That would make a lot of sense and would give the attacker a way to interface with all of the other hardware (network, disk etc.). Do you have a source for this information?
I looked up supermicro blade motherboards, and saw that the chip was right near the IPMI chip's line to spi flash.
And prior to that, there were already persistent rumors in the Chinese interney of certain Chinese mobos sending "weird garbage on ICMP," and "BMCs that somehow boot and work with their flash memory soldered off"
Remembering that, I might even suggest that this is not a modchip that does something with signal on the go, but just a very tiny flash chip that has the modded firmware.
Going further from that, to pack, say, 16 mB on a sandgrain sized chip, the densities need to be like that of best flash chips out there, which also means that they have access to last gen flash fab.
The recovery overrides the primary if detected by default.
The place they put their "filter cap" is right on top the empty TSOP8 pad for the recovery flash. And they probably ordered the factory to sneak the traces just a little bit more, or put hidden vias under it, or simply had somebody very dexterous to solder it to pads with hair thin wires.
This is the most plausible theory I've read in this thread. Assuming the image in the article is a stock image (there isn't yet a clear image of a definitely compromised board), then the added part could simply be another TSOP8 Flash part. This implies the firmware to the AST2400 is unsigned (which it appears to be, as there's coreboot options for it).
That makes the whole thing gloriously simple.
A part "stuck on" afterwards is obvious. A part fitted into a no-fit footprint after optical inspection is not, it looks exactly as if it was meant to be there.
Well, it means that provision for the second flash was already there, and PLA simply exploited that fact that Aspeed chips are virtually omnipresent in higher end servers.
It also means that the extend of intervention into board design was minimal, and that a trivial automatic xray would not have picked it up. And as implied in the article, later they buried the bug to beat the AOI, if it was done higher upstream.
So, they would've been screwed even if they were doing board testing outside of China.
That's a clever trick.
But the sole fact that the chip has "to phone home" makes detection trivial, and puts the usefulness of the method to nil - anybody sees the router blink when it shouldn't and your bug's cover is blown.
There really, really needs to be. If this story is real and things have been known compromised since 2015, it’s unacceptable that this information isn’t shared more widely. The “we’re going to follow them” idea doesn’t hold water over 3 years, countless small vendors using Supermicro and not being well-connected enough to know (like Apple or Amazon) about it.
