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Comment by zrail | Hacker News Reader

Nice. This is a class of error with the delightful acronym TOCTOU (Time of Check to Time of Use) which is present in an astonishing number of places.

https://en.m.wikipedia.org/wiki/Time-of-check_to_time-of-use

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No surprise, those are near-impossible to avoid. I mean, a simple:

  if(check(resource)) {
    use(resource);
  }

is already vulnerable, unless you somehow make the entire piece of code run atomically.

One approach is to copy the data to a location that you control first (eg: RAM) and perform both the check and the use of that data from the trusted location. That can be difficult though on embedded devices with constrained resources.

For a firmware update: read it into memory or copy it to storage you control first.

Correct me if I'm wrong but I think that's how Gameboy carts show custom logos bypassing the Nintendo copyright check thing - one logo to pass the internal check and another for display.

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