CVE-2021-39338: The MyBB Cross-Poster WordPress plugin is vulnerable to Stored Cross-Site Scripting due to insufficient input

Q: How do I fix CVE-2021-39338?

mybb cross-poster: upgrade to newer than 1.0. Exact commands for each OS are on this page.

Q: Is CVE-2021-39338 being exploited?

CVE-2021-39338 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2021-39338 affect?

CVE-2021-39338 affects mybb cross-poster. Severity is medium (CVSS 5.5). 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 adm

Q: How urgent is CVE-2021-39338?

Priority 40/100. Public exploit reference

This month

MEDIUM 5.5UPGRADE

CVE-2021-39338 is a medium-severity flaw (CVSS 5.5) affecting mybb cross-poster. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

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 unfilter

Why this priority

Public exploit reference

A fixed release or distro patch exists.

Risk facts

CVSS score	5.5 (Medium)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	High
User interaction	None
EPSS exploit chance	0.9% (top 39%)
CISA exploited list	No
Weakness	CWE-79
Published	2021-10-15
Last modified	2026-06-17

Publisher (CNA): Wordfence · Vendor: mybb cross-poster project

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2021-10-15 13:15 UTCPublished CVE record published
2026-06-17 04:03 UTCUpdated Record last modified

mybb cross-poster project mybb cross-poster UPGRADE

Affected: <= 1.0

Fixed in: newer than 1.0

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy mybb-cross-poster || rpm -q mybb-cross-poster || mybb-cross-poster --version

2. UPGRADE to newer than 1.0 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade mybb-cross-poster

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade mybb-cross-poster

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2021-39338?

mybb cross-poster: upgrade to newer than 1.0. Exact commands for each OS are on this page.

Is CVE-2021-39338 being exploited?

CVE-2021-39338 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2021-39338 affect?

CVE-2021-39338 affects mybb cross-poster. Severity is medium (CVSS 5.5). 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 adm

How urgent is CVE-2021-39338?

Priority 40/100. Public exploit reference

Software pages

WordPress vulnerabilities · PHP vulnerabilities

References

← Back to latest CVEs