Trail of Bits finds Lean bug that lets a bogus Fermat's Last Theorem proof pass verification
jedisct1 · x · 2026-09-10
Trail of Bits researcher Marc Ilunga disclosed a quirky Lean 4 bug: String.Pos.Raw.extract disagrees between its logical definition (returns empty string for an astronomically large one-byte slice) and the compiled native code (returns the entire original string). Combining the two evaluations lets an absurd "proof" of Fermat's Last Theorem pass Lean's full checker on versions up to 4.33.1.
Key points:
- Found while experimenting with a code-review skill using GPT-5.6
- The team clarifies this is not a kernel soundness issue
- Patch is included in v4.34.0-rc1
- Timely joke: Anthropic recently formalized FLT with 13 million lines of Lean
More from Fun
- StackChan robot performs a drama skit via Hermes agent and GPT-Luna-6 — alexcovo_eth · 2026-10-27
- User spots eerie moment: device starts running on its own with no user action — generativist · 2026-10-27
- 'We're not bad at forecasting—I'm bad at QA-ing my posts,' dlwh wraps up — dlwh · 2026-10-27
- Dev shows striking SD vs HD game texture upgrade alongside a level editor — maxleiter · 2026-10-27
- AI-generated movie-grade clip goes viral, blurring the line with real cinema — cneuralnetwork · 2026-10-27
- Drinking under the JD dog: Beijing's satellite-factory bar next to Moonshot's faux temple — teortaxesTex · 2026-10-27