What is CVE-2021-27279?

CVE-2021-27279 is a medium-severity vulnerability in Mybb, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-02-22.

How severe is CVE-2021-27279?

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

XSS in Mybb

CVE-2021-27279

MyBB before 1.8.25 allows stored XSS via nested [email] tags with MyCode (aka BBCode).

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-02-22 · 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: 0.010 (60.0th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 60th percentile — 60.0% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.010 · newest: 0.010 · change: 0.000

Affected products

Mybb

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

mybb.com/versions/1.8.25/ (Release Notes, Vendor Advisory)
github.com/mybb/mybb/security/advisories/GHSA-6483-hcpp-p75w (Exploit, Third Party Advisory)
github.com/mybb/mybb/commit/cb781b49116bf5c4d8deca3e17498122b701677a (Patch, Third Party Advisory)

Frequently asked questions

What is CVE-2021-27279?: CVE-2021-27279 is a medium-severity vulnerability in Mybb, classified under Cross-site Scripting. CVSS score: 5.4/10. Published 2021-02-22.
How severe is CVE-2021-27279?: Medium severity. CVSS v3 base score is 5.4 out of 10.