What is CVE-2021-39169?

CVE-2021-39169 is a high-severity vulnerability in Misskey, classified under Cross-site Scripting. CVSS score: 8.0/10. Published 2021-08-27.

How severe is CVE-2021-39169?

High severity. CVSS v3 base score is 8.0 out of 10.

XSS in Misskey

CVE-2021-39169

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.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-08-27 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 8.0 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:U/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.007 (52.6th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 53rd percentile — 52.6% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000

Affected products

Misskey
Misskey-Dev Misskey — versions < 12.51.0

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

github.com/misskey-dev/misskey/security/advisories/GHSA-pmmv-jwqh-f5ww (Patch, Third Party Advisory)
github.com/misskey-dev/misskey/commit/ec203f7f795766f76b55fecc9248168c1cdf6c99 (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-39169?: CVE-2021-39169 is a high-severity vulnerability in Misskey, classified under Cross-site Scripting. CVSS score: 8.0/10. Published 2021-08-27.
How severe is CVE-2021-39169?: High severity. CVSS v3 base score is 8.0 out of 10.