CVE-2026-34401 XML Notepad: XML External Entity (XXE) Injection via Unsafe XmlTextReader in XML Diff and Schema Loading
An attacker can craft a malicious XML file to trigger XXE, potentially leaking local files or capturing NTLM credentials via outbound requests.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-34401?
CVE-2026-34401 is a medium-severity vulnerability in microsoft XmlNotepad, published on 31 Mar 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official 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.
Weakness: CWE-611
Which versions are affected by CVE-2026-34401?
It affects 1 product.
Products include Microsoft Xml Notepad.
How do I fix CVE-2026-34401?
No fix has been published for CVE-2026-34401 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-34401 being exploited?
It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-34401 exists.
Sources
Also known as GHSA-5j32-486h-42ch.