Detect Thread Local Storage in Google Chronicle
Adversaries may inject malicious code into processes via thread local storage (TLS) callbacks in order to evade process-based defenses as well as possibly elevate privileges. TLS callback injection involves manipulating pointers inside a portable executable (PE) to redirect a process to malicious code before reaching the code's legitimate entry point. TLS callbacks are normally used by the OS to setup and/or cleanup data used by threads. Manipulating TLS callbacks may be performed by allocating and writing to specific offsets within a process' memory space using other Process Injection techniques such as Process Hollowing.
MITRE ATT&CK
- Technique
- T1055 Process Injection
- Sub-technique
- T1055.005 Thread Local Storage
- Canonical reference
- https://attack.mitre.org/techniques/T1055/005/
YARA-L Detection Query
rule tls_callback_process_immediate_public_network {
meta:
author = "Detection Engineering"
description = "Detects potential TLS callback abuse: Windows host process spawned by unexpected parent makes outbound public network connection within 5 seconds of creation"
technique = "T1055.005"
tactic = "Defense Evasion, Privilege Escalation"
severity = "HIGH"
confidence = "MEDIUM"
reference = "https://attack.mitre.org/techniques/T1055/005/"
events:
$proc.metadata.event_type = "PROCESS_LAUNCH"
$proc.target.process.file.full_path = /(?i)(svchost|rundll32|regsvr32|dllhost|explorer)\.exe$/
not $proc.principal.process.file.full_path = /(?i)(services|svchost|explorer|winlogon)\.exe$/
$net.metadata.event_type = "NETWORK_CONNECTION"
$net.network.ip_protocol = "TCP"
not $net.target.ip = "10.0.0.0/8"
not $net.target.ip = "172.16.0.0/12"
not $net.target.ip = "192.168.0.0/16"
not $net.target.ip = "127.0.0.0/8"
not $net.target.ip = "169.254.0.0/16"
$proc.principal.hostname = $net.principal.hostname
$proc.target.process.pid = $net.principal.process.pid
$proc.metadata.event_timestamp.seconds <= $net.metadata.event_timestamp.seconds
$net.metadata.event_timestamp.seconds <= $proc.metadata.event_timestamp.seconds + 5
condition:
$proc and $net
}Chronicle YARA-L 2.0 rule joining PROCESS_LAUNCH and NETWORK_CONNECTION UDM events by hostname and process PID within a 5-second window. Fires when a Windows system binary (svchost.exe, rundll32.exe, regsvr32.exe, dllhost.exe, explorer.exe) with an anomalous parent process immediately establishes a TCP connection to a public IP — the characteristic signature of TLS callback shellcode executing before the PE main entry point.
Data Sources
Required Tables
False Positives & Tuning
- Windows telemetry and licensing services hosted in svchost.exe that connect to Microsoft endpoints immediately on creation when spawned during atypical boot sequences (e.g., WMI-triggered service restarts, recovery mode).
- Legitimate enterprise software wrapping execution in regsvr32.exe for COM registration that immediately performs cloud-based license activation, particularly in environments with aggressive software deployment pipelines.
- Security or observability agents that intentionally inject into system processes for behavioral monitoring and perform immediate cloud check-ins — such as CrowdStrike Falcon, Carbon Black, or SentinelOne sensor components.
Other platforms for T1055.005
Testing Methodology
Validate this detection against 3 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 1PE with TLS Callback - Compile and Execute
Expected signal: Sysmon Event ID 1: PowerShell execution. In a real TLS callback binary: execution trace would show TLS callback function executing before the PE entry point. Debuggers (x64dbg, WinDbg) would show the TLS callback break before OEP.
- Test 2Ursnif-style TLS Callback Chain Analysis
Expected signal: Sysmon Event ID 1: PowerShell reading PE headers of notepad.exe. Sysmon Event ID 11: No file creation (read-only operation). This is a defensive analysis technique, not an attack.
- Test 3Process Hollowing with TLS Callback Redirection
Expected signal: Sysmon Event ID 1: notepad.exe spawned by PowerShell. When using full technique: Sysmon Event ID 10 (ProcessAccess) with PROCESS_VM_WRITE rights, followed by process resume and immediate malicious activity before expected notepad initialization.
References (4)
- https://attack.mitre.org/techniques/T1055/005/
- https://www.fireeye.com/blog/threat-research/2017/11/ursnif-variant-malicious-tls-callback-technique.html
- https://www.endgame.com/blog/technical-blog/ten-process-injection-techniques-technical-survey-common-and-trending-process
- https://github.com/redcanaryco/atomic-red-team/blob/master/atomics/T1055.005/T1055.005.md
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