CVE-2023-5424

Executive Summary

CVE-2023-5424 is a CSV Injection vulnerability (CWE-1236) affecting the WS Form LITE plugin for WordPress, up to and including version 1.9.217. This vulnerability allows an unauthenticated attacker to inject arbitrary formulas into exported CSV files. When a victim with a vulnerable spreadsheet application (e.g., Microsoft Excel, LibreOffice Calc) opens the crafted CSV file, the injected formulas can execute, leading to potential code execution on the victim's local system. The attack vector is through the web interface, where user-supplied input is not sanitized before being placed into CSV exports. The CVSS score is 4.7 (Medium), with a low EPSS score of 0.0%, indicating low exploitation activity in the wild. No official patch has been released by the vendor at this time.

Supply Chain & Package Security

This vulnerability is specific to the WordPress plugin ecosystem. The WS Form LITE plugin is distributed through the official WordPress plugin repository. Users are advised to monitor the plugin's page for updates and consider disabling CSV export functionality or applying a virtual patch via a Web Application Firewall (WAF) until an official fix is released.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:L
• Base Score: 4.7 (Medium)
• Attack Vector (AV): Network (N) - The vulnerability is exploitable remotely.
• Attack Complexity (AC): Low (L) - No special conditions are required.
• Privileges Required (PR): None (N) - No authentication is needed to trigger the export.
• User Interaction (UI): Required (R) - The victim must open the malicious CSV file.
• Scope (S): Changed (C) - The impact extends beyond the vulnerable component to the user's local system.
• Confidentiality (C): Low (L) - Limited information disclosure possible.
• Integrity (I): Low (L) - Limited ability to modify data.
• Availability (A): Low (L) - Limited impact on availability.

Threat Level: Medium. The requirement for user interaction (opening a CSV file) reduces the immediate risk, but the potential for code execution on a victim's machine makes it a significant concern for organizations using the plugin.

Deep Dive & Mechanics

The vulnerability resides in the CSV export functionality of the WS Form LITE plugin. When a user (typically an administrator) exports form submission data to a CSV file, the plugin does not sanitize or escape user-supplied input that begins with special characters such as =, +, -, or @. These characters are interpreted by spreadsheet applications as formula prefixes.

An attacker can submit a form with a crafted payload in a text field, such as =CMD|' /C calc'!A0. When the administrator exports the submissions and opens the CSV in Microsoft Excel, the formula is executed. In this example, it would launch the Windows Calculator application. More sophisticated payloads can execute arbitrary commands, download malware, or exfiltrate data.

The attack flow:

1. Attacker submits a form with a malicious payload in a text field.
2. Administrator exports form submissions to CSV.
3. Administrator opens the CSV file in a vulnerable spreadsheet application.
4. The spreadsheet application executes the injected formula, leading to code execution.

Vulnerable vs. Patched Code

The vulnerable code likely lacks proper escaping of CSV fields. A patched version would sanitize the output by prefixing fields that start with special characters with a single quote (') or a tab character, which prevents formula execution.

Vulnerable Code (Conceptual):

// No sanitization of $field_value
fputcsv($file, array($field_value));

Patched Code (Conceptual):

// Sanitize fields that start with special characters
if (in_array($field_value[0], array('=', '+', '-', '@'))) {
    $field_value = "'" . $field_value;
}
fputcsv($file, array($field_value));

Exploitation & Lab Setup

Red Team

Prerequisites:

• A WordPress instance with WS Form LITE <= 1.9.217 installed and activated.
• A form with at least one text field that accepts user input.
• An administrator account to export submissions (or a mechanism to trigger the export).

