How private is a private window?

October 15th, 2020

Private window is not helping with shoulder surfing — *Alice and Bob find themselves in a shared living-space, where long-held secrets are at risk of being revealed.*

TL;DR: skip to the conclusions to see what Alice learned.

The Privacy Problem

Alice and Bob recently decided to take their relationship to another level by moving in together. This will undoubtedly lead to many challenges and inevitable discussions about the desired default position of the toilet lid.

More importantly though, the couple have told each other several innocuous white lies about themselves. Bob pretends he’s into classical music, which Alice loves, because he’s ashamed of his deep devotion to Justin Bieber. Alice on the other hand might have overstated her competence in cooking. She found some blogs and YouTubers that teach cooking to beginners, and decided to go with the fake it till you make it strategy, quietly gaining the skills she claims to already have.

The rules of modern romantic comedy dictate that we must swiftly move to the traditional bedroom scene. Here our couple watches romantic movies, which Bob pretends to enjoy, and the occasional Champions League games, which Alice puts up with, all on a shared laptop. That laptop, Alice realizes, could play the role of Judas. If Bob were to peek into the browsing history, he could find such titles as The Complete Beginner’s Guide To Scrambled Eggs.

So, how does Alice organize private browsing sessions, and keep Bob from discovering them by accident?

The Private Window

Alice knows that modern browsers have a so-called private window. But how private is it exactly? What limitations does it have, and how much of the problem does it solve?

Some quick research tells her that any browsing she does in a private window is not recorded in the browsing history. She also learns that cookies are deleted automatically when the session is over, and that form data (e.g. the search query “how to pretend I can cook so my boyfriend won’t find out”) is also removed by the browser.

When she browses, however, the stream of data goes through her home router, the websites she visits, the network of her internet provider, and the networks of the companies that websites use. A lot can happen in the network and a browser VPN like the one available in Opera would be a great way to protect Alice’s session, but that’s a story for another time. Right now she is concerned about local privacy problems, not those that happen on the wire and beyond.

What The Private Windows Don’t Do

Private window does not hide everything Alice does. It does not stop her from bookmarking a website, nor downloading files (with, say, recipes). Bookmarks and files are not deleted by the browser when the session ends. And that’s good that way, since she wouldn’t want to lose them.

Furthermore, if she opens a downloaded file with any program, the file name still ends up on the recently opened list, disclosing the fact that such a file was there. She could also forget to close a private window and leave her laptop sitting open. Finally, if Bob is around when she’s surfing on forbidden pages, he could take a peek over her shoulder.

As we see, active prevention on the part of the user is required as well. What if we define the problem differently though? What if we assume that Bob is actively trying to find out what Alice is hiding?

The Arms Race

With further research, Alice discovers that private browsing can be a much more complicated issue than she originally suspected. Someone with strong motivation, technical skills, and easy access to her computer could find out what she’s up to. Bob could go to a Spy Shop, get a couple of small cameras, and install them in strategic places. It wouldn’t matter how long her passwords are, how expensive her anti-virus software is, or how rigorously she follows safe browsing protocols. All her secrets could be laid bare.

Her computer itself, her home router, or her cell phone could also be used maliciously to snitch on her. It’s just a matter of how much Bob knows about computer hacking, and how willing he is to employ underhanded tactics, rather than simply having an honest conversation.

Alice now knows that she should be wary of who she lets touch her home electronics. Her browser is just one tiny piece in a larger puzzle, and there are many more ducks that she needs to get in a row to ensure her privacy and security.

Conclusions

Private window only eliminates tracks of your browsing history on your computer (websites you have visited, their cookies, and form data).
Browsing can be shoulder-surfed. If you want local privacy, browse with a solid wall behind you, with no cameras on it.
Private window will not stop you from downloading files or creating bookmarks, and neither will be deleted when you close the browser. If you do that, you are leaving strong clues about sites you have visited.
Browser extensions may also access your browsing history and leak it. Avoid enabling extensions when using private window.
No browser will protect you if your computer is already hacked. People with physical access to your computer or living-space have a lot of options if they wish to spy on you.