Security readout for executives and security teams
Plain-English summary
CVE-2021-39318 is a reflected cross-site scripting issue in the H5P CSS Editor WordPress plugin version 1.0 and earlier. An attacker could trick a user into opening a crafted link that causes script to run in that user’s browser. Business impact is mainly account/session abuse or content manipulation, not direct server takeover.
Executive priority
Treat as a moderate web application risk. It deserves timely cleanup during WordPress maintenance, especially on public sites with administrator use, but the provided evidence does not support emergency active-exploitation response.
Technical view
The issue is CWE-79 in h5p-css-editor.php, where the h5p-css-file parameter is reported as injectable for arbitrary web script. CVSS 3.1 is 6.1: network reachable, low complexity, no privileges, user interaction required, changed scope, low confidentiality and integrity impact, no availability impact.
Likely exposure
Exposure is limited to WordPress sites with the H5P CSS Editor plugin installed at version 1.0 or earlier. The source bundle does not identify other affected products, CPEs, or a confirmed patched version.
Exploitation context
The CVE is not listed as CISA KEV in the provided bundle, and no source here states active exploitation. Reflected XSS generally requires social engineering or another path that gets a victim to follow attacker-controlled input.
Researcher notes
Evidence names the vulnerable parameter and file but does not include patch details. Keep analysis scoped to H5P CSS Editor <= 1.0, reflected XSS, and user-interaction-dependent browser impact unless additional vendor or incident data is obtained.
Mitigation direction
- Inventory WordPress sites for H5P CSS Editor version 1.0 or earlier.
- Check Wordfence and WordPress plugin guidance for a fixed or retired release.
- Disable or remove the plugin where no maintained fix is available.
- Prioritize sites with privileged WordPress users or sensitive H5P content workflows.
Validation and detection
- Confirm installed plugin name and version across WordPress assets.
- Review web logs for unusual requests involving h5p-css-file.
- Check whether h5p-css-editor.php is reachable on public sites.
- Verify remediation by confirming the vulnerable plugin version is absent.
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-2021-39318 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
- 6.1 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/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:N/UI:R/S:C/C:L/I:L/A:N2.82.7Primary CVE scoreVulnerability scoring details
Base CVSS 3.1 score
6.1MediumVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Source materials
- CVE List V5 sourceCVE List V5
- https://www.wordfence.com/vulnerability-advisories/#CVE-2021-39318CVE reference · x_refsource_MISC
- https://plugins.trac.wordpress.org/browser/h5p-css-editor/tags/1.0/h5p-css-editor.php#L50CVE 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.