# CVE-2021-39169: XSS vulnerability using dialog

> Attackers can inject malicious scripts via the web client dialog, potentially stealing API tokens and compromising user accounts.

- **Severity:** High 8.0 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H`)
- **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:** No
- **Published:** 27 Aug 2021
- **Updated:** 4 Aug 2024
- **Also known as:** GHSA-pmmv-jwqh-f5ww

## What is CVE-2021-39169?

CVE-2021-39169 is a high-severity cross-site scripting vulnerability in misskey-dev misskey, published on 27 Aug 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.

### Official description

Misskey is a decentralized microblogging platform. In versions of Misskey prior to 12.51.0, malicious actors can use the web client built-in dialog to display a malicious string, leading to cross-site scripting (XSS). XSS could compromise the API request token. This issue has been fixed in version 12.51.0. There are no known workarounds aside from upgrading.

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2021-39169?

It affects 1 product.


Products: Misskey Misskey.

## How do I fix CVE-2021-39169?

No fix has been published for CVE-2021-39169 yet. Limit exposure of the affected software until one is available.

## Is CVE-2021-39169 being exploited?

There is no known exploitation of CVE-2021-39169 in the wild and no public exploit code.

## Other vulnerabilities in misskey-dev misskey

- [CVE-2025-25306](https://insights.sbomify.com/CVE-2025-25306/): Misskey's Incomplete Patch of CVE-2024-52591 Leads to Forgery of Federated Notes (critical 9.3)
- [CVE-2023-49079](https://insights.sbomify.com/CVE-2023-49079/): Misskey's missing signature validation allows arbitrary users to impersonate any remote user. (critical 9.3)
- [CVE-2026-28431](https://insights.sbomify.com/CVE-2026-28431/): Misskey lacks proper authorization checks and input validation (critical 9.2)
- [CVE-2023-52139](https://insights.sbomify.com/CVE-2023-52139/): Misskey vulnerable to improper authorization when accessing with third-party application (critical 9.1)
- [CVE-2024-52590](https://insights.sbomify.com/CVE-2024-52590/): Missing validation allows spoofed profiles in Misskey (high 8.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-39169 and fix it if it is affected.

What it is: CVE-2021-39169 is a high-severity cross-site scripting vulnerability in misskey-dev misskey, published on 27 Aug 2021. Any signed-in user can exploit it over the network, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-39169 in the wild and no public exploit code.

Affected product: misskey-dev misskey. 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-2021-39169/ (Markdown: https://insights.sbomify.com/CVE-2021-39169/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-39169
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-39169
- OSV: https://osv.dev/vulnerability/CVE-2021-39169

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-39169/
Summaries are generated and may contain mistakes; the official description above is the record.