What to Know About the Notepad++ Supply-Chain Attack

The cybersecurity community is still grappling with a sobering realization: one of the most ubiquitous tools in the developer’s toolkit, Notepad++, was hiding a critical vulnerability for over six months. Being so deeply embedded in daily workflows, many organizations did not realize they were vulnerable until a recent security update pulled back the curtain on a sophisticated Chinese state-sponsored campaign, dubbed “Lotus Blossom.”

Investigations have confirmed that the issue wasn’t just a coding error, it was a compromise at the hosting provider level. This means that for much of 2025, even organizations that followed best practices were still potentially open to backdoors from Chinese advanced persistent threat (APT) groups. Here is what you need to know to secure your environment.

Understanding the Notepad++ Vulnerability (CVE-2025-15556)

The vulnerability, tracked as CVE-2025-15556 (VulnDB ID: 430205), exploits a critical flaw in the Notepad++ updater component, WinGUP. In versions prior to the February 2026 patch, the updater failed to verify the file integrity signatures of downloaded installers.

By exploiting this lack of verification, threat actors are able to:

• Intercept legitimate update requests originating from WinGUp servers
• Redirect traffic to malicious servers via Man-in-the-Middle (MitM) attacks or DNS cache poisoning
• Deliver trojanized executables (disguised as update.exe) that appeared to be legitimate software patches

Leveraging this vulnerability, attackers have gained a persistent presence in high-value sectors. According to reports from Kaspersky, the impact has spanned government and telecommunications, critical infrastructure, and financial services.

How CVE-2025-15556 Works

The Lotus Blossom campaign was executed in three attack chains, between July and October 2025. Each phase evolved to evade detection by changing file sizes, IP addresses, and delivery methods.

Phase	Timeline (2025)	Execution Method	Payload
Chain #1	July – August	1MB NSIS installer (update.exe)	Multi-stage attack launching a Cobalt Strike beacon via ProShow.exe.
Chain #2	September	140KB NSIS installer (update.exe)	Rotated C2 URLs to maintain stealth while dropping a Cobalt Strike beacon.
Chain #3	October	Backdoor Deployment	Dropped BluetoothService.exe, log.DLL, and shellcode to establish the Chrysalis backdoor.

Mapping CVE-2025-15556 to MITRE ATT&CK

Flashpoint has mapped Lotus Blossom TTPs (tactics, tools, and procedures) to the MITRE ATT&CK framework. Flashpoint analysts have identified the following techniques:

Execution

Technique Title	ID	Recommendations
User Execution: Malicious File	T1204.002	M1040: Behavior Prevention on Endpoint M1038: Execution Prevention M1017: User Training
Native API	T1106	M1040: Behavior Prevention on Endpoint M1038: Execution Prevention
Command and Scripting Interpreter: Windows Command Shell	T1059.003	M1038: Execution Prevention

Persistence

Technique Title	ID	Recommendations
Hijack Execution Flow: DLL	T1574.002	M1013: Application Developer Guidance M1047: Audit M1038: Execution Prevention M1044: Restrict Library Loading M1051: Update Software
Boot or Logon Autostart Execution: Registry Run Keys / Startup Folder	T1547.001	*MITRE currently does not list any mitigation guidance to combat this attack technique.
Create or Modify System Process: Windows Service	T1543.003	M1047: Audit M1040: Behavior Prevention on Endpoint M1045: Code Signing M1028: Operating System Configuration M1018: User Account Management

Defense Evasion

Technique Title	ID	Recommendations
Masquerading	T1036	M1049: Antivirus/Antimalware M1047: Audit M1040: Behavior Prevention on Endpoint M1045: Code Signing M1038: Execution Prevention M1022: Restrict File and Directory Permissions M1018: User Account Management M1017: User Training
Obfuscated Files or Information	T1027	M1049: Antivirus/Antimalware M1047: Audit M1040: Behavior Prevention on Endpoint M1017: User Training
Obfuscated Files or Information: Dynamic API Resolution	T1027.007	*MITRE currently does not list any mitigation guidance to combat this attack technique.
Deobfuscate/Decode Files or Information	T1140	*MITRE currently does not list any mitigation guidance to combat this attack technique.
Process Injection	T1055	M1040: Behavior Prevention on Endpoint M1026: Privileged Account Management
Reflective Code Loading	T1620	*MITRE currently does not list any mitigation guidance to combat this attack technique.
Execution Guardrails: Mutual Exclusion	T1480.002	M1055: Do Not Mitigate
Indicator Removal: File Deletion	T1070.004	*MITRE currently does not list any mitigation guidance to combat this attack technique.

Discovery

Technique Title	ID	Recommendations
File and Directory Discovery	T1083	*MITRE currently does not list any mitigation guidance to combat this attack technique.
Ingress Tool Transfer	T1105	M1031: Network Intrusion Prevention

Collection

Technique Title	ID	Recommendations
Data from Local System	T1005	M1057: Data Loss Prevention

Command and Control

Technique Title	ID	Recommendations
Application Layer Protocol: Web Protocols	T1071.001	M1031: Network Intrusion Prevention
Encrypted Channel	T1573	M1031: Network Intrusion Prevention M1020: SSL/TLS Inspection

Exfiltration

Technique Title	ID	Recommendations
Exfiltration Over C2 Channel	T1041	M1057: Data Loss Prevention M1031: Network Intrusion Prevention

Protecting Against CVE-2025-15556

Proactive defense requires not only reactive patching of CVE-2025-15556, but also active threat hunting using the TTPs identified by Flashpoint analysts. Flashpoint recommends the following actions:

1. Immediate Update: Ensure all instances of Notepad ++ are updated to v8.9.1 or higher immediately. This version enforces the signature verification that was missing in previous releases.
2. Audit System Paths: Scan for malicious file paths used for persistence.
3. Network Defense: Monitor and block traffic to malicious domains.
4. Endpoint Hardening: Implement Behavior Prevention on Endpoints (M1040) and Audit (M1047) to detect unauthorized registry run keys or new system services.

Outpace Threat Actors Using Flashpoint

Software trust is only as strong as the infrastructure behind it. As organizations respond to these recent updates, having best-in-class vulnerability intelligence and direct visibility into threat actor TTPs is the best defense.

Leveraging Flashpoint vulnerability intelligence, organizations can move beyond CVE and NVD, by gaining deeper technical analysis and MITRE ATT&CK mapping to defend against sophisticated threat actors. Request a demo to learn more.