# CVE-2026-40262: Note Mark has Stored XSS via Unrestricted Asset Upload

> An attacker can upload malicious HTML or SVG files that execute JavaScript in victims' browsers, potentially stealing sessions or performing actions.

- **Severity:** High 8.7 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 16 Apr 2026
- **Updated:** 18 Apr 2026
- **Also known as:** GHSA-9pr4-rf97-79qh, GO-2026-5295

## What is CVE-2026-40262?

CVE-2026-40262 is a high-severity cross-site scripting vulnerability in enchant97 note-mark, published on 16 Apr 2026. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

Note Mark is an open-source note-taking application. In versions 0.19.1 and prior, the asset delivery handler serves uploaded files inline and relies on magic-byte detection for content type, which does not identify text-based formats such as HTML, SVG, or XHTML. These files are served with an empty Content-Type, no X-Content-Type-Options: nosniff header, and inline disposition, allowing browsers to sniff and render active content. An authenticated user can upload an HTML or SVG file containing JavaScript as a note asset, and when a victim navigates to the asset URL, the script executes under the application's origin with access to the victim's authenticated session and API actions. This issue has been fixed in version 0.19.2.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation), CWE-434 (Unrestricted Upload of File with Dangerous Type)

## Which versions are affected by CVE-2026-40262?

3 open source packages ship an affected version, including github.com/enchant97/note-mark/backend.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/enchant97/note-mark/backend` | Go | All versions | 0.0.0-20260411145018-6bb62842ccb9 |
| `github.com/enchant97/note-mark/backend` | Go | All versions | 0.0.0-20260411145018-6bb62842ccb9 |
| `github.com/enchant97/note-mark/backend` | Go | All versions | 0.0.0-20260411145018-6bb62842ccb9 |

## How do I fix CVE-2026-40262?

Upgrade github.com/enchant97/note-mark/backend to 0.0.0-20260411145018-6bb62842ccb9 or later.

## Is CVE-2026-40262 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-40262 exists.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40262 and fix it if it is affected.

What it is: CVE-2026-40262 is a high-severity cross-site scripting vulnerability in enchant97 note-mark, published on 16 Apr 2026. Any signed-in user can exploit it over the network, 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-2026-40262 exists.

Affected packages and fixed versions:
- github.com/enchant97/note-mark/backend (Go): affected all versions; fixed in 0.0.0-20260411145018-6bb62842ccb9
- github.com/enchant97/note-mark/backend (Go): affected all versions; fixed in 0.0.0-20260411145018-6bb62842ccb9
- github.com/enchant97/note-mark/backend (Go): affected all versions; fixed in 0.0.0-20260411145018-6bb62842ccb9

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-2026-40262/ (Markdown: https://insights.sbomify.com/CVE-2026-40262/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40262
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40262
- OSV: https://osv.dev/vulnerability/CVE-2026-40262

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40262/
Summaries are generated and may contain mistakes; the official description above is the record.