CVE-2026-107300

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.

msgpack5 before 6.1.0 treats the reserved byte 0xc1 as incomplete input instead of invalid. When a stream begins with 0xc1, subsequent data remains buffered while waiting for bytes that cannot make the value valid, allowing a remote peer to exhaust memory.

msgpack5 before 6.1.0 has no nesting-depth limit for arrays and maps, allowing an attacker who can provide MessagePack input to submit deeply nested containers that exhaust the JavaScript call stack and interrupt a process, worker, or request handler.

In msgpack5 before 6.1.0, the streaming decoder reparses an incomplete array or map from the beginning whenever another chunk arrives. A remote peer can split one valid MessagePack container across many small chunks, causing completed elements to be decoded repeatedly, producing quadratic CPU use and blocking the event loop.

msgpack5 is a msgpack v5 implementation for node.js and the browser. Prior to 6.1.0, decoding a negative signed 64-bit integer modifies the corresponding bytes in the caller-provided input buffer while computing the value. Applications that retain or reuse encoded input for integrity checks, logging, or later processing can observe silently corrupted data, while positive integers and other MessagePack value types are unaffected. This issue is fixed in version 6.1.0.