Microsoft XmlNotepad prior 2.9.0.21 HTTP/SMB xml external entity reference

Summaryinfo

A vulnerability was found in Microsoft XmlNotepad. It has been rated as problematic. Affected by this vulnerability is an unknown functionality of the component HTTP/SMB. Performing a manipulation results in xml external entity reference. This vulnerability is known as CVE-2026-34401. Remote exploitation of the attack is possible. No exploit is available. Upgrading the affected component is advised.

Detailsinfo

A vulnerability, which was classified as critical, was found in Microsoft XmlNotepad. This affects an unknown function of the component HTTP/SMB. The manipulation with an unknown input leads to a xml external entity reference vulnerability. CWE is classifying the issue as CWE-611. The product processes an XML document that can contain XML entities with URIs that resolve to documents outside of the intended sphere of control, causing the product to embed incorrect documents into its output. This is going to have an impact on confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at github.com. This vulnerability is uniquely identified as CVE-2026-34401 since 03/27/2026. The exploitability is told to be easy. It is possible to initiate the attack remotely. No form of authentication is needed for exploitation. It demands that the victim is doing some kind of user interaction. Neither technical details nor an exploit are publicly available. The price for an exploit might be around USD $0-$5k at the moment (estimation calculated on 04/01/2026).

Upgrading to version 2.9.0.21 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 3665603d61ba10b7827a3724e854748cb780140c is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

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Productinfo

Vendor

• Microsoft

Name

• XmlNotepad

License

• commercial

Website

• Vendor: https://www.microsoft.com/
• Product: https://github.com/microsoft/XmlNotepad/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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