How is Craigslist tracking my actions in Firefox?

Question

Clearing cookies does not prevent Craigslist from tracking certain actions, not using cookies, and I'm trying to figure out what technique is being used.

You can follow the following steps to see what I mean:

1. Go to Craigslist.org and click one of the categories. This will display a list of ads.

2. Hide an ad by clicking the X to the right of the ad (or under the ad in gallery view) and notice that on the top of the page, the count increments next to the red "X". for example, next to the red X on top, it might say "1 hidden" now. Click another X to hide another ad, and notice the hidden count at the top increments.

3. Now, go into firefox options -> "privacy" and click "remove individual cookies". Type "Craigslist" in the search box, and click "remove all" to remove all displayed cookies matching craigslist.

4. Now notice that if you were logged into Craigslist, you are now logged out. But go back to Craigslist.org, and click the same category you clicked before and notice that the hidden count at the top of the screen has not been cleared. If you click on the hidden count you see the ads you hid earlier.

Thing is, if you had clicked “Clear your recent history;” instead and cleared everything that way, rather than filtering by Craigslist, the hidden history is erased.

So what technique is Craigslist using to track your deleted ads, and how can that history be deleted by only clearing Craigslist cookies instead of clearing all sites?

Answer 1

Craigslist is using local web storage. In Firefox, you can use the Storage Inspector developer tool to view and manage this storage. As that link mentions, you do have to open the developer tools settings and enable the Storage Inspector tool by checking the Storage box in the upper left of the settings page. There might also be Firefox extensions that can manage web storage, but I haven't looked into it.

Here's an example of what Storage Inspector shows. If you delete the value associated with the banish key, Craigslist should lose track of which posts you've hidden.