Nimiq is a Rust implementation of the Nimiq Proof-of-Stake protocol based on the Albatross consensus algorithm. Prior to 1.6.0, a malicious state-sync peer can crash a syncing node by sending a crafted TrieChunk whose proof contains a TrieNodeChild suffix that is individually valid but exceeds the 63-byte KeyNibbles backing array when combined with the parent key. KeyNibbles::Add in primitives/src/key_nibbles.rs performs the combined slice operation without checking the total length, and the input reaches put_chunk, TrieNodeChild::key, and TrieNodeChild::is_stump before proof.verify, so the attacker does not need a valid cryptographic proof. Exploitation requires the attacker to be selected as the victim's sync peer during state sync. The resulting out-of-bounds panic is transient because the node restarts and resynchronizes. This issue is fixed in version 1.6.0.

Affected: crates.io: nimiq-primitives

### Impact

A malicious peer acting as a state-sync source can crash a syncing node with a crafted `TrieChunk` whose proof contains a `TrieNodeChild` whose `suffix`, when concatenated with the parent key via `KeyNibbles::Add`, exceeds the fixed 63-byte backing array. `Add` (`primitives/src/key_nibbles.rs:332` / `:341`) indexes `bytes[self.bytes_len()..self.bytes_len() + other.bytes_len()]` with no combined-length check, causing an out-of-bounds slice panic (both the even- and odd-length branches).

`KeyNibbles` deserialization validates only the individual `length <= 126`, not the combined parent + suffix length. The panic occurs at `put_chunk` → `child.key()` → `is_stump()` → `+`, i.e. **before** `proof.verify()`, so no valid proof is required. As with the related `child_index` issue, exploitation requires being the victim's sync peer during state sync, and the resulting crash is transient (the node restarts and re-syncs).

Affected: core-rs-albatross <= 1.5.1 (`nimiq-primitives`).

### Patches

Fixed in **1.6.0** via https://github.com/nimiq/core-rs-albatross/pull/3790 (commit `eabfc3e2`), which guards key-nibble concatenation against exceeding the maximum length instead of indexing out of bounds.

### Workarounds

None other than syncing only from trusted peers. Upgrade to 1.6.0.