Exploitation Steps:

1. Craft Payload: Create a payload that will execute a command when the CSV is opened. For example, to launch Calculator on Windows:

   =CMD|' /C calc'!A0
   

   For a reverse shell (PowerShell):

   =CMD|' /C powershell -e <base64_encoded_payload>'!A0
   

2. Submit Form: Navigate to the public form and submit the payload in a text field.
3. Trigger Export: As an administrator, go to the WS Form submissions page and export the data to CSV.
4. Deliver CSV: Send the CSV file to the victim (e.g., via email or a download link).
5. Execute: When the victim opens the CSV in Excel (with macros enabled or default settings), the payload executes.

Note: Modern versions of Excel (2016+) show a warning before executing DDE formulas. Older versions or LibreOffice may execute without warning.

Blue Team

Detection:

• Monitor for form submissions containing special characters (=, +, -, @) at the beginning of text fields.
• Alert on CSV file downloads from the WordPress admin area.
• Use endpoint detection and response (EDR) to monitor for suspicious processes spawned by spreadsheet applications (e.g., cmd.exe, powershell.exe).

Prevention:

• Disable CSV export functionality in WS Form LITE until a patch is available.
• Implement a Web Application Firewall (WAF) rule to block form submissions containing formula injection patterns.
• Educate users to open CSV files in a text editor first, or use a spreadsheet application with macros disabled.
• Apply a virtual patch by modifying the plugin code to sanitize CSV output (see patched code above).

Indicators of Compromise (IOCs)

• Network:
  • Form submissions containing strings starting with =, +, -, or @ followed by function names like CMD, DDE, HYPERLINK, COM, etc.
  • CSV files downloaded from the WordPress admin area with suspicious content.
• Host:
  • Spreadsheet application (e.g., excel.exe, soffice.bin) spawning child processes like cmd.exe, powershell.exe, wscript.exe.
  • Unexpected network connections from spreadsheet applications.
• File:
  • CSV files containing formulas such as =CMD|..., =DDE|..., =HYPERLINK(...).

Detection Engineering

YARA Rule

rule CVE_2023_5424_CSV_Injection {
    meta:
        description = "Detects CSV files with potential formula injection patterns from CVE-2023-5424"
        author = "Valters IT"
        date = "2024-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2023-5424"
    strings:
        $formula1 = /^[=+\-@](CMD|DDE|HYPERLINK|COM|EXEC|SHELL)/ nocase
        $formula2 = /[=+\-@].*!A0/ nocase
        $formula3 = /[=+\-@].*'\/C / nocase
    condition:
        any of them
}

Sigma Rule

title: CVE-2023-5424 CSV Injection via WS Form LITE
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects potential CSV injection attempts targeting WS Form LITE plugin
author: Valters IT
date: 2024-01-01
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2023-5424
logsource:
    category: webserver
    product: apache
    service: access_combined
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/wp-json/ws-form/v1/submit'
        sc-status: 200
    keywords:
        - '=CMD|'
        - '=DDE|'
        - '=HYPERLINK('
        - '=COM|'
        - '=EXEC('
        - '=SHELL('
    condition: selection and keywords
falsepositives:
    - Legitimate use of special characters in form fields
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-5424 WS Form LITE CSV Injection Attempt"; flow:to_server,established; content:"POST"; http_method; content:"/wp-json/ws-form/v1/submit"; http_uri; pcre:"/(=CMD\|)|(=DDE\|)|(=HYPERLINK\()|(=COM\|)|(=EXEC\()|(=SHELL\()/i"; classtype:attempted-user; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable CSV Export: Temporarily disable the CSV export functionality in WS Form LITE settings.
2. Virtual Patching: Implement a WAF rule to block form submissions containing formula injection patterns.
3. User Education: Warn administrators not to open CSV files from WS Form LITE in spreadsheet applications.

Long-Term Fix:

• Monitor the WS Form LITE plugin page for an official patch. Once released, update to the patched version immediately.
• If no patch is forthcoming, consider replacing the plugin with an alternative that properly sanitizes CSV output.

Workaround:

• Modify the plugin code to sanitize CSV fields by prefixing special characters with a single quote ('). This can be done by editing the export function in the plugin's source code.

Valters IT