CVE-2026-33471: nimiq-block has skip block quorum bypass via out-of-range BitSet indices & u16 truncation

Executive brief

### 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](https://github.com/nimiq/core-rs-albatross/pull/3657) is included as part of [v1.3.0](https://github.com/nimiq/core-rs-albatross/releases/tag/v1.3.0).

### Workarounds No known workarounds.

Affected products

• crates.io nimiq-block

References

• https://api.github.com/users/1seal
• https://github.com/1seal
• https://api.github.com/users/1seal/gists%7B/gist_id%7D
• https://api.github.com/users/1seal/repos
• https://avatars.githubusercontent.com/u/239602944?v=4
• https://api.github.com/users/1seal/events%7B/privacy%7D