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Yep, when using an untrusted USB power source, you can use what is called a "USB condom": either an adapter or a USB cable with the data wires cut letting only power go through. You can search the web for tutorial on which wires to cut in your USB cable, or for already made USB condoms to buy for less than $10 a piece.

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You need to buy good ones otherwise the power negotiation doesn't happen and you get a slow charge.

Any way to sandbox or proxy the power negotiation through the sheath? Laptop's USB controller chip could be vulnerable.

Not sure of any that do that, but I've seen quite a few that support most of the charging standards and negotiate the highest current available (still at 5V) so that the phone can usually charge reasonably fast.

There are chips you can buy that do power negotiation. You could design your own PCB that does the job.

> when using an untrusted USB power source...

Don't do that. Ever. Always use your own charger and cable, preferably the ones that came with the phone. If not make sure you're sourcing them from a reputable company and not an anonymous Chinese importer.

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