What is the ruling on accessing the internet for free by exploiting vulnerabilities in mobile companies, under the pretext of "as they steal from us, we steal from them"?

It is not permissible to transgress against people's rights and properties in any way, including exploiting loopholes to obtain a service without compensation. This applies to very specific cases, such as "the issue of the claw" (mas'alat al-dhafar), where a person may take their right from another's property if the right is established and certain, and cannot be obtained except through this hidden method. Scholars have differed on this issue; some have expanded upon it to permit extensive transgression against property, while others have prohibited it entirely. Some have chosen - and this is the most just opinion - that the reason should be apparent and clear, such as a wife taking her maintenance from her husband's money if he withholds it. However, for what is not apparent, such as a loan and the like, it is not permissible to take without informing.