The vulnerability CVE-2026-27206 affects the zumba json-serializer PHP library, specifically versions prior to 3.2.3. This library serializes PHP variables into JSON format and supports deserialization of PHP objects from JSON using a special @type field that specifies the class to instantiate. The core issue is that the deserializer does not restrict which classes can be instantiated based on the @type field, allowing an attacker to supply JSON that causes the deserializer to instantiate arbitrary classes available in the application or its dependencies. If these classes implement magic methods such as __wakeup() or __destruct(), which can execute code during object lifecycle events, this can be exploited to perform PHP Object Injection. This injection can lead to Remote Code Execution (RCE) if the attacker can chain gadgets—classes and methods that together perform malicious actions—within the application or its dependencies. The vulnerability is analogous in risk to PHP's native unserialize() function when used without the allowed_classes option, which is known to be dangerous when handling untrusted input. The vulnerability requires that the application passes attacker-controlled JSON into the JsonSerializer::unserialize() method. The fix, released in version 3.2.3, presumably restricts or disables the unsafe @type-based instantiation. Until upgrading, mitigations include never deserializing untrusted JSON, validating and sanitizing JSON inputs before deserialization, and disabling the @type-based object instantiation feature if possible. There are no known exploits in the wild at this time, but the high CVSS score of 8.1 reflects the potential severity of exploitation.

If exploited, this vulnerability can have severe consequences for affected organizations. Successful exploitation can lead to Remote Code Execution, allowing attackers to execute arbitrary code on the server hosting the vulnerable application. This can result in full system compromise, data theft, data manipulation, or service disruption. The confidentiality, integrity, and availability of affected systems are all at risk. Organizations running PHP applications that use the vulnerable versions of zumba json-serializer and that deserialize untrusted JSON input are particularly at risk. Attackers can leverage this vulnerability to bypass security controls, escalate privileges, and move laterally within networks. The impact is heightened in environments where the application has access to sensitive data or critical infrastructure. Since the vulnerability depends on the presence of exploitable gadget chains in the application or its dependencies, the actual risk varies by application but remains significant. The lack of known exploits in the wild suggests limited active exploitation currently, but the vulnerability is likely to attract attacker interest given its high severity and potential for RCE.

The primary and most effective mitigation is to upgrade the zumba json-serializer library to version 3.2.3 or later, where the vulnerability is fixed. If immediate upgrading is not feasible, organizations should implement the following mitigations: 1) Avoid deserializing JSON input from untrusted or unauthenticated sources using JsonSerializer::unserialize(). 2) Implement strict validation and sanitization of all JSON inputs before deserialization to ensure they do not contain malicious @type fields or unexpected data. 3) Disable or restrict the @type-based object instantiation feature if the library or application configuration allows it. 4) Conduct a thorough audit of the application and its dependencies to identify classes with dangerous magic methods that could be leveraged as gadget chains, and consider removing or hardening such classes. 5) Employ runtime application self-protection (RASP) or web application firewalls (WAFs) with rules to detect and block suspicious JSON payloads containing unexpected @type fields. 6) Monitor application logs for unusual deserialization activity or errors related to object instantiation. 7) Educate developers about the risks of unsafe deserialization and encourage secure coding practices, including the use of allowlists for deserialization where possible. These targeted mitigations go beyond generic advice by focusing on the specific deserialization mechanism and attack vectors involved.