Sorry, I'm not sure I follow. We are talking about bugs in a verified system, that is, in this case, a verified implementation of a zlib-based compression tool. Did it have bugs? yes. Several in fact. I'd recommend reading the article for a detailed listing of the bugs in the tool.

Further to my earlier reply - a more succinct way to look at it might be:

- When they fix the run time, bug A goes away. So the proof still holds and the zlib code is still correct.

- When they add a system of proofs for the parser and modify that, then bug B goes away. So the proof still holds and the zlib code is still correct; and now more of the library is proven correct.

The formulation of the title is "I was told X but that's not true"... but that's not true. You were told X, and X is true, but you found Y and Z which are also important.

You give the answers to this in the artcle:

> The most substantial finding was a heap buffer overflow! but, not in lean-zip's code, but in the Lean runtime itself. (emphasis mine)

> The OOM denial-of-service is straightforward: the archive parser was never verified. (me again)

"Lean proved this program correct" vs "I found bugs in the parts that Lean didn't prove was correct". The failure was not in Lean's proof (which as I said is heavily implied by the title), but in ancillary or unverified code.

Do I misunderstand how Lean works? I am by no means an expert (or even an amateur) on Lean or formal systems in general. Surely the first class of bug could be found in any code that uses the framework, and the second class of bug could happen to any system that isn't proven? Does that make sense? Otherwise where's the boundary? If you find a bug in the OS, does that mean Lean failed somehow? How about the hardware? If your definition of a 'formally verified system' goes beyond the code being verified and the most important thing is whether or not you can make it crash, then the OS and the hardware are also part of the system.

Of course bugs are important to users regardless of the cause, but the audience for your article seems to be software engineers and as a software engineer, your article was interesting and you found something useful, but the title was misleading; that's all I'm saying.

Say you study some piece of software. And it happens that it has an automated suite of tests. And it happens that some files aren't covered by the test suite. And you happen to find a bug in one of those files that were not covered.

Would you publish a blog post titled "the XXX test suite proved there was no bug. And then I found one"?

It would be a bit silly, right?