fix: Reject an inner node claimed at leaf depth in verifyProofPath

[bthomee]

Part 3/8 of a stack. Base: part 2 (bthomee/shamap-stack-02-shamapnodeid-isprefixof).

A SHAMap has 65 levels, and nibbles run out at level 64: selectBranch indexes
the key byte at depth / 2, so at depth 64 it reads byte 32 of a 32-byte key.
Only the leaf terminating a path may sit at that depth, but proof path nodes
come off the wire, so a peer could send 65 hash-chained inner nodes and drive
that read past the end of the buffer.

The existing length bound cannot be tightened to catch this: a path for two
keys sharing all 63 leading nibbles legitimately holds 64 inner nodes plus a
leaf, so 65 elements is valid. The claimed node type at the final depth is the
thing to reject, using >= rather than == to match the convention every
other kLeafDepth comparison in this codebase already follows.

Reachable from TMProofPathResponse via LedgerReplayMsgHandler; confirmed
under ASan with asserts compiled out that the unguarded read lands one byte
past a 32-byte heap allocation. Also closes a second gap: nothing checked the
terminal leaf's own key against key, so the hash chain alone let a peer
substitute any leaf whose subtree hashes matched at every level above it.
Pinned by tests: the 65-element path that must verify for both keys sharing
the deep prefix, and the forged all-inner path that must not.

[Copilot]

Pull request overview

Hardens SHAMap proof-path verification against malformed network-supplied paths by rejecting an inner node claimed at leaf depth (preventing an out-of-bounds key read in selectBranch) and by binding the terminal leaf to the requested key (preventing leaf substitution when the hash chain alone matches).

Changes:

• Reject inner proof-path nodes at depth >= kLeafDepth in SHAMap::verifyProofPath to avoid reading past the 32-byte key.
• Verify the terminal leaf’s key matches key to tie the proof to the intended leaf.
• Add regression tests covering (1) the legitimate 65-element deep path case and (2) the forged all-inner 65-element path rejection.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/tests/libxrpl/shamap/SHAMap.cpp	Adds tests asserting valid 65-element deep proof paths and rejecting forged all-inner paths at leaf depth.
src/libxrpl/shamap/SHAMapSync.cpp	Updates verifyProofPath to reject inner nodes at leaf depth and validate the terminal leaf key matches the requested key.

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