Users must have a router that supports either Universal Plug and Play (UPnP) or NAT Port Mapping Protocol (NAT-PMP) with either of those features enabled in their connected router.
For example, Adobe Flash programs running outside the sandbox (specific version needed with security issues) are capable of generating a specific type of HTTP request which allows a router implementing the UPnP IGD protocol to be controlled by a malicious web site when someone with a UPnP-enabled router simply visits that web site.
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