[runtime]: Pin the anchor slot in non-existence proof sibling verification#992
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…tion verify_non_existence_proof's non-empty-terminal branch set parent_end = proof_nodes.len() - 1, excluding the terminal/anchor from parent_nodes. Each sibling's combined walk (parent_nodes ++ sib.proof_path) therefore reconnected at the terminal's parent and could descend through an unrelated parent slot instead of anchor.slot[idx]. Combined with SkipEmpty internal-node hashing (position-independent over the ordered non-zero slots), this let an attacker relabel the queried slot's child into a free slot and "pin" it with a leaf from a different subtree — forging non-membership of a key that genuinely exists, which on the timeout path refunds an already-delivered cross-chain message. Include the terminal in parent_nodes (proof_nodes.len()) so every sibling walk is forced through anchor.slot[idx], matching the all-zero-terminal branch. Adds a regression test asserting a divergent (parent-routed) sibling is rejected while the honest sibling still verifies. Reported via HackenProof (HYPERBR-1962).
Wizdave97
changed the title
…pharos-spv-nonexistence-anchor-pin
dharjeezy
approved these changes
Jun 25, 2026
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verify_non_existence_proof's non-empty-terminal branch set parent_end = proof_nodes.len() - 1, excluding the terminal/anchor from parent_nodes. Each sibling's combined walk (parent_nodes ++ sib.proof_path) therefore reconnected at the terminal's parent and could descend through an unrelated parent slot instead of anchor.slot[idx]. Combined with SkipEmpty internal-node hashing (position-independent over the ordered non-zero slots), this let an attacker relabel the queried slot's child into a free slot and "pin" it with a leaf from a different subtree — forging non-membership of a key that genuinely exists, which on the timeout path refunds an already-delivered cross-chain message.
Include the terminal in parent_nodes (proof_nodes.len()) so every sibling walk is forced through anchor.slot[idx], matching the all-zero-terminal branch. Adds a regression test asserting a divergent (parent-routed) sibling is rejected while the honest sibling still verifies.