9.6

CVSS Score

3.1

-

CVSS Score

Basic Information

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CVE-2026-33471

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

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 is included as part of v1.3.0.

Workarounds

No known workarounds.

(GitHub Advisory)

Miggo Vulnerability Database

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CVE-2026-33471

CVE-2026-33471:

9.6

CVSS Score

3.1

-

CVSS Score

Basic Information

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Technical Details

Package Name	Ecosystem	Vulnerable Versions	First Patched Version
nimiq-block	rust	<= 0.2.0

Technical Details

Vulnerability Intelligence
Miggo AI

Root Cause Analysis

The vulnerability, identified as GHSA-6973-8887-87ff, stems from an integer truncation and lack of bounds checking in the nimiq-block crate. The core issue resides in how several proof verification functions handled signer information stored in a BitSet. An attacker could craft a proof with signer indices outside the valid range (e.g., slot and slot + 65536).

The vulnerable functions, SkipBlockProof::verify, TendermintProof::verify, and DoubleVoteProof::verify, would first perform a quorum check by reading the .len() of the BitSet. This length could be artificially inflated by the out-of-range indices. Subsequently, when aggregating the public keys of the signers, these functions would cast the usize index to a u16. This truncation caused the out-of-range indices to collide with valid in-range indices (e.g., slot + 65536 becomes slot after as u16). This flaw allowed a malicious validator to bypass the 2f+1 quorum check with significantly fewer real signatures than required, as a single signature could be counted multiple times.

The patch addresses this by introducing a new function, checked_signer_slots, which explicitly validates that all signer indices in the BitSet are within the allowed range (< Policy::SLOTS and <= u16::MAX) before they are used. The vulnerable verify methods in SkipBlockProof, TendermintProof, and DoubleVoteProof were all updated to use this new validation function. This ensures that the quorum check is performed on a sanitized list of signers and that integer truncation can no longer lead to a bypass. Any proof containing out-of-range signer indices is now correctly rejected.

Vulnerable functions

SkipBlockProof::verify

primitives/block/src/skip_block.rs

The function performed a quorum check using `self.sig.signers.len()` and then iterated over the signers, casting each index to a `u16`. This allowed an attacker to use out-of-range indices to inflate the length for the quorum check, which would then collide with in-range indices due to the `u16` truncation, effectively bypassing the quorum verification.

TendermintProof::verify

primitives/block/src/tendermint.rs

This function was vulnerable to the same integer truncation and quorum bypass issue as `SkipBlockProof::verify`. It used the length of the `signers` BitSet for a quorum check without validating that the indices were within the expected range. This allowed an attacker to pass the check with fewer signatures than required.

DoubleVoteProof::verify

primitives/block/src/equivocation_proof.rs

This function verified equivocation proofs by checking signatures. It was vulnerable because it did not validate the range of signer indices from the `BitSet` before using them to aggregate public keys for signature verification. This exposed it to the same truncation and quorum bypass vulnerability.

Vulnerability Intelligence
Miggo AI

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WAF Rule

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Reasoning

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