CVE-2026-73364 in Flexible Subscriptions Plugin
Summary
by MITRE • 08/19/2026
Customer PHP Object Injection in Flexible Subscriptions <= 1.8.1 versions.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability identified as Customer PHP Object Injection within the Flexible Subscriptions plugin, affecting versions up to and including 1.8.1, represents a critical server-side security flaw rooted in improper handling of user-supplied input during object deserialization processes. This type of vulnerability arises when an application accepts data from untrusted sources, such as HTTP request parameters or cookies, and passes it directly into PHP's unserialize function without adequate validation or sanitization. In the context of WordPress plugins like Flexible Subscriptions, this often occurs when processing subscription-related data that may be manipulated by authenticated users or, in some configurations, potentially accessible to lower-privilege accounts depending on specific implementation details. The core technical flaw lies in the assumption that incoming serialized strings are safe and originate from trusted internal sources rather than external actors who can craft malicious payloads designed to exploit known gadget chains within PHP libraries included in the WordPress environment.
From a technical perspective, this vulnerability allows an attacker to inject arbitrary PHP objects into the application's execution flow through crafted input fields associated with subscription management features. When the vulnerable code executes unserialize on this tainted data, it triggers the instantiation of specified classes and their magic methods, such as __wakeup or __destruct. If these instantiated classes have side effects in their destructors or other lifecycle methods—such as file system operations, database queries, or remote API calls—an attacker can leverage these behaviors to achieve Remote Code Execution (RCE). This is particularly dangerous because it bypasses traditional input validation mechanisms that focus on string escaping and instead targets the object instantiation logic itself. The exploitation typically requires an authenticated user with at least subscriber-level privileges in many WordPress setups, although certain configurations or specific endpoints might allow unauthenticated access depending on how the plugin handles session data or nonce verification for subscription modifications.
The operational impact of this vulnerability is severe, as successful exploitation can lead to complete compromise of the underlying web server and the entire WordPress installation hosting the application. An attacker gaining RCE through PHP Object Injection can execute arbitrary system commands, read sensitive configuration files such as wp-config.php containing database credentials, exfiltrate customer payment data stored in the subscription tables, or install persistent backdoors for long-term access. This directly violates confidentiality, integrity, and availability principles central to information security frameworks like ISO 27001. Furthermore, because Flexible Subscriptions handles financial transactions and recurring billing data, a breach could result in significant regulatory compliance issues under standards such as PCI DSS if payment card details are exposed or manipulated during the exploitation process. The ability to modify subscription states arbitrarily also poses a direct business impact, allowing attackers to grant premium access without payment or disrupt service for legitimate customers by corrupting subscription records.
Mitigation strategies must prioritize immediate remediation through software updates and defensive coding practices. The primary recommendation is to upgrade Flexible Subscriptions to version 1.8.2 or later, where the developers have addressed this vulnerability by implementing strict input validation before deserialization operations. Developers should avoid using unserialize on any data derived from user input; instead, they should utilize safer alternatives such as json_decode for structured data exchange or implement allowlists that restrict which classes can be instantiated during unserialization. For organizations unable to patch immediately due to compatibility constraints, deploying a Web Application Firewall (WAF) with rules specifically tuned to detect PHP Object Injection patterns in HTTP requests can provide an additional layer of defense by blocking malicious serialized payloads before they reach the application logic. Additionally, enforcing least-privilege principles for WordPress user accounts and regularly auditing plugin code for unsafe deserialization calls are essential steps in reducing the attack surface associated with such vulnerabilities aligned with CWE-502 (Deserialization of Untrusted Data) and MITRE ATT&CK technique T1059 (Command and Scripting Interpreter).