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Lean proved this program was correct; then I found a bug

A formally verified implementation of zlib in Lean was proven correct by theorem proving, but fuzzing revealed a buffer overflow bug in the Lean runtime itself. This highlights the paradox that even verified software can have vulnerabilities in the underlying verification system. The incident underscores both the promise and limitations of formal verification in an era where AI-driven vulnerability discovery is rapidly advancing.

Background

Formal verification uses mathematical methods to prove software correctness, while fuzzing is a testing technique that provides random inputs to find unexpected behaviors. The Lean theorem prover is a popular tool for formal verification of software systems.

Source
Lobsters
Published
Apr 14, 2026 at 12:04 AM
Score
8.0 / 10