Decoder accepts non-minimal encoding #297

Summary

The decoder accepts input that violates the specification's canonicalization requirements. Specifically, it does not enforce: msgpack-shortest-integer.

Specification

msgpack-shortest-integer: The MessagePack specification directs an encoder to use the shortest representation that holds the value, so a decoder that accepts a wider encoding admits a second encoding of a value its own encoder would never emit.

Source: https://github.com/msgpack/msgpack/blob/master/spec.md

Affected Version

• @msgpack/msgpack 3.1.3

Reproduction

The following hex-encoded byte strings demonstrate the issue. Each should be rejected by a strict decoder but is currently accepted:

Witnesses (should be rejected):

cc6c
ce00000006

Canonical controls (correctly accepted):

9334406d
93376c5a

Detection Method

Decode with the library's public API, re-encode the decoded value with the SAME library's encoder, and compare bytes. The library is its own reference, so no second implementation is needed: accepting bytes its own encoder would never emit is the defect.

Impact

A decoder that admits a non-canonical encoding of a value its own encoder would never produce breaks the one-encoding-per-value property that content addressing and signature verification rely on. No chain-level reachability is claimed here.