Is identity theft possible?
Identity theft (as in "borrowing" or "stealing" someone else's SIN) is not a "thing" in Shadowrun.
But it is also not needed since SIN verification systems are used to verify that you are a legit citizen.
If you wish to pretend that you are a legit citizen then you get a fake SIN (any fake SIN, of high enough rating, will do)
The result of a SIN verification system is one out of the following three:
1. There is a SIN and it is legit
2. There is a SIN and it appear to be legit, but you might or might not want to investigate it further.
3. There is a SIN but it is fake. SIN have been automatically burned and local authorities have been notified.
SIN verification systems are not there to verify who you really are or if you belong.
To do that corps will use all kinds of security devices (facial recognition, palm prints, key codes, ID badges, proximity RFID cards and card readers, retinal scanners, DNA scanners, voice recognition etc).
There are a lot rules on how to successfully breach each of them.
Could you simply pose as a legit user by simply having a mark and a know persona icon?
There is actually an advanced action called Masquerade found at Kill Code p. 40 that let you temporary impersonate another
persona. Intercepting calls, using their social media, read their email history.... (but you are not considered the matrix owner of his devices and financial transactions have too many double checks to work). This is an action that a social face/hacker will have a lot of use for.
If you know anything about real life hacking, the primary skill in most hacking is social engineering. You talk to authorized users, get them to give you their legit credentials, and you use them.
It seem as if 'ownership' is a big thing in the SR5 matrix and that files and devices always 'know' if it is really you or not. Marks are also individual. In order to gain access on something you need to have your own mark on it.
There are only three ways to get your mark on a file or icon:
1. Owner invite you to place mark
2. You force the icon to accept your mark (by taking the illegal brute force action)
3. You trick the icon to accept your mark (by taking the illegal hack on the fly action).
Having said that; You can totally 'influence' the legit owner to invite you to place mark on their icons. For example by using Commanding Voice, Control Actions or Puppeteer to 'force' the owner by using magic or resonance. By using Con, Leadership or Intimidate to trick, command or force the user to invite you to place your mark on his icons by using social skills.
But it does not seem as if you are allowed to 'log in' AS the owner (in a traditional sense anyway) and through that gain all access that he normally have. The closest thing you get to that is the Masquerade action from Kill Code that let you
impersonate the owner for awhile.