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Advanced Custom FieldsWordPress extension · Advancedcustomfields

CVE-2021-20866

MEDIUM · 6.5 CVSS v3.1 Published 2021-12-13
Fix available
A fix is available. Upgrade to 5.11 or later.
See remediation →
71/100
Remediation priority · Elevated
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Advanced Custom Fields versions prior to 5.11 and Advanced Custom Fields Pro versions prior to 5.11 contain a missing authorization vulnerability in obtaining the user list which may allow a user to obtain the unauthorized information via unspecified vectors.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Advanced Custom Fields (both free and Pro versions) before 5.11 lacks proper authorization checks when returning user list data, allowing authenticated users to access user information they should not have permission to view.

MitigationUpgrade Advanced Custom Fields to version 5.11 or later to receive the authorization fix.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Advanced Custom FieldsWordPress extension
Affected:< 5.11

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

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

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Confirm Advanced Custom Fields is installed
    Check the WordPress plugins directory, wp-content/plugins/ folder, or run 'wp plugin list' via WP-CLI to list installed plugins
    Affected if Advanced Custom Fields plugin is not found in the installation
  2. Identify the installed version
    Check the plugin header in main PHP file (wp-content/plugins/advanced-custom-fields/acf.php) or run 'wp plugin get advanced-custom-fields --field=version' via WP-CLI
    Affected if Version is lower than 5.11 (e.g., 5.10.x, 5.9.x, etc.)
  3. Verify WordPress user authentication is enabled
    Check WordPress general settings at /wp-admin/options-general.php or inspect wp_options table for 'users_can_register' setting
    Affected if User registration is enabled or the site has user accounts beyond the administrator
  4. Confirm ACF user list endpoint is accessible
    Test access to wp-json/acf/v2/users or similar ACF user-related REST API endpoints using a low-privilege authenticated account
    Affected if An authenticated user with limited permissions can retrieve user data they should not have access to

The environment is affected if Advanced Custom Fields version is below 5.11 AND the site has authenticated user accounts, allowing a low-privilege user to access restricted user information through ACF endpoints.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 5.11 or later
Fixed in 5.11
Interim mitigation

Upgrade Advanced Custom Fields to version 5.11 or later to receive the authorization fix.

Recommended fix High confidence

Advanced Custom Fields 5.11 or later (including ACF Pro 5.11)

  1. Log in to WordPress admin dashboard
  2. Navigate to Plugins > Installed Plugins
  3. Locate 'Advanced Custom Fields' (or 'Advanced Custom Fields Pro') in the plugin list
  4. If an update is available, click 'Update Now' to upgrade to version 5.11 or later
  5. Alternatively, go to Plugins > Add New > Upload Plugin and upload the latest version from wordpress.org
  6. Verify the update completed successfully by checking the plugin version number
  7. Test that the plugin functions correctly after the update

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Advanced Custom Fields Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

Scan for this in your stack

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-20866 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data