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Upgrade to ProDetect Ransomware-Affiliate Custom Exfiltration Tooling (StealBit & Exmatter) in Google Chronicle
Ransomware affiliates increasingly deploy purpose-built exfiltration utilities instead of general-purpose tools like rclone, separating the data-theft channel from the encryption payload's C2 infrastructure. StealBit, distributed by LockBit affiliates, is a standalone binary (originally HTTP/HTTPS-based, later versions moved to a raw multi-threaded TCP protocol) that is dropped with a randomized hex/GUID filename into ProgramData, AppData\Local\Temp, or Users\Public, then run with no command-line arguments. It enumerates local and mapped drives, filters for high-value file types (databases, documents, spreadsheets, credential stores), and spawns dozens of concurrent upload threads to a single hardcoded external host to maximize throughput before the encryptor executes. Exmatter, used by BlackMatter/Noberus, BlackCat/ALPHV, and BlackByte affiliates, is a .NET utility that prioritizes specific extensions (.sql, .mdf, .bak, .pdf, .doc/x, .xls/x, .rdp, .kdbx) and exfiltrates over SFTP, FTPS, or WebDAV to attacker-controlled infrastructure using credentials embedded in its configuration; several observed samples overwrite or truncate a subset of already-uploaded source files with random data immediately after a successful transfer, an anti-recovery and extra-leverage behavior distinct from the ransomware's own encryption routine. Both tools are typically staged and executed in the hours immediately preceding encryption, making the exfiltration phase a high-value, narrow detection window before impact.
MITRE ATT&CK
- Tactic
- Exfiltration
YARA-L Detection Query
rule ransomware_affiliate_exfil_tooling {
meta:
author = "df00tech"
description = "Detects StealBit/Exmatter-style ransomware-affiliate exfiltration tooling via loader drop and network fan-out"
severity = "HIGH"
mitre_attack_tactic = "Exfiltration"
mitre_attack_technique = "T1048.002"
threat_actors = "LockBit, BlackMatter, BlackCat/ALPHV, BlackByte"
reference = "https://attack.mitre.org/techniques/T1048/002/"
events:
$proc.metadata.event_type = "PROCESS_LAUNCH"
re.regex($proc.target.process.file.full_path, `(?i)\\(ProgramData|AppData\\Local\\Temp|Users\\Public)\\`)
(
re.regex($proc.target.process.file.full_path, `(?i)[0-9a-f]{8}-?[0-9a-f]{4}-?[0-9a-f]{4}-?[0-9a-f]{4}-?[0-9a-f]{12}\.exe$`)
or re.regex($proc.target.process.file.full_path, `(?i)[0-9a-f]{16,32}\.exe$`)
)
$net.metadata.event_type = "NETWORK_CONNECTION"
$net.target.port = 443 or $net.target.port = 21 or $net.target.port = 990 or $net.target.port = 22
$proc.principal.hostname = $net.principal.hostname
match:
$proc.principal.hostname over 15m
condition:
$proc and $net
}Chronicle YARA-L 2.0 rule matching PROCESS_LAUNCH and NETWORK_CONNECTION UDM events within a 15-minute window per host, detecting a randomly-named argument-less executable dropped in a common staging directory followed by an outbound connection on an FTPS/SFTP/HTTPS port — the shared drop-and-transfer pattern for StealBit (LockBit) and Exmatter (BlackMatter/BlackCat/BlackByte affiliates).
Data Sources
Required Tables
False Positives & Tuning
- Installer temp binaries with a single outbound connection for update/license checks — narrow the match window or require the HighFanoutExfilConnections-equivalent volume threshold before escalating
- Authorized backup agents installed under ProgramData with legitimate scheduled outbound FTPS/SFTP connections — maintain a Chronicle reference list of authorized agent principal.process.file.full_path values and exclude matches
Other platforms for THREAT-Ransomware-AffiliateExfilTooling
Testing Methodology
Validate this detection against 2 adversary techniques from Atomic Red Team. Each test below lists the behaviour to exercise and the telemetry you should expect to see. Executable commands and cleanup steps are available with Pro.
- Test 1Simulate StealBit-Style Multi-Threaded Exfil Fan-Out
Expected signal: Sysmon Event ID 1 for a randomly-named executable created in ProgramData; Sysmon Event ID 3 showing 20+ outbound connections to the test endpoint on port 443 within a short window.
- Test 2Simulate Exmatter-Style Staging and Post-Upload File Corruption
Expected signal: Sysmon Event ID 11 for 60 file creations matching the .pdf extension within a short window; Sysmon Event ID 3 for the outbound SFTP connection on port 22; a second Event ID 11/2 for the modification of staged_1.pdf within minutes of the SFTP connection.
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