CVE-2026-41128: Craftcms/cms Craft CMS Has A Missing Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jq2f-59pj-p3m3",
  "modified" : "2026-04-24T20:42:09Z",
  "published" : "2026-04-14T23:34:52Z",
  "aliases" : [ "CVE-2026-41128" ],
  "summary" : "Craft CMS has a Missing Authorization Check on User Group Removal via save-permissions Action",
  "details" : "## Summary\n\nThe `actionSavePermissions()` endpoint allows a user with only `viewUsers` permission to remove arbitrary users from all user groups. While `_saveUserGroups()` enforces per-group authorization for additions, it performs no equivalent authorization check for removals, so submitting an empty `groups` value removes all existing group memberships.\n\n## Affected Versions\n\n- Craft CMS 5.6.0 through 5.9.14 (latest release at time of report)\n- Regression introduced in 5.6.0 when the `viewUsers` permission was added\n- Prior to 5.6.0, `editedUser()` required `editUsers`, which implicitly protected this endpoint\n- Requires Pro edition or higher (the vulnerable code path is gated by `CmsEdition::Pro`)\n\n## Vulnerability Details\n\n### Root Cause\n\nThis is a **regression** introduced in Craft CMS 5.6.0 when the `viewUsers` permission was added. Before that change, `editedUser()` required `editUsers` permission for accessing other users’ data, which implicitly protected `actionSavePermissions()`. After the change, `actionSavePermissions()` became reachable for users with read-only access to other users, but the underlying group-saving logic still lacked authorization for group removals.\n\nThe vulnerability has two components:\n\n1. **`actionSavePermissions()` reachable with read-only access**: The action only requires a control panel request and delegates to `editedUser()`, which now only checks `viewUsers` — a permission explicitly documented as \"read-only access to user elements.\"\n\n2. **Asymmetric authorization in `_saveUserGroups()`**: The method checks `assignUserGroup` permission only when **adding** a user to a new group. When the `groups` parameter is an empty string (resulting in an empty array), the loop is skipped entirely, no authorization checks are run, and all group memberships are removed.\n\n### Prerequisites\n\n- Attacker has a control panel account with `accessCp` and `viewUsers` permissions only\n- Target user belongs to one or more user groups that grant additional permissions\n- Pro edition or higher\n\n### Attack Steps\n\n1. Attacker authenticates to the Control Panel\n2. Attacker sends a POST request to `actions/users/save-permissions` with:\n   - `userId` = target user's ID\n   - `groups` = `` (empty string)\n3. All group memberships for the target user are removed\n4. All permissions inherited from those groups are immediately revoked\n\n### Impact\n\n- **Privilege revocation**: An attacker can strip group-based permissions from arbitrary users, including accounts whose effective access derives from group membership\n- **Denial of access**: Users lose access to sections, volumes, and features that were granted through group membership\n- **Bypass of elevated session requirement**: Group removal does not trigger `requireElevatedSession()` (which is only triggered when new groups are added)",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "craftcms/cms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "5.6.0"
      }, {
        "fixed" : "5.9.15"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/craftcms/cms/security/advisories/GHSA-jq2f-59pj-p3m3"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-41128"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/craftcms/cms/commit/b135384808ad43fcf8836a9dd9b877fb0087bc27"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/craftcms/cms"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-862" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-04-14T23:34:52Z",
    "nvd_published_at" : "2026-04-22T00:16:28Z"
  }
}