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I refer to computer parts or devices like Routers or Motherboards where vendors release newer firmware over a period of time. However, for some upgrades vendors clearly mention that once you upgrade to this version, you will not be able to downgrade anymore.

e.g. TPLink Archer C6 Firmware Rev 1.3 or ASRock X470 Taichi Ultimate motherboard BIOS Rev 3.3

What is the technical justification for preventing users from downgrading to an earlier version of firmware once you upgrade to a specific version? Is there any specific technical concept that governs this behavior?

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Here are two reasons that come to my mind immediately:

  • A security bug was fixed and going back to an old version would result in a security regression.
  • Data has been manipulated and cannot be transformed back (e.g. the supervisor password was stored as cleartext and needs to be hashed for the new version. Since a cryptographic hash function is a one-way function, this cannot be reverted).
  • A bunch of other reasons could apply.
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The new firmware writes a different version, in a different way, and the old firmware cannot completely overwrite. Some routers , some of my electronic instrumentation, and some computers are like this. Apple iPhones are notorious for only going forward

I think a workable picture would be that there is space at the end to handle larger firmware. Then going backward would not write out to the new longer end. So then the old update fails. Just a picture, please understand.

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