CVE-2026-5525: Notepad++ Buffer Overflow Vulnerability

CVE-2026-5525 Overview

A stack-based buffer overflow vulnerability exists in Notepad++ version 8.9.3 in the file drop handler component. When a user drags and drops a directory path of exactly 259 characters without a trailing backslash, the application appends a backslash and null terminator without proper bounds checking, resulting in a stack buffer overflow and application crash (STATUS_STACK_BUFFER_OVERRUN).

Critical Impact

This buffer overflow vulnerability can cause application crashes and potential denial of service when processing specially crafted directory paths through drag-and-drop operations.

Affected Products

• Notepad++ version 8.9.3
• Notepad++ file drop handler component
• Windows installations running vulnerable Notepad++ versions

Discovery Timeline

• 2026-04-10 - CVE CVE-2026-5525 published to NVD
• 2026-04-13 - Last updated in NVD database

Technical Details for CVE-2026-5525

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow). The root cause lies in the file drop handler component's failure to properly validate the length of directory paths before performing string concatenation operations.

When a user drags and drops a directory path into Notepad++, the application processes the path and, if the path does not end with a backslash, automatically appends one. The vulnerability occurs specifically when the input directory path is exactly 259 characters long. In this edge case, appending a backslash character followed by a null terminator exceeds the allocated buffer size on the stack, causing a buffer overflow condition.

The local attack vector requires user interaction, as a victim must drag and drop a maliciously crafted directory path. While exploitation requires local access and user participation, successful exploitation could lead to application crashes and potential memory corruption scenarios.

Root Cause

The vulnerability stems from insufficient bounds checking in the path string manipulation logic. The file drop handler allocates a fixed-size buffer for directory paths but fails to account for the additional characters (backslash and null terminator) that may be appended during path normalization. This off-by-one (or off-by-two) error is a classic buffer overflow pattern where boundary conditions at maximum string lengths are not properly validated.

Attack Vector

The attack requires local access to the target system and user interaction. An attacker would need to convince a user to drag and drop a specially crafted directory path of exactly 259 characters (without a trailing backslash) into the Notepad++ application window. When the vulnerable code attempts to normalize this path by appending a backslash and null terminator, the resulting string exceeds the allocated buffer size, triggering a STATUS_STACK_BUFFER_OVERRUN exception.

The vulnerability manifests in the file drop handler's path processing logic when normalizing directory paths. The exact mechanism involves string concatenation operations that fail to verify available buffer space before appending characters. For technical implementation details, see the GitHub Commit Update which addresses this issue.

Detection Methods for CVE-2026-5525

Indicators of Compromise

• Application crashes with STATUS_STACK_BUFFER_OVERRUN error codes in Notepad++ process
• Windows Event Log entries indicating buffer overflow exceptions in notepad++.exe
• Crash dump files showing stack corruption in file drop handler functions
• Unexpected Notepad++ process terminations following drag-and-drop operations

Detection Strategies

• Monitor for repeated Notepad++ crash events in Windows Event Logs with buffer overflow indicators
• Implement endpoint detection rules that alert on STATUS_STACK_BUFFER_OVERRUN exceptions in text editor applications
• Use application crash monitoring tools to identify patterns consistent with exploitation attempts
• Deploy SentinelOne agents to detect and alert on memory corruption behavior in endpoint applications

Monitoring Recommendations

• Enable Windows Error Reporting to capture crash dumps for analysis
• Configure endpoint protection platforms to monitor for abnormal application terminations
• Implement file system monitoring to detect creation of directory paths at boundary lengths
• Review application stability logs for patterns indicating exploitation attempts

How to Mitigate CVE-2026-5525

Immediate Actions Required

• Update Notepad++ to the latest patched version that addresses this vulnerability
• Review the GitHub Pull Request #17930 for patch details
• Audit systems for installations of Notepad++ version 8.9.3 and prioritize remediation
• Consider temporarily restricting drag-and-drop functionality if updates cannot be immediately applied

Patch Information

The Notepad++ development team has addressed this vulnerability through GitHub Commit bfe7514d68bc559534c046c4ef2d1865267aa2b0. Users should update to the latest version that includes this fix. The issue was tracked and resolved through GitHub Issue #17921.

Workarounds

• Avoid using drag-and-drop functionality for directory paths until the application is patched
• Use manual file opening methods (File > Open) instead of drag-and-drop operations
• Implement application whitelisting policies to ensure only patched versions of Notepad++ can execute
• Deploy endpoint protection solutions capable of detecting and preventing buffer overflow exploitation

# Verify Notepad++ version on Windows systems
# Check if vulnerable version 8.9.3 is installed
wmic product where "name like 'Notepad++%%'" get name,version

# Alternative: Check via registry
reg query "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall" /s /f "Notepad++" | findstr /i "displayversion"