SPIP – Insecure Deserialization Leading to Remote Code Execution

Field	Value
CVE ID	CVE-2026-27475
Affected Product	SPIP CMS
Vulnerability Type	Insecure Deserialization (CWE-502)
CVSS v3.1	8.1 – High
CVSS v4.0	9.2 – Critical
Severity	High to Critical
Exploitability	Network exploitable; No authentication required (if exposed); No user interaction required; Requires ability to inject serialized data
Exploit Availability	No confirmed public exploit module; PoC discussions observed; Exploitation feasible in misconfigured environments

Overview

A vulnerability was identified in SPIP CMS where serialized PHP data could be processed through components such as the table_valeur filter and the DATA iterator in public-facing templates.

If attacker-controlled serialized input reaches PHP’s unserialize() function without restriction, object instantiation may occur. When certain classes with exploitable magic methods (__wakeup(), __destruct(), __toString()) are present in memory, arbitrary method execution can be triggered. In certain configurations, this condition may result in Remote Code Execution (RCE).

The issue stems from unsafe handling of untrusted serialized input in the public rendering logic.

Technical Root Cause

The vulnerability exists due to:

Acceptance of serialized PHP data in public components
Use of unserialize() without strict class restrictions
Absence of validation or filtering before deserialization
Potential presence of gadget chains within loaded PHP classes

When PHP unserializes object data, it reconstructs object instances. If class definitions contain dangerous logic in magic methods, execution may occur automatically during deserialization or destruction phases.

This creates a classic PHP Object Injection scenario.

Attack Preconditions

Successful exploitation requires:

Ability to inject serialized content into a SPIP data path
Public-facing endpoint processing serialized data
Presence of exploitable gadget chains in loaded classes
Unrestricted unserialize() behavior

Common injection vectors could include:

Syndicated content feeds
Public content fields
Template rendering inputs
File import mechanisms
Database fields exposed to template iterators

Exploitation Scenario (Educational)

In a vulnerable environment:

Serialized object payload is crafted.
Payload is injected into a content source processed by DATA or table_valeur.
Application processes content.
unserialize() reconstructs object.
Magic method executes unintended logic.
Arbitrary command execution may occur.

Example of Serialized Object Structure (Educational)

O:8:"ClassName":1:{s:4:"prop";s:10:"malicious";}

This demonstrates structure only. It does not represent a working exploit.

Potential Impact

If exploited successfully:

Remote code execution on web server
Web shell deployment
Credential theft
Database compromise
Lateral movement inside internal network
Persistent backdoor installation
Defacement or ransomware deployment

Impact severity depends on server privileges and isolation controls.

MITRE ATT&CK Mapping

Technique ID	Description
T1190	Exploit Public-Facing Application
T1059	Command and Scripting Interpreter
T1505.003	Web Shell
T1105	Ingress Tool Transfer
T1021	Remote Services

Indicators of Compromise

Presence of serialized object strings in HTTP POST bodies
Unexpected PHP errors referencing unserialize()
New PHP files appearing in web directories
Suspicious outbound connections from web server
Unexpected cron jobs
Modified .htaccess or template files
High entropy POST bodies

Detection Strategy

Detection should focus on identifying serialized object patterns in inbound traffic and suspicious deserialization behavior.

Web Server Log Detection (Regex-Based)

Pattern to Detect Serialized PHP Objects

O:\d+:"[A-Za-z0-9_\\]+":\d+:\{

Additional Patterns

a:\d+:\{
s:\d+:"

Splunk Query

index=web_logs
| search method=POST OR method=PUT
| regex _raw="O:\d+:\"[A-Za-z0-9_\\\\]+\":\d+:\{"
| stats count by src_ip, uri, user_agent

Elastic / Kibana KQL

http.request.method : ("POST" or "PUT") and
http.request.body : /O:\d+:"[A-Za-z0-9_\\]+":\d+:\{/

Suricata IDS Rule

alert http any any -> $HOME_NET any 
(msg:"Possible PHP Object Injection Attempt"; 
flow:to_server,established; 
content:"O:"; 
pcre:"/O:\d+:\"[A-Za-z0-9_\\\\]+\":\d+:\{/"; 
classtype:web-application-attack; 
sid:202627475; 
rev:1;)

NGINX Log Hunting

grep -E 'O:[0-9]+:"[A-Za-z0-9_\\]+"' /var/log/nginx/access.log

Apache Log Hunting

grep -E 'O:[0-9]+:"' /var/log/apache2/access.log

PHP Error Log Monitoring

grep -i unserialize /var/log/php*.log

Recommended Log Sources

Web server access logs (NGINX / Apache)
Web server error logs
PHP-FPM logs
Application logs
WAF logs
IDS/IPS alerts
EDR telemetry on web server
File integrity monitoring logs

Defensive Hardening

Immediate Action

Upgrade to patched version immediately.

Restrict Deserialization

Where deserialization is unavoidable:

unserialize($data, ['allowed_classes' => false]);

WAF Filtering

Block inbound requests containing:

O:\d+:
a:\d+:{
s:\d+:"

Input Validation

Reject unexpected serialized patterns in:

Public forms
Syndicated feeds
Data imports

Principle of Least Privilege

Ensure web server:

Does not run as root
Cannot write outside designated directories
Has restricted outbound internet access

Disable Dangerous PHP Functions (if possible)

exec
system
shell_exec
passthru
popen
proc_open

Post-Exploitation Detection

Search for:

Files containing eval(, base64_decode(, gzinflate(
Unexpected outbound traffic
Reverse shell patterns
Suspicious .php files in uploads directory
Recently modified files

Example:

find /var/www -type f -mtime -3 -name "*.php"

Remediation

Official Patch / Upgrade Link

Upgrade to SPIP 4.4.9 or later

Official Release Page:

https://www.spip.net/en_article6651.html

All vulnerable installations should be updated immediately.

Serialized data handling in public areas has been hardened and deprecated in newer branches.

Risk Assessment Summary

Category	Rating
Attack Vector	Network
Privileges Required	None (if injection path exists)
User Interaction	None
Complexity	Medium to High
Impact	Complete system compromise possible
Business Risk	High

Executive Summary

A deserialization vulnerability in SPIP allows attacker-controlled serialized PHP data to be processed without adequate validation. When specific conditions are met, arbitrary code execution may occur. The vulnerability carries high impact due to its potential to grant full server control. While exploitation requires a controllable input path, exposure risk is significant for publicly accessible instances.

Immediate upgrade is strongly recommended. Continuous monitoring for serialized object patterns and anomalous behavior should be implemented across all SPIP deployments.

CVE-2026-27475: Critical SPIP Deserialization Flaw Exposes Websites to Remote Code Execution