CVE-2026-66565 in FC United Plugin
Summary
by MITRE • 10/10/2026
Unauthenticated PHP Object Injection in FC United <= 1.1.1 versions.
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Analysis
by VulDB Data Team • 10/11/2026
The vulnerability identified as an unauthenticated PHP object injection flaw within FC United versions up to and including 1.1.1 represents a critical security deficiency that allows attackers to execute arbitrary code or manipulate application state without valid credentials. This type of vulnerability arises from the improper handling of user-supplied input during the deserialization process, where data is converted back into executable PHP objects. In many modern web applications built with frameworks like Laravel, Symfony, or CodeIgniter, serialization is often used for session management, caching mechanisms, or passing complex state between requests. When these systems accept serialized strings from untrusted sources such as HTTP parameters, cookies, or POST bodies without rigorous validation and type checking, they create a direct pathway for malicious actors to inject crafted payloads that trigger unintended object instantiation with dangerous properties.
The technical root cause lies in the application's failure to validate the integrity of deserialized data before processing it. PHP provides several magic methods such as __wakeup(), __destruct(), __toString(), and __call() which are automatically invoked when an object is unserialized or destroyed. If a vulnerable class contains one of these methods that interacts with external resources, executes system commands, or performs file operations based on property values set during deserialization, an attacker can exploit this by crafting a serialized string that instantiates the target class and sets its properties to malicious values. Since the vulnerability is unauthenticated, there are no access control checks preventing remote attackers from initiating this attack sequence directly through standard HTTP requests, significantly lowering the barrier for exploitation compared to authenticated variants.
From an operational perspective, the impact of successful exploitation can be severe, ranging from complete server compromise to data exfiltration and service disruption. An attacker leveraging PHP object injection may achieve Remote Code Execution (RCE) by chaining multiple gadget classes available within the application or its dependencies, a technique often referred to as a Gadget Chain. This allows for arbitrary command execution on the underlying operating system, potentially leading to full control over the web server environment. Additionally, even if RCE is not immediately achievable through direct method calls, attackers can manipulate object states to cause Denial of Service conditions by triggering infinite loops or exhausting memory resources during deserialization. The lack of authentication means that this vulnerability affects all users and automated scanners indiscriminately, making it a high-priority target for opportunistic attacks and botnet activity.
This flaw aligns with Common Weakness Enumeration (CWE) ID 502, which describes Deserialization of Untrusted Data, as well as CWE 470 regarding the use of externally-controlled input in object instantiation. In terms of offensive security frameworks, this vulnerability maps to MITRE ATT&CK technique T1059 Command and Scripting Interpreter if it leads to RCE via gadget chains, or potentially T1190 Exploit Public-Facing Application for initial access. The absence of authentication further exacerbates the risk profile, as noted in CWE 306 Missing Authentication for Critical Function, allowing unauthenticated actors to interact with critical internal logic that should be restricted.
Mitigation strategies must focus on eliminating the root cause by avoiding deserialization of user-controlled data entirely where possible. If serialization is necessary, developers should implement strict allow-listing of expected classes and properties rather than relying on deny lists or generic validation. Utilizing JSON for data interchange instead of PHP's native serialize function can significantly reduce risk as it does not support object instantiation directly. For existing deployments running FC United versions 1.1.1 or earlier, immediate patching to the latest stable version is required if a fix has been released by the vendor. In cases where an upgrade is not immediately feasible, implementing Web Application Firewall rules that detect and block common serialization payloads containing PHP magic method signatures can provide temporary protection against exploitation attempts while longer-term architectural changes are implemented.