GHSA-6973-8887-87ff nimiq-block has skip block quorum bypass via out-of-range BitSet indices & u16 truncation

• crates.io nimiq-block: < 1.7.2 → fixed in 1.7.2

Impact `SkipBlockProof::verify` computes its quorum check using `BitSet.len()`, then iterates `BitSet` indices and casts each `usize` index to `u16` (`slot as u16`) for slot lookup. If an attacker can get a `SkipBlockProof` verified where `MultiSignature.signers` contains out-of-range indices spaced by 65536, these indices inflate `len()` but collide onto the same in-range `u16` slot during aggregation. This makes it possible for a malicious validator with far fewer than `2f+1` real signer slots to pass skip block proof verification by multiplying a single BLS signature by the same factor. Patches The patch for this vulnerability is included as part of v1.3.0. Workarounds No known workarounds.