Hope of Delivery: Extracting User Locations From Mobile Instant Messengers
Theodor Schnitzler, Katharina Siobhan Kohls, Evangelos Bitsikas, Christina Pöpper · 2023
Mobile instant messengers such as WhatsApp use delivery status notifications in order to inform users if a sent message has successfully reached its destination.This is useful and important information for the sender due to the often asynchronous use of the messenger service.However, as we demonstrate in this paper, this standard feature opens up a timing side channel with unexpected consequences for user location privacy.We investigate this threat conceptually and experimentally for three widely spread instant messengers.We validate that this information leak even exists in privacy-friendly messengers such as Signal and Threema.Our results show that, after a training phase, a messenger user can distinguish different locations of the message receiver.Our analyses involving multiple rounds of measurements and evaluations show that the timing side channel persists independent of distances between receiver locations -the attack works both for receivers in different countries as well as at small scale in one city.For instance, out of three locations within the same city, the sender can determine the correct one with more than 80 % accuracy.Thus, messenger users can secretly spy on each others' whereabouts when sending instant messages.As our countermeasure evaluation shows, messenger providers could effectively disable the timing side channel by randomly delaying delivery confirmations within the range of a few seconds.For users themselves, the threat is harder to prevent since there is no option to turn off delivery confirmations.1) Signal: For Signal, two specific IPv4 addresses are in use.Reverse DNS lookups point to the same domain name operated by Amazon Web Services (AWS), also when we perform these lookups from different geographical locations.When we resolve the resulting domain name, the same two IP addresses are returned, irrespective of the location.Even though the order of the two addresses varies, there is no indication that one address is preferred over the other at specific locations.2) Threema: For Threema, we identify two similar IP addresses from the same IPv4/24 address range, for one of which the reverse DNS lookup points to a threema.chdomain name.Reverse lookup fails for the other address.We manually identify several more IP addresses whose domain names are resolved to threema.ch,resulting in an extended set of 12 IP addresses.However, it is unclear if all these IP