What’s Vulnerable › Vulnerabilities

CVE-2021-39338: MyBB Cross-Poster <= 1.0 Authenticated Stored Cross-Site Scripting

• Severity: medium (CVSS 4.8 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.9% probability of exploitation in the next 30 days
• Public exploit or detection template: yes
• Published: 2021-10-15 · updated 2026-06-17
• Weakness: CWE-79

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.

Affected products

• MyBB Cross-Poster: through 1.0

References

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

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