GCHQ report on 'MULLENIZE' program to 'stain' anonymous electronic traffic
This 2012 report from the NSA's British counterpart, the General Communications Headquarters, describes a method of "staining" electronic traffic to distinguish otherwise anonymous users. It is used to unmask Tor users and devices on networks in which many computers share the same Internet address. The method relies on planting code in a web browser to change the "user agent," which is the way the browser identifies itself on the web. In a two month period, the report says, GCHQ managed to implant about 200 computers with uniquely identifying stains.