SIERRAMONTANA: NSA Exploit of the Day

Today's implant from the NSA's Tailored Access Operations (TAO) group implant catalog:

SIERRAMONTANA (TS//SI//REL) SIERRAMONTANA provides persistence for DNT implants. The DNT implant will survive an upgrade or replacement of the operating system -- including physically replacing the router's compact flash card.
(TS//SI//REL) Currently, the intended DNT Implant to persist is VALIDATOR, which must be run as a user process on the target operating system. The vector of attack is the modification of the target's BIOS. The modification will add the necessary software to the BIOS and modify its software to execute the SIERRAMONTANA implant at the end of its native System Management Mode (SMM) handler.
(TS//SI//REL) SIERRAMONTANA must support all modern versions of JUNOS, which is a version of FreeBSD customized by Juniper. Upon system boot, the JUNOS operating system is modified in memory to run the implant, and provide persistent kernel modifications to support implant execution.
(TS//SI//REL) SIERRAMONTANA is the cover term for the persistence technique to deploy a DNT implant to Juniper M-Series routers.
Unit Cost: $
Status: (U//FOUO) SIERRAMONTANA under development and is expected to be released by 30 November 2008.

Page, with graphics, is here. General information about TAO and the catalog is here.
We have already seen the codename VALIDATOR. It's the code name for a default, or basic, NSA exploit. It's the exploit that FOXACID defaults to using.
In the comments, feel free to discuss how the exploit works, how we might detect it, how it has probably been improved since the catalog entry in 2008, and so on.