CVE-2026-21861: Baserproject/basercms Remote Code Execution Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-qxmc-6f24-g86g",
  "modified" : "2026-03-31T22:27:05Z",
  "published" : "2026-03-31T22:27:05Z",
  "aliases" : [ "CVE-2026-21861" ],
  "summary" : "baserCMS has OS Command Injection Leading to Remote Code Execution (RCE)",
  "details" : "## Summary\n\nIn the core update functionality of baserCMS, some parameters sent from the admin panel are passed to the `exec()` function without proper validation or escaping. This issue allows **an authenticated CMS administrator to execute arbitrary OS commands on the server (Remote Code Execution, RCE)**.\n\nThis vulnerability is not a UI-level issue such as screen manipulation or lack of CSRF protection, but rather stems from **a design that directly executes input values received on the server side as OS commands**. Therefore, even if buttons are hidden in the UI, or even if CakePHP's CSRF/FormProtection (SecurityComponent) ensures that only legitimate POST requests are accepted, **an attack is possible as long as a request containing a valid token is processed within an administrator session**.\n\n---\n\n## Vulnerability Information\n\n| Item | Details |\n| ---- | ------- |\n| CWE | CWE-78: Improper Neutralization of Special Elements used in an OS Command |\n| Impact | Remote Code Execution (RCE) |\n| Severity | Critical |\n| Attack Requirements | Administrator privileges required |\n| Reproducibility | Reproducible (confirmed multiple times) |\n| Test Environment | baserCMS 5.2.2 (Docker / development environment) |\n\n---\n\n## Affected Areas\n\n- **Controller**\n  - `PluginsController::get_core_update()`\n- **Service**\n  - `PluginsService::getCoreUpdate()`\n- **Affected Endpoint**\n  - `/baser/admin/baser-core/plugins/get_core_update`\n\n---\n\n## Technical Details\n\n### Vulnerable Code Flow\n\n```text\nPluginsController::get_core_update()\n  ↓ Retrieves php parameter from POST data\nPluginsService::getCoreUpdate($targetVersion, $php, $force)\n  ↓ Concatenates $php into command string without validation or escaping\nexec($command)\n```\n\n### Relevant Code (Excerpt)\n\n**PluginsController.php**\n\n```php\n$service->getCoreUpdate(\n    $request->getData('targetVersion') ?? '',\n    $request->getData('php') ?? 'php',\n    $request->getData('force'),\n);\n```\n\n**PluginsService.php**\n\n```php\n$command = $php . ' ' . ROOT . DS . 'bin' . DS . 'cake.php composer ' .\n           $targetVersion . ' --php ' . $php . ' --dir ' . TMP . 'update';\n\nexec($command, $out, $code);\n```\n\nThe `$php` parameter is user input, and **none** of the following countermeasures are in place:\n\n- Restriction via allowlist\n- Validation via regular expression\n- Escaping via `escapeshellarg()` or similar\n\n---\n\n## Attack Scenario\n\n1. The attacker logs in as a CMS administrator\n2. Sends a POST request to the core update functionality in the admin panel\n3. Specifies a string containing OS commands in the `php` parameter\n4. `exec()` is executed on the server side, running the arbitrary OS command\n\n### Example Attack Input (Conceptual)\n\n```text\nphp=php;id>/tmp/rce_test;#\n```\n\n---\n\n## Verification Results (PoC)\n\n### Execution Result\n\n```bash\n$ docker exec bc-php cat /tmp/rce_test\nuid=1000(www-data) gid=1000(www-data) groups=1000(www-data)\n```\n\nThe above confirms that OS commands can be executed with `www-data` privileges.\n\n### Additional Notes\n\n- Reproducible through the legitimate flow in the admin panel (browser)\n- Succeeds even with CSRF/FormProtection tokens included in a legitimate request\n- Failure cases (400/403) have also been investigated and differentiated\n- Confirmed reproducible via resending HTTP requests with tools such as curl (resending the same request containing valid tokens)\n\n---\n\n## Impact\n\nIf this vulnerability is exploited, the following becomes possible:\n\n- Retrieval of server information\n- Reading/writing arbitrary files\n- Retrieval of application configuration information (DB credentials, etc.)\n- OS-level operations beyond application permission boundaries\n\nAlthough administrator privileges are required, **this is a design issue where the impact extends from the application layer to the OS layer**, and the impact is considered significant.\n\n---\n\n## Recommended Fix\n\n### Primary Recommendation\n\n- Do not accept the PHP executable path from user input\n- Fix the PHP executable on the server side using the `PHP_BINARY` constant\n\n```php\n$php = escapeshellarg(PHP_BINARY);\n```\n\n### Supplementary Fix Recommendations\n\n- Apply `escapeshellarg()` escaping to other command-line arguments (version number, directory, etc.) as well\n- If possible, consider using execution methods that do not involve shell interpretation (array format, Process class, etc.)\n\n### Alternative (Not Recommended)\n\n- Allowlist validation for the PHP executable path\n- Combined use of regex validation and `escapeshellarg()`\n\nHowever, **from the perspective of reducing the attack surface, a design that eliminates user input entirely is recommended**.\n\n---\n\n## Additional Notes\n\n- This issue is independent of UI display controls (showing/hiding buttons)\n- As long as the endpoint exists, an attack is possible if a request containing valid tokens is processed\n- This is a problem stemming from the design-level handling of input, and cannot be prevented by CSRF or UI controls alone\n\n---\n\n## Conclusion\n\nDue to a design issue in baserCMS's core update functionality where user input is passed to `exec()` without validation, **Remote Code Execution (RCE) is achievable with administrator privileges**. This vulnerability can be fixed through input validation and design review, and prompt remediation is recommended.\n\nThis advisory was translated from Japanese to English using GitHub Copilot.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:H/UI:N/S:C/C:H/I:H/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Packagist",
      "name" : "baserproject/basercms"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.2.3"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 5.2.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/baserproject/basercms/security/advisories/GHSA-qxmc-6f24-g86g"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-21861"
  }, {
    "type" : "WEB",
    "url" : "https://basercms.net/security/JVN_20837860"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/baserproject/basercms"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/baserproject/basercms/releases/tag/5.2.3"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-78" ],
    "severity" : "CRITICAL",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-31T22:27:05Z",
    "nvd_published_at" : "2026-03-31T01:16:35Z"
  }
}