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

Source: https://junglewise.ai/threats/cve-2026-33471-nimiq-block-has-skip-block-quorum-bypass-via-out-of-range-bitset  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-22. Updated: 2026-05-11  
License: CC BY 4.0 with a link back

CVE-2026-33471 · Severity: critical · CVSS 9.6 · Published 2026-04-22

nimiq-block contains block primitives to be used in Nimiq's Rust implementation. `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. Prior to version 1.3.0, 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. The patch for this vulnerability is included as part of v1.3.0. No known workarounds are available.

Technologies: [Nimiq Proof-Of-Stake](https://junglewise.ai/threats/technologies/nimiq-proof-of-stake). Vendors: [crates.io](https://junglewise.ai/threats/vendors/crates-io), [Nimiq](https://junglewise.ai/threats/vendors/nimiq).

## Quick answers

**Is CVE-2026-33471 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-33471 being exploited in the wild.

**How severe is CVE-2026-33471?** Critical severity, with a CVSS score of 9.6, published 22 April 2026.

**Which products and versions are affected?** nimiq-block (crates.io) <= 0.2.0, SEMVER:introduced=0,last_affected=0.2.0.

**Is there a fix for CVE-2026-33471?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**Am I affected by CVE-2026-33471?** Check whether you run nimiq-block (crates.io) or Nimiq Proof-Of-Stake at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Affected products

- nimiq-block (crates.io) <= 0.2.0, SEMVER:introduced=0,last_affected=0.2.0
- Nimiq Proof-Of-Stake

## Identifiers

- CVE-2026-33471
- GHSA-6973-8887-87ff

## References

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

## Related threats

- [nimiq-transaction: UpdateValidator transactions allows voting key change without proof-of-knowledge](https://junglewise.ai/threats/cve-2026-34068-nimiq-transaction-updatevalidator-transactions-allows-voting-key)
- [nimiq-transaction: Panic via `HistoryTreeProof` length mismatch](https://junglewise.ai/threats/cve-2026-34067-nimiq-transaction-panic-via-historytreeproof-length-mismatch)
- [nimiq-blockchain: Peer-triggerable panic during history sync](https://junglewise.ai/threats/cve-2026-34066-nimiq-blockchain-peer-triggerable-panic-during-history-sync)
- [nimiq-primitives: Node crash due to missing interlink validation in election macro block proposals](https://junglewise.ai/threats/cve-2026-34065-nimiq-primitives-node-crash-due-to-missing-interlink-validation)
- [nimiq-account: Vesting insufficient funds error can panic](https://junglewise.ai/threats/cve-2026-34064-nimiq-account-vesting-insufficient-funds-error-can-panic)