Can you explain this vulnerability to me?

CVE-2021-47870 is a Stored Cross-Site Scripting (XSS) vulnerability in the GetSimple CMS My SMTP Contact Plugin version 1.1.2. The plugin tries to sanitize user input using the PHP function htmlspecialchars(), but this can be bypassed by encoding dangerous characters as escaped hexadecimal bytes. This allows attackers to inject arbitrary client-side scripts that execute in the administrator's browser when they visit a maliciously crafted page. Additionally, this vulnerability can be chained with a Cross-Site Request Forgery (CSRF) vulnerability to achieve Remote Code Execution (RCE) on the hosting server by exploiting the administrator's session and uploading a webshell. [1, 2]

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How can this vulnerability impact me? :

This vulnerability can allow attackers to execute arbitrary client-side code in the administrator's browser, potentially leading to Remote Code Execution (RCE) on the hosting server. By exploiting this, attackers can upload malicious webshells to the CMS, gaining full control over the server, compromising confidentiality, integrity, and availability of the system. This can result in unauthorized access, data theft, defacement, or further attacks on the network. [1, 2]

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How can this vulnerability be detected on my network or system? Can you suggest some commands?

Detection of this vulnerability involves identifying if the GetSimple CMS My SMTP Contact Plugin version 1.1.2 or earlier is installed and checking for signs of stored XSS exploitation. Since the vulnerability involves bypassing htmlspecialchars() sanitization by encoding dangerous characters as escaped hex bytes, monitoring HTTP POST requests to the plugin's contact form for suspicious encoded payloads can help. Additionally, inspecting administrator browser activity for unexpected script execution or unusual XMLHttpRequests (XHR) may indicate exploitation. Specific commands are not provided in the resources, but general approaches include using web application scanners to detect stored XSS, reviewing web server logs for suspicious POST requests, and using browser developer tools to monitor XHR activity during admin sessions. [1, 2]

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What immediate steps should I take to mitigate this vulnerability?

Immediate mitigation steps include updating or patching the GetSimple CMS My SMTP Contact Plugin to a version that fixes the stored XSS vulnerability if available. If no patch is available, restrict administrative access to trusted users only and avoid visiting untrusted or suspicious websites while logged in as an administrator. Additionally, implement web application firewall (WAF) rules to block suspicious POST requests containing encoded payloads targeting the plugin. Monitoring and restricting the use of the vulnerable plugin or disabling it temporarily can also reduce risk. [1, 2]

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