A vulnerability in nimiq-block's Rust implementation allows malicious validators to bypass skip block proof verification. The issue occurs in SkipBlockProof::verify where usize indices are cast to u16, causing out-of-range indices spaced by 65536 to collide onto the same slot during aggregation. This enables attackers with fewer than 2f+1 real signer slots to pass verification by multiplying a single BLS signature. The vulnerability affects versions prior to 1.3.0 and has been patched in that release with no available workarounds.