Nicholas Weaver from the UCSD International Computer Science Institute wrote a guest editorial for Wired about the NSA's weaponization of the Internet, explaining in detail how the agency targets individuals with malware payloads injected straight from the backbone.
The NSA has a collection of FOXACID servers, designed to exploit visitors. Conceptually similar to Metasploit's WebServer browser autopwn mode, these FOXACID servers probe any visiting browser for weaknesses to exploit.
All it takes is a single request from a victim passing a wiretap for exploitation to occur. Once the QUANTUM wiretap identifies the victim, it simply packet injects a 302 redirect to a FOXACID server. Now the victim's browser starts talking to the FOXACID server, which quickly takes over the victim's computer. The NSA calls this QUANTUMINSERT.
The NSA and GCHQ used this technique not only to target Tor users who read Inspire (reported to be an Al-Qaeda propaganda magazine in the English language) but also to gain a foothold within the Belgium telecommunication firm Belgacom, as a prelude to wiretapping Belgium phones.
One particular trick involved identifying the LinkedIn or Slashdot account of an intended target. Then when the QUANTUM system observed individuals visiting LinkedIn or Slashdot, it would examine the HTML returned to identify the user before shooting an exploit at the victim. Any page that identifies the users over HTTP would work equally well, as long as the NSA is willing to write a parser to extract user information from the contents of the page.
Other possible QUANTUM use cases include the following. These are speculative, as we have no evidence that the NSA, GCHQ, or others are utilizing these opportunities. Yet to security experts they are obvious extensions of the logic above.