What is CVE-2021-41866?

CVE-2021-41866 is a medium-severity vulnerability in Mybb, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-10-26.

How severe is CVE-2021-41866?

Medium severity. CVSS v3 base score is 5.4 out of 10.

XSS in Mybb

CVE-2021-41866

MyBB before 1.8.28 allows stored XSS because the displayed Template Name value in the Admin CP's theme management is not escaped properly.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-10-26 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.4 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.005 (39.9th percentile), scored 2026-09-30.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 40th percentile — 39.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · change: 0.000

Affected products

Mybb

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

github.com/mybb/mybb/security/advisories/ (Third Party Advisory)
github.com/mybb/mybb/security/advisories/GHSA-gxhv-r3m5-6qv7 (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-41866?: CVE-2021-41866 is a medium-severity vulnerability in Mybb, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-10-26.
How severe is CVE-2021-41866?: Medium severity. CVSS v3 base score is 5.4 out of 10.