What is CVE-2021-27949?

CVE-2021-27949 is a medium-severity vulnerability in Mybb, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2021-03-15.

How severe is CVE-2021-27949?

Medium severity. CVSS v3 base score is 6.1 out of 10.

XSS in Mybb

CVE-2021-27949

Cross-site Scripting vulnerability in MyBB before 1.8.26 via Custom moderator tools.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-03-15 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.1 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N.

EPSS exploit prediction

EPSS: 0.006 (47.8th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 48th percentile — 47.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000

Affected products

Mybb

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

github.com/mybb/mybb/security/advisories/GHSA-cmmr-39v8-8rx2 (Patch, Third Party Advisory, Release Notes)

Frequently asked questions

What is CVE-2021-27949?: CVE-2021-27949 is a medium-severity vulnerability in Mybb, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2021-03-15.
How severe is CVE-2021-27949?: Medium severity. CVSS v3 base score is 6.1 out of 10.