As Google wanted users to be able to browse the web privately and for their browsing mode choices to be private as well, they have closed a loophole by making the API available in both browsing modes. As part of this fix, instead of using disk storage for the FileSystem API, when in Incognito mode they are using a transient memory filesystem that gets cleared when a session is closed.
The use of a memory filesystem, though, create two new loopholes that could be used to detect Incognito mode, which are described below.
Detecting Incognito mode through filesystem quotas
When Google made it so that Incognito mode uses a temporary filesystem using the computer's RAM, it opened up a new method of detecting it based on the amount of storage set aside for the internal filesystem used by the browser.In research presented by security research Vikas Mishra, he found that when Chrome allocates storage for the temporary memory filesystem used by Incognito mode, it will have a maximum quota of 120MB. "Based on the above observations, key differences in TEMPORARY storage quota between incognito and non-incognito mode are that in case of incognito mode, there’s a hard limit of 120MBwhile this is not the case for non-incognito window. And from the above table it’s clear that for the temporary storage quota to be less than 120MB in case of non-incognito mode the device storage has to be less than 2.4GB. However for all practical purposes it is safe to assume that the majority of the devices currently in use have more than 2.4GB of storage." Using this knowledge, Mishra came up with a script that would query the quota allocated to the browser's filesystem and if its 120MB or less, then the browser is in incognito mode.
Using Mishra's script, BleepingComputer came up with a simple PoC that demonstrates this technique. The PoC can be found here.
Detecting Incognito based on filesystem sizeDetecting Incognito mode through access timings
When it comes to reading and writing data, memory filesystems are always faster than disk filesystems. As Chrome switched to a memory filesystem in Incognito mode, it is now possible to detect private browsing by measuring the speed of writing to the filesystem. This new detection method was discovered by researcher Jesse Li that measures a series of writes to the browser's filesystem. Based on the speed of these writes, a web site could theoretically determine if the browser is using Incognito mode.
Normal vs Incognito writing timingsFurthermore, Li's approach requires many writes to determine the speed of the filesystem, which would cause the detection process to take quite a bit of time.
If you would like to measure the filesystem writes in both Incognito and regular browsing mode, Li created a script you can play with to show the write speed differences.
Write Speed TestSites already using new detection methods
Unfortunately, sites have already started to use Mishra's filesystem quota detection method to determine if a visitor is in Incognito mode.
As noted by Microsoft Edge developer Eric Lawrence, the New York Times, is testing this method to detect when a visitor in in private mode.
New York Times detecting incognito modeThis is done through a script that clearly shows Mishra's research being used.
Script used to detect Incognito modeWhen asking Google about these two new detection methods, Google has told BleepingComputer that they stand by their statement that they will "work to remedy any other current or future means of Incognito Mode detection."
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