CVE-2026-72711: Lean 4 kernel soundness bug — inference cache poisoning via opaque declarations (0 = 1 demonstrated)

Posted by Jonathan Brossard on Oct 25 1. Summary ========== A soundness bug in the Lean 4 kernel allows construction of a checked proof of False — and by extension, any proposition, including 0 = 1 — without axioms, sorry, unsafeCast, debug.skipKernelTC, or modified .olean files. The proof passes the kernel at maximum checking (--trust=0). The kernel's add_opaque path skips the closure check (check_no_metavar_no_fvar) that the definition and theorem paths perform.

Published 26-Oct-2026 00:59 UTC · First seen 26-Oct-2026 00:58 UTC

Sources

• Open Source Security · 26-Oct-2026