HTTP header enrichment allows mobile operators to annotate HTTP connections via the use of a wide range of request headers. Operators employ proxies to introduce such headers for operational purposes, and-as recently widely publicized-also to assist advertising programs in identifying the subscriber responsible for the originating traffic, with significant consequences for the user's privacy. In this paper, we use data collected by the Netalyzr network troubleshooting service over 16 months to identify and characterize HTTP header enrichment in modern mobile networks. We present a timeline of HTTP header usage for 299 mobile service providers from 112 countries, observing three main categories: (1) unique user and device identifiers (e.g., IMEI and IMSI), (2) headers related to advertising programs, and (3) headers associated with network operations.
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