What’s Vulnerable › Vulnerabilities

CVE-2026-34401: XML Notepad: XML External Entity (XXE) Injection via Unsafe XmlTextReader in XML Diff and Schema Loading

• Severity: medium (CVSS 6.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.0% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-03-31 · updated 2026-07-24
• Weakness: CWE-611

Description

XML Notepad is a Windows program that provides a simple intuitive User Interface for browsing and editing XML documents. Prior to version 2.9.0.21, XML Notepad does not disable DTD processing by default which means external entities are resolved automatically. There is a well known attack related to malicious DTD files where an attacker to craft a malicious XML file that loads a DTD that causes XML Notepad to make outbound HTTP/SMB requests, potentially leaking local file contents or capturing the victim's NTLM credentials. This issue has been patched in version 2.9.0.21.

Affected products

• microsoft XmlNotepad: before 2.9.0.21 — fixed in 2.9.0.21
• microsoft XmlNotepad: before 2.9.0.21

How to fix

Upgrade to a fixed version: microsoft XmlNotepad 2.9.0.21. Check the vendor advisory for mitigations.

References

• https://github.com/microsoft/XmlNotepad/commit/3665603d61ba10b7827a3724e854748cb780140c
• https://github.com/microsoft/XmlNotepad/commit/c03ab2311ac6960452eb1ab49098768f851dcc53
• https://github.com/microsoft/XmlNotepad/releases/tag/2.9.0.21
• https://github.com/microsoft/XmlNotepad/security/advisories/GHSA-5j32-486h-42ch
• https://github.com/microsoft/XmlNotepad/security/advisories/GHSA-5j32-486h-42ch

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