The quintessential idiomatic term in this context is leech ( vt , or as n the person doing it).
Per Wiktionary , (figuratively) A person who derives profit from others in a parasitic fashion.
Urban Dictionary's top definition lists "parasite" and "freeloader" as synonyms. Leech in this sense is very common slang in technical circles and especially file-sharing, where it refers specifically to downloading files but not staying online to share them.
A Google search for "leeching Wi-Fi" turns up a full page of results describing the exact activity in the question.
ell.stackexchange.com/users/2536/chrylis -cautiouslyoptimistic-
One word used is piggybacking , given by Wikipedia as Piggybacking on Internet access is the practice of establishing a wireless Internet connection by using another subscriber's wireless Internet access service without the subscriber's explicit permission or knowledge.
One definition of piggyback given by Lexico is piggyback VERB 1.1 Link to or take advantage of (an existing system or body of work)
they have piggybacked their own networks on to the system
ell.stackexchange.com/users/56820/Weather Vane
The best [American, maybe not British] English word for this is probably mooching .
It specifically refers to using a resource that's not well-guarded, either without permission or overstepping intended level of permission, in a way that's annoying or inconvenient to others but not severe enough to be treated as theft.
Other examples of mooching include: Consuming snacks from employee break area as meals.
Dropping in a bank that has free lolipops for customers' children with no actual business to conduct, just to get lolipops.
Consuming a roommate's food from their [section of the] refrigerator, or consistently consuming more than your fair share from food purchased jointly for use by oneself and roommates.
Using a roommate's car that they always fill with gas.
The term mooching is also used fairly often for legitimate use of social welfare programs for their intended purposes, when the speaker either doesn't understand or doesn't accept those intended purposes.
ell.stackexchange.com/users/6186/R.. GitHub STOP HELPING ICE
One could say such people are free riders , a noun derived from free ride : 1 : a benefit obtained at another's expense or without the usual cost or effort (source: Merriam Webster ) and in the same spirit (thanks @PeterJennings), freeloaders , derived from the verb freeload : to impose upon another's generosity or hospitality without sharing in the cost or responsibility involved (source: Merriam Webster ) which nicely 'rhymes' with upload & download, which is what you do when using the network.
These are certainly not limited to using WiFi, as @Lambie suggests.
ell.stackexchange.com/users/18673/Glorfindel
Honestly the word I have heard used for this is just "stealing".
This is fairly common in everyday use, because "stealing" is a quite general term for taking something without authorization.
ell.stackexchange.com/users/19148/Mark Foskey
TL;DR
It's helpful to think about the way you're framing this.
If you're viewing bandwidth or the Wi-Fi network as personal property, then the matching term or phrase should have a negative connotation that carries a sense of theft or unauthorized use.
Some Common Terms
They might be “stealing bandwidth,” which is a limited resource on Wi-Fi networks.
They may also be “trespassing” (in several senses of the word, and especially in the sense of unlawful transit of property).
Taking possession of another’s property (in this case, bandwidth) is “squatting.”
The person squatting on the network is a “squatter.”
If it wasn’t an open network, then someone had to “hack” it or “crack” it to gain access.
The former term is more common in colloquial speech, but the latter more strongly connotes “breaking” (or "breaking into") the system.
If it was an open network, and no money exchanged hands, then the unpaid use of the Wi-Fi would be “freeloading.”
The one freeloading would therefore be a “freeloader.”
It's definitely worth pointing out the terms above are closely related, but aren't identical.
For example, "trespassing" and "stealing" are both crimes, but stealing carries a stronger sense of criminal intent .
ell.stackexchange.com/users/25285/CodeGnome
I'm going to suggest the obvious: thief .
It clearly indicates the "without consent" part of the question.
Or perhaps the more specific term: bandwidth thief .
If a company is paying for Internet access by the megabyte then the person using it without permissions is certainly stealing.
If the company pays a flat fee for a limited amount of bandwidth then the unapproved person is still stealing - if they use 200 MB of a company's 2GB plan, that is 200 MB the company is now deprived of using.
ell.stackexchange.com/users/27474/Greenstone Walker
Is there a word or phrase that means "use other people's wifi or Internet service without consent"? I think some people are able to use other people's wifi without consent even if they're password protected, is there a word that means exactly that? I don't know any word used to say that.
Is there a word or phrase that means "use other people's wifi or Internet service without consent"?
ell.stackexchange.com
<word-request>
ell.stackexchange.com/questions/222294
ell.stackexchange.com/users/99845/Methuselah
One word used is  piggybacking , given by  Wikipedia  as   Piggybacking on Internet access is the practice of establishing a wireless Internet connection by using another subscriber's wireless Internet access service without the subscriber's explicit permission or knowledge.   They might be “stealing bandwidth,” which is a limited resource on Wi-Fi networks.  If it was an open network, and no money exchanged hands, then the unpaid use of the Wi-Fi would be “freeloading.” The correct possibilities in this respect are: "leech", "piggyback", "steal", "squat", "hack", "crack", "freeload" or "thieve".
The correct possibilities in this respect are: "leech", "piggyback", "steal", "squat", "hack", "crack", "freeload" or "thieve".