What is CVE-2021-29399?

CVE-2021-29399 is a medium-severity vulnerability in PHP, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2021-04-19.

How severe is CVE-2021-29399?

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

XSS in PHP

CVE-2021-29399

XMB is vulnerable to cross-site scripting (XSS) due to inadequate filtering of BBCode input. This bug affects all versions of XMB. All XMB installations must be updated to versions 1.9.12.03 or 1.9.11.16.

Vulnerability class: XSS (Cross-Site Scripting)

Published 2021-04-19 · 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: 0.008 (55.4th percentile), scored 2026-09-29.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 55th percentile — 55.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.008 · newest: 0.008 · change: 0.000

Affected products

PHP — versions 5.0.0, 7.0.0, 8.0.0
Xmbforum2 Xmb

Weakness classification (CWE)

CWE-79 — Cross-site Scripting

References

www.xmbforum2.com/ (Vendor Advisory)
docs.xmbforum2.com/index.php (Vendor Advisory)
forums.xmbforum2.com/viewthread.php (Patch, Vendor Advisory)

Frequently asked questions

What is CVE-2021-29399?: CVE-2021-29399 is a medium-severity vulnerability in PHP, classified under Cross-site Scripting. CVSS score: 6.1/10. Published 2021-04-19.
How severe is CVE-2021-29399?: Medium severity. CVSS v3 base score is 6.1 out of 10.