What is Underconstrained circuit? Definition and why it matters
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A circuit whose constraints admit witnesses outside the specification, typically because a value computed during witness generation is never pinned down by a constraint. It is the dominant class of exploitable ZK bug.

Detail: In Circom the classic form is a `<--` assignment without a matching `===` constraint; in Halo2 it is a cell that is assigned but not gated; in any system it is a missing range check, a missing boolean check or a division by a possibly-zero value. Automatic tools (Picus, CIVER, NAVe) detect many instances by checking that outputs are uniquely determined by inputs; a soundness proof rules out all of them.

Source page: https://sorryfree.com/glossary/underconstrained-circuit/
Compiled by: sorryfree editors (https://sorryfree.com/about/)
Last reviewed: 2026-09-13