CVE-2026-27206

Description

The zumba/json-serializer library allows deserialization of PHP objects from JSON using a special @type field.

Prior to version 3.2.3, the deserializer would instantiate any class specified in the @type field without restriction. When processing untrusted JSON input, this behavior may allow an attacker to instantiate arbitrary classes available in the application.

If a vulnerable application passes attacker-controlled JSON into JsonSerializer::unserialize() and contains classes with dangerous magic methods (such as __wakeup() or __destruct()), this may lead to PHP Object Injection and potentially Remote Code Execution (RCE), depending on available gadget chains in the application or its dependencies.

This behavior is similar in risk profile to PHP's native unserialize() when used without the allowed_classes restriction.

Workarounds

If upgrading is not immediately possible, applications should ensure that:

• JsonSerializer::unserialize() is never called on untrusted or attacker-controlled JSON.
• JSON input is validated and sanitized before deserialization.
• Object instantiation via @type is disabled in application logic where possible.

After upgrading, users can mitigate risk by explicitly configuring:

$serializer->setAllowedClasses([]);

to disable all object instantiation, or by providing a strict allowlist of safe classes.

References

• CWE-502: https://cwe.mitre.org/data/definitions/502.html
• PHP unserialize() documentation: https://www.php.net/manual/en/function.unserialize.php
• OWASP PHP Object Injection: https://owasp.org/www-community/vulnerabilities/PHP_Object_Injection

This vulnerability allows instantiation of arbitrary PHP classes via the @type field when deserializing JSON.

Applications are impacted only if:

• Untrusted or attacker-controlled JSON is passed into JsonSerializer::unserialize(), and
• The application or its dependencies contain classes that can be leveraged as a gadget chain.

Successful exploitation may lead to:

• Arbitrary code execution
• Data exfiltration
• File manipulation
• Denial of service

Applications that only deserialize trusted data are not affected.

Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.

CVE-2026-27206 has a CVSS score of 8.1 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.2.3); upgrading removes the vulnerable code path.

Kodem intelligence

This issue is mitigated in version 3.2.3.

Version 3.2.3 introduces the method: setAllowedClasses(?array $allowedClasses)

This allows applications to restrict which classes may be instantiated during deserialization, similar to PHP's native unserialize() allowed_classes option.

Users should upgrade to version 3.2.3 or later and configure an appropriate class allowlist.

1. What is CVE-2026-27206? CVE-2026-27206 is a high-severity insecure deserialization vulnerability in zumba/json-serializer (composer), affecting versions < 3.2.3. It is fixed in 3.2.3. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
2. How severe is CVE-2026-27206? CVE-2026-27206 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of zumba/json-serializer are affected by CVE-2026-27206? zumba/json-serializer (composer) versions < 3.2.3 is affected.
4. Is there a fix for CVE-2026-27206? Yes. CVE-2026-27206 is fixed in 3.2.3. Upgrade to this version or later.
5. Is CVE-2026-27206 exploitable, and should I be worried? Whether CVE-2026-27206 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27206 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27206? Upgrade zumba/json-serializer to 3.2.3 or later.