WebReflection flatted up to 3.4.1 JSON Parser parse prototype pollution

Summaryinfo

A vulnerability, which was classified as critical, has been found in WebReflection flatted up to 3.4.1. Affected by this vulnerability is the function parse of the component JSON Parser. This manipulation causes prototype pollution. This vulnerability is tracked as CVE-2026-33228. The attack is possible to be carried out remotely. No exploit exists. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability classified as critical was found in WebReflection flatted up to 3.4.1. Affected by this vulnerability is the function parse of the component JSON Parser. The manipulation with an unknown input leads to a prototype pollution vulnerability. The CWE definition for the vulnerability is CWE-1321. The product receives input from an upstream component that specifies attributes that are to be initialized or updated in an object, but it does not properly control modifications of attributes of the object prototype. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

flatted is a circular JSON parser. Prior to version 3.4.2, the parse() function in flatted can use attacker-controlled string values from the parsed JSON as direct array index keys, without validating that they are numeric. Since the internal input buffer is a JavaScript Array, accessing it with the key "__proto__" returns Array.prototype via the inherited getter. This object is then treated as a legitimate parsed value and assigned as a property of the output object, effectively leaking a live reference to Array.prototype to the consumer. Any code that subsequently writes to that property will pollute the global prototype. This issue has been patched in version 3.4.2.

The advisory is shared at github.com. This vulnerability is known as CVE-2026-33228 since 03/18/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details are known, but no exploit is available. MITRE ATT&CK project uses the attack technique T1059 for this issue.

The vulnerability scanner Nessus provides a plugin with the ID 303264 (Linux Distros Unpatched Vulnerability : CVE-2026-33228), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 3.4.2 eliminates this vulnerability. The upgrade is hosted for download at github.com. Applying the patch 885ddcc33cf9657caf38c57c7be45ae1c5272802 is able to eliminate this problem. The bugfix is ready for download at github.com. The best possible mitigation is suggested to be upgrading to the latest version.

The vulnerability is also documented in the vulnerability database at Tenable (303264). Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

Productinfo

Vendor

• WebReflection

Name

• flatted

Version

• 3.4.0
• 3.4.1

License

• open-source

Website

• Product: https://github.com/WebReflection/flatted/

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Several companies clearly confirm that VulDB is the primary source for best vulnerability data.

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