Cyber Resilience

CVE-2026-6539

Notepad-Plus-Plus Notepad\+\+ 8.9.3

Public PoC
Published
30 April 2026
Modified
01 May 2026
CVSS Score v4 4.6
Click a component to see what it means
Raw vectorCVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:A/VC:L/VI:N/VA:L/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
EPSS Score 0.0019 9th percentile
Risk Priority 35 floored blend · peak EPSS

Summary

CVE-2026-6539 is a medium-severity Use of Externally-Controlled Format String (CWE-134) vulnerability in Notepad-Plus-Plus Notepad\+\+. Its CVSS base score is 4.6 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Malicious File (T1204.002); ranked at the 9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) and SI-3 (Malicious Code Protection) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

Notepad++ 8.9.3 contains a format string injection vulnerability in the Find Results panel handler that allows attackers to cause denial of service and information disclosure by crafting a malicious nativeLang.xml language pack file. Attackers can distribute a poisoned language pack…

more

through community channels that triggers format string interpretation when a user performs search operations, leading to access violations and potential leakage of stack or register contents.

CWE(s)
CWE-134

Related Threats

MITRE ATT&CK Enterprise TechniquesAI

T1204.002 Malicious File Execution
An adversary may rely upon a user opening a malicious file in order to gain execution.
attack.mitre.org →
Why these techniques?

Vulnerability is triggered by user loading a crafted malicious nativeLang.xml language pack file, directly enabling exploitation via malicious file execution.

Confidence: MEDIUM · MITRE ATT&CK Enterprise v19.0

Likely ATT&CK TechniquesAI

Techniques this vulnerability likely enables, inferred from its description, weakness type, and attributed-actor tradecraft. Confidence is per-technique.

T1005 Data from Local System Collectionconfidence: HIGH
Format string injection directly enables leakage of stack/register contents from the local process memory.
attack.mitre.org →
T1212 Exploitation for Credential Access Credential Accessconfidence: MEDIUM
Exploitation of the format string bug can yield sensitive data usable for credential access.
attack.mitre.org →
inferred from description + CWE · MITRE ATT&CK Enterprise v19.0

CVEs Like This One

CVE-2026-48770Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2026-5525Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2023-6401Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2026-25926Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2023-40166Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2026-48778Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2026-52884Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2023-40036Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2023-47452Same product: Notepad-Plus-Plus Notepad\+\+
CVE-2023-40164Same product: Notepad-Plus-Plus Notepad\+\+

Affected Assets

notepad-plus-plus
notepad\+\+
8.9.3

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)
  • SI-10 Information Input Validation
  • SI-7 Software, Firmware, and Information Integrity
  • SI-3 Malicious Code Protection
Detect
Catch it (NIST detect / respond)
  • SI-7 Software, Firmware, and Information Integrity
  • SI-3 Malicious Code Protection
Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)

—

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match
prevent

Directly blocks exploitation of the externally-controlled format string in nativeLang.xml by requiring validation/sanitization of all inputs before they reach the Find Results panel handler.

SI-7 Software, Firmware, and Information Integrity partial match
preventdetect

Requires integrity verification of language-pack files before loading, preventing a poisoned nativeLang.xml from being interpreted as trusted input.

SI-3 Malicious Code Protection partial match
preventdetect

Treats the malicious language pack as untrusted code/content and applies scanning or execution restrictions before the format-string payload can be processed during search operations.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly prevent external format strings via code review, static analysis, and safe APIs.

ID.RA-01 partial match
prevents

Vulnerability identification can discover existing format-string flaws but does not prevent their introduction.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
detects

Security testing in development can detect format-string vulnerabilities before release.

A.8.25 Secure development life cycle partial match
prevents

Secure SDLC mandates input validation and safe API usage that can prevent externally-controlled format strings.

A.8.26 Application security requirements partial match
prevents

Application security requirements can specify safe formatting functions and ban uncontrolled format strings.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles discourage dangerous string-handling patterns that lead to format-string issues.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly forbid the use of untrusted format strings and prescribe safe alternatives.

References