A validator can propose a macro block with an interlink that does not match the canonical next interlink. The block passes early verification in verify_macro_block_proposal, so honest validators prevote and precommit the proposal hash. Later, when the block is pushed, verify_block rejects it with an InvalidInterlink error, forcing the validator to discard a block it has already voted for. This mismatch can lead to temporary consensus instability or a denial of service condition, as validators waste time voting for a block that will eventually be rejected.

The vulnerability affects the Rust implementation of the Nimiq Proof‑of‑Stake protocol, core‑rs‑albatross, in all releases prior to 1.3.0. The issue was fixed in the 1.3.0 release. Nodes running older versions should be upgraded to address the flaw.

The CVSS score of 4.9 indicates moderate severity, and the EPSS score of less than 1% suggests a low likelihood of exploitation. The flaw is not yet marked as a known exploit in the KEV catalog. An attacker would need control over an elected validator to craft a malformed macro block; ordinary network or external attackers cannot trigger the issue. Consequently, the risk is moderate but the exploitability window is narrow.