Vulnerability · Published Apr 22, 2026 · Assigned by GitHub_M
CVE-2026-33471nimiq-block has skip block quorum bypass via out-of-range BitSet indices & u16 truncation
Critical 9.6
In plain English
A critical-severity improper input validation in nimiq-block from nimiq. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 12% of all known vulnerabilities).
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.
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| nimiq | nimiq-block | < 1.3.0 |
Weakness: CWE-20, CWE-190, CWE-345, CWE-1284 · Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:N/I:H/A:H