Security readout for executives and security teams
Plain-English summary
CVE-2021-39338 is a stored cross-site scripting issue in the MyBB Cross-Poster WordPress plugin through version 1.0. It matters mainly where an attacker already has administrator access but should not be able to add unrestricted HTML or scripts.
Executive priority
Treat as a medium-priority cleanup item, not an emergency, unless administrator accounts may be compromised or the plugin is deployed broadly across managed WordPress sites.
Technical view
The vulnerability is CWE-79 caused by insufficient validation and sanitization of several settings parameters in classes/MyBBXPSettings.php. An authenticated administrator could store script content that later runs in a privileged browser context. CVSS is 5.5 with high privileges required and changed scope.
Likely exposure
Exposure is limited to WordPress sites running MyBB Cross-Poster version 1.0 or earlier, especially multisite installations or environments where unfiltered_html is disabled for administrators.
Exploitation context
The source bundle does not show CISA KEV listing or active exploitation evidence. Exploitation requires authenticated administrator access, so risk depends heavily on admin account trust, compromise likelihood, and multisite policy.
Researcher notes
Evidence identifies affected version 1.0 and vulnerable settings handling, but the bundle does not provide a confirmed patched version or active exploitation. Keep remediation tied to vendor guidance and local exposure validation.
Mitigation direction
- Inventory WordPress sites for MyBB Cross-Poster version 1.0 or earlier.
- Check vendor or WordPress plugin guidance for fixed versions or replacement advice.
- Disable or remove the plugin if it is not business-critical.
- Restrict and review administrator accounts on affected WordPress sites.
- Prioritize sites where unfiltered_html is disabled or multisite is enabled.
Validation and detection
- Confirm plugin presence and version on each WordPress installation.
- Check whether the WordPress site is multisite.
- Verify whether unfiltered_html is disabled for administrators.
- Review plugin settings for unexpected script-like content.
- Review administrator account activity for suspicious configuration changes.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-79: User-session and phishing behavior lookup
Client-side and session-facing weaknesses should be reviewed alongside initial-access and user-execution behaviors. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2021-39338 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Medium
- CVSS
- 5.5 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N2.32.7Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
5.5MediumVector: CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:L/I:L/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://www.wordfence.com/vulnerability-advisories/#CVE-2021-39338CVE reference · x_refsource_MISC
- https://github.com/BigTiger2020/word-press/blob/main/MyBB%20Cross-Poster.mdCVE reference · x_refsource_MISC
- https://plugins.trac.wordpress.org/browser/mybb-cross-poster/trunk/classes/MyBBXPSettings.phpCVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.