CVE-2025-15556 Overview
CVE-2025-15556 is a critical update integrity verification vulnerability affecting Notepad++ versions prior to 8.8.9 when using the WinGUp updater component. The vulnerability stems from a failure to cryptographically verify downloaded update metadata and installers, allowing attackers who can intercept or redirect update traffic to deliver and execute malicious installers with the privileges of the current user.
This vulnerability falls under CWE-494 (Download of Code Without Integrity Check), representing a significant supply chain security risk. Attackers positioned to perform man-in-the-middle attacks on update traffic can leverage this flaw to achieve arbitrary code execution on vulnerable systems.
Critical Impact
This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, indicating active exploitation in the wild. Attackers can achieve arbitrary code execution by intercepting Notepad++ update traffic and delivering malicious installers without cryptographic verification.
Affected Products
- Notepad++ versions prior to 8.8.9
- WinGUp updater component (bundled with Notepad++)
Discovery Timeline
- 2026-02-03 - CVE-2025-15556 published to NVD
- 2026-02-13 - Last updated in NVD database
Technical Details for CVE-2025-15556
Vulnerability Analysis
The vulnerability exists in the WinGUp updater component used by Notepad++ for automatic updates. Prior to version 8.8.9, the updater downloaded update metadata and installer binaries without performing cryptographic verification of their authenticity or integrity. This means the application trusted any content received over the network without validating that it originated from the legitimate Notepad++ developers or that it hadn't been tampered with in transit.
The lack of code signing verification creates a classic supply chain attack surface. When a user triggers an update check, the WinGUp component fetches update information and, if available, downloads the new installer. Without cryptographic verification, an attacker who can manipulate the network traffic—through DNS hijacking, BGP hijacking, compromised CDN, or local network attacks—can substitute the legitimate installer with a malicious payload.
Root Cause
The root cause is the absence of cryptographic integrity verification for downloaded binaries in the WinGUp updater. The original implementation called verifySignedLibrary() which did not properly validate the code signing certificate and signature on update installers. The fix introduced verifySignedBinary() along with proper certificate validation infrastructure including signer display name, subject, key ID, and authority key ID verification.
Attack Vector
The attack requires network-level access to intercept or redirect update traffic between the victim's Notepad++ installation and the update servers. This can be accomplished through:
- Man-in-the-Middle (MitM) attacks on local networks or compromised network infrastructure
- DNS hijacking to redirect update server lookups to attacker-controlled servers
- Compromised update infrastructure if attackers gain access to distribution points
- BGP hijacking for larger-scale attacks targeting specific IP ranges
Once positioned, the attacker serves a malicious installer that the WinGUp component downloads and executes without verification, resulting in arbitrary code execution with the user's privileges.
// Security patch in verifySignedfile.cpp - Changed from verifySignedLibrary to verifySignedBinary
// Source: https://github.com/notepad-plus-plus/notepad-plus-plus/commit/bcf2aa68ef414338d717e20e059459570ed6c5ab
{
#ifndef _DEBUG
if (_securityMode == sm_certif)
- return verifySignedLibrary(filePath);
+ return verifySignedBinary(filePath);
else if (_securityMode == sm_sha256)
return checkSha256(filePath, module2check);
else
The fix also added proper certificate verification infrastructure:
// Security patch in verifySignedfile.h - Added certificate verification fields
// Source: https://github.com/notepad-plus-plus/notepad-plus-plus/commit/bcf2aa68ef414338d717e20e059459570ed6c5ab
{
public:
SecurityGuard();
+
bool checkModule(const std::wstring& filePath, NppModule module2check);
+ std::wstring signer_display_name() { return _signer_display_name; }
+ std::wstring signer_subject() { return _signer_subject; }
+ std::wstring signer_key_id() { return _signer_key_id; }
+ std::wstring authority_key_id() { return _authority_key_id; }
private:
// SHA256
static SecurityMode _securityMode;
Detection Methods for CVE-2025-15556
Indicators of Compromise
- Unexpected network connections from GUP.exe (WinGUp updater) to non-standard update servers
- Notepad++ installer files with invalid or missing code signatures
- Unusual process spawning from GUP.exe or notepad++.exe during update operations
- Modified or unsigned binaries in Notepad++ installation directories
Detection Strategies
- Monitor for GUP.exe process activity and validate destination IPs against known legitimate Notepad++ update servers
- Implement network-based detection for update traffic to unexpected destinations
- Use endpoint detection to alert on execution of unsigned binaries from Notepad++ directories
- Deploy file integrity monitoring on Notepad++ installation paths
Monitoring Recommendations
- Enable logging for application updates and verify installer signatures before deployment
- Monitor DNS queries for Notepad++ update domains for potential hijacking attempts
- Track process execution chains involving GUP.exe for anomalous child processes
- Review network traffic patterns for update-related communications
How to Mitigate CVE-2025-15556
Immediate Actions Required
- Upgrade Notepad++ to version 8.8.9 or later immediately
- Verify installed Notepad++ binary signatures using Windows certificate verification
- Review systems for indicators of compromise if previously exposed
- Consider blocking automatic updates until systems are upgraded, then perform manual verified updates
Patch Information
The vulnerability has been addressed in Notepad++ version 8.8.9 with the introduction of proper code signing certificate verification. The fix modifies the WinGUp updater to validate both the certificate and signature of downloaded installers before execution.
Key fixes were implemented in:
Additional details are available in the Notepad++ v8.8.9 vulnerability fix announcement and the official security incident clarification.
Workarounds
- Disable automatic updates in Notepad++ preferences until the application can be upgraded
- Download Notepad++ installers only from the official website and manually verify digital signatures
- Use network segmentation to limit potential MitM attack vectors
- Deploy application whitelisting to prevent execution of unsigned binaries
# Verify Notepad++ installer signature on Windows using PowerShell
Get-AuthenticodeSignature "C:\Path\To\npp.8.8.9.Installer.exe" | Format-List
# Check for valid signature status - should show "Valid" for legitimate installers
# SignerCertificate should show "Notepad++" as the subject
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
