Please make sure you properly understand all the risks involved before attempting to install it!
Ensure you have an SLC backup at hand and a safe way to restore it before proceeding.
Thanks to shinyquagsire123's de_Fuse vulnerability it should now be possible to restore SLC without the need for an SLC hardmod, which should make repairs at least somewhat less troublesome.

You find a full guide for setting it up here: How to set up ISFShax on GBAtemp.net

isfshax

isfshax is a coldboot boot1 exploit for the Wii U.
This repository contains the main isfshax exploit and stage2loader. It will use minute_minute as a stage2
It will produce an (unencrypted) ISFS superblock image, meant to be installed through isfshax_installer.
The minute_minute stage2 payload will attempt to load (in order):

• SD (5 times)
  • sd:/minute.img
  • sd:/fw.img
• SLC
  • slc:/sys/hax/minute.img
  • slc:/sys/hax/fw.img
• slc:/sys/title/00050010/1000400a/code/fw.img + patches

The first two locations are supposed to hold the full minute. The third location is the OSv10 IOSU as a fallback. A minimal set of paches will be applied to the IOSU to make it boot with ISFShax and mitigate side effects of ISFShax and to block system updates.

In case a broken fw.img gets installed to the slc, the SLC load can be skipped completely by spamming the power buttion. In that case only the SD will be tried and it will be retried indefinitely.

When using minute with stroopwafel the wafel_isfshax_patch is required or else IOSU would crash because of ISFShax.

Status

isfshax superblock repairs have also not been tested extensively.

Other developer information

isfshax must be the newest ISFS superblock (with the highest superblock generation number) to be loaded by boot1. This creates a problem: unpatched IOSU will also attempt to load the isfshax superblock. IOS-FS contains the same unchecked recursion bug that makes boot1 isfshax possible (though it's probably not as easily exploitable in IOSU, due to stack checks and memory protection), so the loading attempt will normally result in a crash (which is better than the alternative, IOSU attempting to "repair" the patched block and potentially overwriting real superblocks).
To avoid an uncontrolled crash and ensure IOSU stops as soon as possible in a somewhat controlled manner, we deliberately insert an FST entry pointing to a 0xFFFC cluster, in order to break an assertion only present in IOSU.

To boot IOSU or other CFWs (including minute) with isfshax installed, ISFS superblock loading requires patches to prevent attempts to load the isfshax superblock (this can usually be archieved by reducing the maximum allowed superblock generation number; in IOSU a very small 4 byte patch in IOS-FS is sufficient, see the iosuhax repository... well, probably sufficient at least, none of this was significantly tested so, while it appears to work, it could still fail in unexpected ways). A stroopwafel plgingin version of that patch can be found here: wafel_isfshax_patch

Another thing to take into account is the (admittedly moderately low) possibility of SLC corruption. Normally, SLC repairs are handled by boot1 or IOSU. This is undesirable for isfshax superblocks, due to the usual risk of accidentally overwriting good superblocks, and the potential for generation number overflows (due to the very high generation numbers used).

To dissuade boot1 or IOSU from repairs, all other superblocks are marked as bad NAND clusters inside of each isfshax superblock (isfshax superblocks are also marked as bad inside of all normal superblocks to ensure they are not accidentally overwritten).

This leaves to stage2 the burden of actually repairing superblocks, should cluster corruption occur. We also store a number of duplicate copies of isfshax to tackle NAND clusters eventually becoming unrepairable.

Credits

Huge thanks to:

• rw-r-r-0644 for finding and implementing isfshax
• GaryOderNichts and ashquarky for their direct contributions and help
• vgmoose for all the support and for the Wii U that replaced the one I destroyed during early isfshax testing
• Maschell for his help and all other contributions to the Wii U scene
• hexkyz for the warmboot boot1 exploit which made all of this possible
• shinyquagsire123 for de_Fuse and minute
• Salt Team for the original minute CFW
• dimok789, FIX94 and others for the iosuhax CFW
• fail0verflow for mini
• and all other contributors to the Wii U scene!