CVE-2021-39338

Name: CVE-2021-39338
Creator: NVD / NIST
Published: 2021-10-15T13:15:07.893+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

MEDIUMCVSS 4.8/10EPSS 0.92%

Last modified Jun 17, 2026

CVE-2021-39338 is a medium-severity vulnerability rated 4.8/10 on the CVSS scale. The MyBB Cross-Poster WordPress plugin is vulnerable to Stored Cross-Site Scripting due to insufficient input validation and sanitization via several parameters found in the ~/classes/MyBBXPSettings.php file which allowed attackers with administrative user access to inject arbitrary web scripts, in versions up to and including 1.0. This affects multi-site installations where unfiltered_html is disabled for administrators, and sites where unfiltered_html is disabled.. EPSS estimates a 0.92% chance of exploitation in the next 30 days.

Description

The MyBB Cross-Poster WordPress plugin is vulnerable to Stored Cross-Site Scripting due to insufficient input validation and sanitization via several parameters found in the ~/classes/MyBBXPSettings.php file which allowed attackers with administrative user access to inject arbitrary web scripts, in versions up to and including 1.0. This affects multi-site installations where unfiltered_html is disabled for administrators, and sites where unfiltered_html is disabled.

Metrics

CVSS 3.1

4.8/10

CVSS:3.1/AV:N/AC:L/PR:H/UI:R/S:C/C:L/I:L/A:N

EPSS Probability

0.92%

55.6th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-79

Affected Software

Vendor	Product	Versions
Mybb Cross-Poster Project	Mybb Cross-Poster	<= 1.0

References

https://github.com/BigTiger2020/word-press/blob/main/MyBB%20Cross-Poster.mdExploit, Third Party Advisory
https://plugins.trac.wordpress.org/browser/mybb-cross-poster/trunk/classes/MyBBXPSettings.phpPatch, Third Party Advisory
https://www.wordfence.com/vulnerability-advisories/#CVE-2021-39338Third Party Advisory
https://github.com/BigTiger2020/word-press/blob/main/MyBB%20Cross-Poster.mdExploit, Third Party Advisory
https://plugins.trac.wordpress.org/browser/mybb-cross-poster/trunk/classes/MyBBXPSettings.phpPatch, Third Party Advisory
https://www.wordfence.com/vulnerability-advisories/#CVE-2021-39338Third Party Advisory

Timeline

Published: Oct 15, 2021
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2021-39338?

How severe is CVE-2021-39338?

CVE-2021-39338 has a CVSS score of 4.8/10 (MEDIUM severity). The EPSS model estimates a 0.92% probability of exploitation in the next 30 days.

How do I fix CVE-2021-39338?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2021-39338?

Run a free Strix scan to check your systems for this vulnerability.

Scan your code now

Source: NVD / NIST