TLDR: The Lib_MerkleTrie._walkNodePath function conflates “short” node IDs and “long” nodeIDs in a manner that can lead to a collision and proof forgery when the attacker controls the value of some key “near” the target key.
It is possible to construct two different RLP items, r1 and r2 such that
keccak256(rlp.encode(r1)) == bytes32(rlp.encode(r2))
with len(rlp.encode(r1)) >= 32 and len(rlp.encode(r2)) < 32.
Note that the cast on the RHS pads the encoding with zeroes.
Thus if r1 is a node in the true trie, the prover may instead prove the existence of node r2, and vice-versa.
An example of such r1 and r2 can be found via a small brute force search (implemented in forgery.c):
keccak256(rlp.encode(b' ', b'\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00Qku\x8b\x00\x00\x00\x00'])) == bytes32(rlp.encode([b' ', b'\xd2\xf1\x89~\xe7[\x1a\x9b\xc6\x1c].%\xf1\xa5X\x0e\xcb#\xc5\x98\x89Ndh>\x832']))
This bug is not easily exploitable in tries which use hashes for all keys - in the example above we use a leaf node with empty key - this assumes the parent node is a branch and thus that there is some other key in the tree which matches on all by the last nibble. In general we will either need to find a collision which matches on a much longer prefix, which is impractical for 256-bit hashes. I believe the search complexity for an arbitrary collision is 2^64, and for a targeted collision is 2^128.
FIX:
One easy fix is to compare the hashes of the nodes, rather than the nodes themselves. Because the hash is length-dependent the hash of a <32 byte node will be different from the hash of a 32-byte hash. Even better, throw a domain separator in there so they are oboviously incapable of colliding.