Overview

Backdoor:Win32/Yunsip is a highly evasive, targeted Remote Access Trojan (RAT) engineered to provide attackers with persistent, interactive control over compromised endpoints for long-term espionage.

Understanding Yunsip
To the victim, Yunsip provides zero visual indicators of infection. For threat intelligence analysts, Yunsip represents a capable, mid-tier backdoor frequently utilized in targeted corporate espionage and data theft campaigns. It is designed to bypass initial security controls, harvest system information, and establish a highly covert Command-and-Control (C2) channel.

Execution and Evasion Strategies
Yunsip is typically delivered via spearphishing campaigns containing malicious attachments. Upon execution, it utilizes heavy packing and obfuscation to evade static antivirus signatures. It drops a randomized executable into the %AppData% directory. Persistence is achieved by creating a hidden scheduled task or modifying the Registry Run keys. Yunsip frequently injects its core modules into legitimate system processes (like svchost.exe) to mask its outbound network traffic. Its C2 communications are heavily encrypted, often mimicking legitimate TLS traffic to evade DPI (Deep Packet Inspection).

Indicators of Compromise (IoCs)
Threat hunters should investigate EDR alerts for 'Suspicious Process Injection' or 'Anomalous Child Process Spawning'. Network logs will often reveal Yunsip reaching out to compromised domains using encrypted HTTPS traffic. The presence of unexpected, hidden scheduled tasks designed to execute randomly named binaries in the user's profile is a strong IoC. Memory analysis is crucial to extract the injected modules and identify the specific C2 infrastructure.

MITRE ATT&CK Techniques

Observed techniques used by this family, mapped to the MITRE ATT&CK framework:

Technique	Name	Tactic
`T1055`	Process Injection	Defense Evasion
`T1105`	Ingress Tool Transfer	Command and Control
`T1573`	Encrypted Channel	Command and Control
`T1547.001`	Boot or Logon Autostart Execution: Registry Run Keys / Startup Folder	Persistence
`T1027`	Obfuscated Files or Information	Defense Evasion

Generated Detections (Boilerplate)

These YARA and Sigma rules are auto-generated based on the family name and aliases. They must be heavily tuned before deployment in a production environment.

YARA Rule

rule MALWARE_WIN_YUNSIP {
    meta:
        description = "Detects Yunsip (trojan_generic)"
        author = "SystemHelpdesk Boilerplate Generator"
        date = "2026-07-06"
    strings:
        $s1 = "yunsip" ascii wide nocase
    condition:
        uint16(0) == 0x5a4d and any of them
}

Sigma Rule

title: Suspicious Yunsip Activity
id: 9743f91fd7ccac513de8fbde14cc8261
status: experimental
description: Detects generic indicators of the yunsip malware family.
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
        CommandLine|contains:
            - "*yunsip*"
    condition: selection
level: medium

Containment & Response Steps

Ordered checklist for responders. Adapt to your environment and engage professional support for active incidents.

Instantly isolate the endpoint from the network to sever the attacker's interactive, remote-control session.
Capture a full forensic memory image of the machine to extract the decrypted Yunsip payload and its C2 configuration.
Audit the Windows Task Scheduler and Registry Run keys to identify and remove the Yunsip persistence mechanisms.
Assume total endpoint compromise; perform a clean OS rebuild and force password resets for all accounts that accessed the machine.

What to Avoid

Common mistakes during response to this family that can destroy evidence, spread the infection, or worsen recovery.

Do not leave the machine connected to the network during triage; the attacker has live access and will likely destroy evidence or move laterally.
Avoid relying solely on static signatures, as Yunsip is frequently re-packed and obfuscated for each targeted campaign.

Yunsip

Overview

MITRE ATT&CK Techniques

Generated Detections (Boilerplate)

YARA Rule

Sigma Rule

Containment & Response Steps

What to Avoid

References & External Analysis

Related Families (Category: trojan_generic)

Machine-readable

Ecosystem & Interactive Environments