The hidden perils of cookie syncing

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Online tracking is fitting more complicated and thus increasingly challenging to dam. Modern browsers expose many surfaces that enable users to be uniquely diagnosed, adding Flash cookies and browser fingerprints. In a new paper that may appear at ACM CCS, we current the 1st large scale study of 3 advanced monitoring mechanisms — canvas fingerprinting, evercookies, and cookie syncing. We built novel dimension methods and located that these monitoring mechanisms are used on a large number of sites. Our findings on canvas fingerprinting, in selected, have been in the scoop Propublica, BBC, EFF.

In this blog post I’ll center around a different a part of our paper that checked out cookie syncing, the procedure in which two diverse trackers link the IDs they’ve given to the same user. The most common use of cookie syncing is to enable real time bidding between a number of entities in an ad public sale. It allows the bidder and the ad network to discuss with the user by the same ID in order that the bidder can place bids on a distinctive user in present and future auctions. Cookie syncing raises subtle yet severe privacy concerns, but because of the technical complexity of explaining it, didn’t accept much press coverage. In this post I’ll clarify cookie syncing and why it’s worrisome — even more so than canvas fingerprinting.

In our study, we measured the incidence of cookie syncing using our newly built web size platform, OpenWPM. The platform allows us to automate visits to a site and record all HTTP site visitors and adjustments to the browser’s state that result from the visit. We can use this information to track the flow of third party cookies that include unique identifiers1. We found that just about 40% of all monitoring IDs are synced between as a minimum two entities, so it is a ubiquitous observe. How cookie syncing works. The process begins when a user visits a site say example.

com, not shown in the figure, which contains A. com as an embedded third party tracker. 1 The browser makes a request to A. com, and covered during this request is the monitoring cookie set by A. com.

2 A. com retrieves its monitoring ID from the cookie, and redirects the browser to B. com, encoding the tracking ID into the URL. 3 The browser then makes a request to B. com, which includes the complete URL A.

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com redirected to in addition to B. com’s monitoring cookie. 4 B. com can then link its ID for the user to A. com’s ID for the user2. All of this is invisible to the user.

Once two trackers sync cookies, they can exchange user data between their servers. This data can be shopping histories and even PII3. This trade doesn’t go through the browser, and so it cannot be noticed by experiments like those in our study. To be clear, we don’t know if here’s a standard observe. But this is accurately my point: cookie syncing permits a world of back end data sharing, and there’s so little oversight of the monitoring surroundings that we just don’t know what’s happening behind the scenes.

And here is a problem. Based on the proof of what we can examine in the browser, it seems that every avenue for data assortment and sharing does appear to at last get utilized. Cookie syncing can also negate the effect of users clearing cookies. How might this happen?Some trackers “respawn” cleared cookies, typically by abusing browser qualities. This is named an “evercookie.

” We studied and measured evercookies in a separate phase of the paper. Fortunately, the observe is taken into account an egregious privacy breach and not one of the major tracking agencies do it. But here’s the kicker: we found some trackers respawning their cookies after the user clears all cookies, and passing these respawned cookies to other trackers via cookie syncing!Thus, even trackers that don’t employ respawning/evercookies nevertheless gain the capability to constantly track users who clear cookies, as we clarify below. The diagram shows the information stored in tracker. com’s databases as a user browses numerous sites: 1 tracker.

com tracks the user with cookie ID 123. 2 tracker. com receives the synced ID ABC from partner. com. 3 The user clears her cookies and tracker. com begins tracking the user with a new cookie ID 456.

However accomplice. com respawns an identical ID of ABC not shown. Without external suggestions, tracker. com is unable to link IDs 123 and 456 to a similar user. 4 associate. com syncs the cost ABC with tracker.

com 5 tracker. com knows 123 and 456 correspond to a similar user since they are linked with partner. com’s cookie ID ABC. Now tracker. com has the capacity to link the user’s histories under IDs 123 and 456, if it chooses to do so.

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This graph shows what occurs when a user visits a couple of hundred of the head 1500 sites in a random order but clears cookies midway5. As before, nodes constitute pages visited or embedded third party trackers and edges represent a tracked visit. We highlight the same tracker both before and after the user clears cookies blue and orange respectively. The graph on the left is absolutely disconnected, meaning the user easily seems as two assorted users to the trackers. But when this tracker gets respawned cookies via cookie syncing, they gain the potential to connect the user’s visits to websites they track from before and after cookie clearing. Technical solutions like cookie double keying or list based and heuristic based blocking off tools may help prevent cookie syncing to the level they forestall tracking altogether.

However, the company model of the general public of the internet will likely stop the common deployment of those answers as on by default for the traditional client. Instead, these patrons must simply trust that companies aren’t misusing tracking data. Transparency of data use practices in an externally verifiable manner would go a great distance toward repairing customer distrust of online tracking and advertisements. 1 ID cookies are made up our minds using the tactic defined in Section 5. 1 of our paper.

2 Alternative syncing mechanisms exist; A. com can build the request to B. com in javascript requiring a single request, or B. com can generate the preliminary request to A. com with its monitoring ID embedded in the url thus acting a two directional sync. 3 Onboarding businesses are an instance of trackers that be given PII from other domains.

4 This graph shows real data collected during a dimension, where front page of the worldwide top 3000 Alexa sites were visited in descending order. Nodes are Public Suffix + 1, edges are the lifestyles of an HTTP referrer back to the visited site and an opting for cookie. For graph simplicity Facebook. com is excluded and only a subset of page visits are protected. 5 We use an analogous data as before, but simulate this by splitting tracker nodes into pre clear and post clear.