I'm not very familiar with the ritual magic part of SR5, but I've run in to a situation in the game I GM that I could do with some advice on.

In short: The main opponent in the last run the team did is a mage with heavy corp and organised crime resources. The team managed to take him prisoner, but then released him - the runners have items the corp wants, the corp has a hold over some of the runners' families. So, to try and keep the bad guy on the level during the upcoming deals, the runner mage made sure the team cut off and kept his finger to provide a material link for ritual magic.

My question is, can that material link work in reverse? Can the corp mage cast ritual magic to zap a curse down the astral link to his own severed finger? Can he follow his own astral signature to find where his severed finger is stashed? (And, side question - the material link can be "frozen for later use" (p298 SR5) - would that affect the astral signature?) Thanks!

The best use of the spell Astral Message is to mess with anyone you have studied astrally or have a material link. By casting the spell even to a target that doesn't perceive astrally you create an astral link that can be followed using the astral tracking rules in SR5. Alternately you can start a ritual with a material link obviously. I would assume you would be keeping the material link behind a barrier to make it hard to use in reverse. Most likely the material link could be tracked back using a ritual designed to do so. There are also rituals that can scramble material links to limit their effectiveness in previous editions and the astral doppleganger in this edition can make astral tracking more difficult.

It doesn't answer the OP question: he asks if the PC-opponent could use his own material link to affect them. they got a link to him. He has none.

AFAIK, no you can't.

If you could do that, you could simply destroy any link at will which would be inconsistent with certain techniques described in the SR-universe.

Maybe I would consider an idea that says: I affect myself and hence, I affect all links to myself. Which would mean that he could curse himself to make the finger cursed.
Could make an interesting story if he had a mercyless mage-boss that would be ready to do anything and sacrifice him.

No, they're asking if a mage that has a sample taken can use himself as a link to target his sample. Yes, yes you can. This is why such links are likely stored in heavily warded areas.

A better question is does the mage survive the ritual as the SR5 book fails to mention what happens to the material link when a ritual fires off, whether due to changes in how rituals work or just general missing bits thing,

Back in SR4, page 29 of Street Magic, the process of ritual spellcasting destroys the material link, which in this case could be the mage himself.
This was done this way you couldn't just keep using the same link over and over though I don't think they anticipated mage targeting himself, or at least of part of himself by using himself.
If a spellcasting ritual is interrupted and has to be aborted, the material link is also destroyed, so no take backs.

So an evil GM could let the player cast the ritual, get a brief attachment back to the stored sample to throw out a departing curse and then have him go poof when the ritual consumes the material link, aka him.

Well then, any mage PC with any ranks in magical theory or Ritual Spellcasting would know this, and they'd use their nail clippings or a blood sample instead.

I hate GMs being dicks.

Playing Devil's Advocate, here...
The Nail Clippings and Blood are a link to the living body, not to another material link.

I agree that you could use your Body (potentially) as a link to a (all?) Material Link(s), but that flies against the narrative of the Shadowrun world as a whole. If it was that simple, there would be no runs against Corps to recover your Links. There would be no Links at all, in fact, because it would be such a simple matter to eliminate them that they no longer serve a purpose at all.

It is not about being a Dick, but about reinforcing the narrative of the world as a whole.

OP here - let's say I don't pull the dick move and rule that the mage could remotely destroy the link, 'cause that's too easy. But what about following his own astral signature to locate where his missing finger is stashed? At the moment it's sitting in a bag of peas in the PCs freezer in his appartment, and it isn't protected by a mana barrier or ward or anything. Do you reckon the evil mage could get a fix on the location and send in the goons? It just seems to me that if the astral signature links the finger to the guy, there should probably be at least a weak connection running the other way.

It certainly seems well within the realm of possibility.

Plus if the mage doesn't want to go himself, he could send a spirit to go look since he normally has to be able to share an image or signature with the spirit to be able to have it use it's search ability and he has his own right there to share.

May take some careful wording to the spirit since it's first response otherwise it just says you are right here, so you have to point out you want it to find the smaller bit of you that is about pinky sized.

It does raise interesting possibilities, though it could cut both ways so don't go storing body parts of your enemies in your own freezer, use the Sammie's.

Oh, like.... Sympathetic Links, page 29 of Street Magic? I'm using my shavings/clippings/whatever as a sympathetic link to my other sample the other organization took, since both their material link to me and my sympathetic link to it both shared my body at one time.

By the way, nice to see you again.

Sympathetic Links requires additional training, and I have no issues with that (though I still think it stretches the world narrative a bit). Of course, I would make the Link to your remote Ritual Sample a Symbolic Link (Much harder than Sympathetic) to represent the difficulty in targeting a specific Ritual Sample that you left as a link somewhere, not something that represented ALL your Ritual Samples. You want to get rid of them all, you have to go in series, with separate Symbolic Links to each of them.

You too - Semper Fidelis Marine.