = "Detects NET ofbuscated Stealer used loaded by KoiLoader"
		author = "Sekoia.io"
		id = "deb06e2a-848c-44b3-be95-017ebccf11f8"
		date = "2024-03-20"
		modified = "2024-12-19"
		reference = "https://github.com/SEKOIA-IO/Community"
		source_url = "https://github.com/SEKOIA-IO/Community/blob/a438a5901a211c2abdf75da9c93921ef187473d1/yara_rules/koi_netstealer.yar#L1-L20"
		license_url = "https://github.com/SEKOIA-IO/Community/blob/a438a5901a211c2abdf75da9c93921ef187473d1/LICENSE.md"
		logic_hash = "814db1092820ff1ed9e592dc92c72ad73643eb6d68df9f593ed637434373e41b"
		score = 75
		quality = 80
		tags = "FILE"
		version = "1.0"
		classification = "TLP:CLEAR"

	strings:
		$name_1 = "pg20"
		$name_2 = "pg40"
		$s1 = "Curve25519"
		$s2 = "ConsoleApp"
		$s3 = "e0d2eec7-eb14-48ba-8709-dcc9de65947d"

	condition:
		uint16be( 0 ) == 0x4d5a and filesize < 150KB and any of ( $name_* ) and all of ( $s* )
}

Strings (5 patterns)

IdentifierTypeValueModifiers
$name_1text"pg20"
$name_2text"pg40"
$s1text"Curve25519"
$s2text"ConsoleApp"
$s3text"e0d2eec7-eb14-48ba-8709-dcc9de65947d"

Condition

uint16be( 0 ) == 0x4d5a and filesize < 150KB and any of ( $name_* ) and all of ( $s* )

In plain words (Heuristic)

  • The file must start with “MZ” — a Windows executable (PE).
  • The file must be smaller than 150KB (a cheap guard that skips large files).
  • All patterns in ( $s* ) must match.
  • At least one pattern in ( $name_* ) must match.

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References

Licence & attribution

Author: Sekoia.io

Source repository: SEKOIA.IO Community @ a438a5901a

Licence: Detection Rule License 1.1 (repository licence)

Original rule

Mirrored via YARA Forge 20260927, which prefixes rule names with the source and adds normalised meta fields.

Rule id: deb06e2a-848c-44b3-be95-017ebccf11f8

Metadata

version1.0
classificationTLP:CLEAR

Tags:FILE

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