Lev Lazinskiy

That UI Bug with Missing Data is a Security Issue

This is (sometimes) a development blog, so I am going to write about some failed development of mine since writing about success is much less interesting. You know that UI bug that someone added to your GitHub Issues where there is some missing data? You know the one, it only happens in production, all of your tests pass, and you marked it as a low priority. Yeah, that one. It’s probably a security bug and you should look into it right away. At least, that is the lesson I taught myself yet again when I began to research this bug.

before

<a id="shared_note.id)" href=""{{">
{{current_user.notes.filter_by(id=shared_note.id).first().title}}
</a>

after

<a href="{{ url_for('main.note', id=shared_note.note_id) }}">
{{current_user.notes.filter_by(id=shared_note.note_id).first().title}}
</a>

The difference is very subtle, but the key issue here is shared_note.id vs shared_note.note_id; I released a feature a few weeks ago that showed you all of the notes that you have shared. Locally everything worked fine, but I noticed later on, once it was in production, that the note title was not showing up. This is, of course, due to the fact that rather than showing the title of the note with the ID shared_note.note_id (the foreign key linking to the note) I was showing the title for the note with the primary key of shared_note.id. The reason why this is a security issue is because this allows someone to share a bunch of notes and start seeing the titles for notes that they do not own. The reason why this worked locally is because I am only testing with a single user, with a single notebook, with a single note, and with a single shared note. This means that in this specific case all of the Primary Keys and Foreign Keys are usually “1” so everything just happens to work.

Key Takeaways

• Always test with multiple users, make your local environment as similar to production as possible
• Consider using UUID instead of Auto Incrementing Integers, this would have been immediately caught if that was the case.
• “Partial Missing Data” == Security Bug (most of the time)