# 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.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N`)
- **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 available:** No
- **Published:** 31 Mar 2026
- **Updated:** 1 Apr 2026
- **Also known as:** GHSA-5j32-486h-42ch

## 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 (Improper Restriction of XML External Entity Reference)

## Which versions are affected by CVE-2026-34401?

It affects 1 product.


Products: 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-34401 and fix it if it is affected.

What it is: 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.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-34401 exists.

Affected product: microsoft XmlNotepad. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-34401/ (Markdown: https://insights.sbomify.com/CVE-2026-34401/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-34401
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-34401
- OSV: https://osv.dev/vulnerability/CVE-2026-34401

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-34401/
Summaries are generated and may contain mistakes; the official description above is the record.