# CVE-2026-33471: nimiq-block has skip block quorum bypass via out-of-range BitSet indices & u16 truncation > A malicious validator can bypass skip block proof verification in nimiq-block by exploiting integer truncation to inflate quorum counts with out-of-range indices. - **Severity:** Critical 9.6 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Any signed-in user - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** No - **Published:** 22 Apr 2026 - **Updated:** 23 Apr 2026 - **Also known as:** GHSA-6973-8887-87ff ## What is CVE-2026-33471? CVE-2026-33471 is a critical vulnerability in nimiq-block, published on 22 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim. ### Official 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. Weakness: CWE-345 (Insufficient Verification of Data Authenticity), CWE-20 (Improper Input Validation), CWE-190 (Integer Overflow or Wraparound), CWE-1284 (Improper Validation of Specified Quantity in Input) ## Which versions are affected by CVE-2026-33471? 2 open source packages ship an affected version, including nimiq-block. It affects 1 product. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `nimiq-block` | crates.io | All versions to 0.2.0 | None | | `nimiq-block` | crates.io | All versions to 0.2.0 | None | Products: Nimiq Nimiq Proof-of-Stake. ## How do I fix CVE-2026-33471? No fix has been published for CVE-2026-33471 yet. Limit exposure of the affected software until one is available. ## Is CVE-2026-33471 being exploited? There is no known exploitation of CVE-2026-33471 in the wild and no public exploit code. ## Other vulnerabilities in nimiq-block - [CVE-2026-40093](https://insights.sbomify.com/CVE-2026-40093/): nimiq-blockchain is missing a wall-clock upper bound on block timestamps (high 8.1) - [CVE-2026-34063](https://insights.sbomify.com/CVE-2026-34063/): network-libp2p: Peer can crash the node by opening discovery protocol substream twice (high 7.5) - [CVE-2026-34065](https://insights.sbomify.com/CVE-2026-34065/): nimiq-primitives: Node crash due to missing interlink validation in election macro block proposals (high 7.5) - [CVE-2026-32605](https://insights.sbomify.com/CVE-2026-32605/): Nimiq: Remote crash via off-by-one signer bounds check in proposal buffer (high 7.5) - [CVE-2026-33184](https://insights.sbomify.com/CVE-2026-33184/): nimiq/core-rs-albatross: Discovery handshake limit could underflow and later provoke a deterministic overflow panic (high 7.5) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-33471 and fix it if it is affected. What it is: CVE-2026-33471 is a critical vulnerability in nimiq-block, published on 22 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-33471 in the wild and no public exploit code. Affected packages and fixed versions: - nimiq-block (crates.io): affected all versions up to 0.2.0; no fix listed - nimiq-block (crates.io): affected all versions up to 0.2.0; no fix listed Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-33471/ (Markdown: https://insights.sbomify.com/CVE-2026-33471/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-33471 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-33471 - OSV: https://osv.dev/vulnerability/CVE-2026-33471 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-33471/ Summaries are generated and may contain mistakes; the official description above is the record.