CVE-2026-107301 in msgpack5
Summary
by MITRE • 10/08/2026
msgpack5 is a msgpack v5 implementation for node.js and the browser. Prior to 6.1.0, constructing msgpack5 with an empty or partial options object disables the default protoAction: 'error' protection. A decoded map containing a __proto__ key can then replace the decoded object's prototype, potentially changing inherited properties or downstream behavior, although Object.prototype is not modified globally. This issue is fixed in version 6.1.0.
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Analysis
by VulDB Data Team • 10/08/2026
The msgpack5 library serves as an implementation of MessagePack v5 for Node.js and browser environments, facilitating efficient binary serialization and deserialization of JavaScript objects. A critical security vulnerability exists in versions prior to 6.1.0 related to the handling of configuration options during object construction. Specifically, when a user instantiates msgpack5 with an empty or partially populated options object, the library fails to enforce its default security policy regarding prototype manipulation. By default, msgpack5 is configured with protoAction set to error, which is designed to prevent attacks that attempt to modify JavaScript objects via their prototypes during deserialization. However, this safeguard is inadvertently disabled if the initialization process does not explicitly and completely define all necessary options, leaving a gap in the security boundary.
This technical flaw allows an attacker who controls or influences the input data to inject a map containing a key named _proto_ into the serialized payload. During the decoding phase, instead of rejecting this entry as per standard secure practices, the vulnerable version processes it by setting the prototype of the decoded object to match the properties defined in that malicious map. This behavior aligns with CWE-1321: Improperly Controlled Modification of Object Prototype Attributes, a category of vulnerability where an attacker can alter the prototype chain of an object, thereby affecting all objects created from that constructor or inheriting from it. Although this attack does not modify the global Object.prototype directly, it compromises the integrity and expected behavior of specific application instances by injecting unintended properties into their inheritance chain.
The operational impact of this vulnerability is significant in contexts where untrusted data is deserialized using msgpack5. An attacker could exploit this to change inherited properties, potentially leading to unauthorized access control bypasses, logic flaws, or denial of service conditions depending on how the application utilizes these objects downstream. For instance, if an object represents a user session or administrative role, prototype pollution might allow an attacker to elevate privileges by injecting admin flags into the prototype chain that are then picked up by all subsequent instances derived from that class structure. This type of attack is often associated with CWE-915: Improperly Controlled Modification of Dynamically-Determined Object Attributes and can be mapped to MITRE ATT&CK technique T1068, which covers Exploitation for Privilege Escalation, as the attacker manipulates object state to gain unintended capabilities.
To mitigate this vulnerability, organizations must immediately upgrade msgpack5 to version 6.1.0 or later, where the default protoAction: error protection is robustly enforced regardless of how the options object is constructed. In environments where upgrading is not immediately feasible, developers should implement strict input validation and ensure that all deserialization processes explicitly pass a fully defined options object with protoAction set to error. Additionally, adopting secure coding practices such as using Object.create(null) for data structures or employing libraries specifically designed to prevent prototype pollution can further reduce the risk surface. Regular security audits of dependency configurations are essential to ensure that default protections remain active and are not inadvertently disabled by incomplete configuration objects.