> What is the main technical cause that prevents to jailbreak updates firmware ?
Lack of known exploits.
*Guessing*, possibly very wrongly, how the process used in this thread works - It looks like factory-shipped devices are able to be flashed with non-signed builds which is what coplate abuses by creating a custom build of the firmware with a vulnerability.
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If you care about exploiting Linux systems, you could play Linux-based CTFs to gain at least basic understanding of things. And pick up a book like TLCL.
http://overthewire.org/wargames/