On startup, Catalina has been displaying brief flashes of pop-ups stating, "Do you want the application ________ to accept incoming network connections? Accept or Deny" The applications mentioned are netbiosd, mediasharingd, and rapportd. How do I

I am having this same exact issue. No matter what I do, everytime I restart my mac I am getting "Do you want the application mediasharingd to accept incoming network connections? Accept or Deny" I have no access to any mediasharingd app on firewall or mac applications.

from system preferences/sharing:

Turn on Home Sharing

With Home Sharing, you can access your media library on all your devices where you’re signed in with the same Apple ID. 

Note: If you already have five Mac computers authorized to play content purchased from the iTunes Store or the Apple TV app, you can’t turn on Home Sharing for another Mac. See Authorize a Mac to play iTunes Store purchases.

1. On your Mac, choose Apple menu  > System Preferences, then click Sharing.
2. Select Media Sharing.
3. Select the Home Sharing checkbox.
4. Enter your Apple ID and password, then click Turn On Home Sharing.
5. Select any of the following options:

• • Devices update play counts: Select this checkbox if you want to update play counts (the number of times an item has been played) when the items are played on other computers in your Home Sharing network, or on your iPhone, iPad, or iPad touch.
  • Share photos with Apple TV: Select this checkbox if you want to view your photos on Apple TV.

1. If you select the “Share photos with Apple TV” option, click Choose, then select which photos you want to share:

• • Select a Photos library or a folder: Click the “Share Photos from” pop-up menu, then choose Photos or a folder that contains the photos you want to share.
  • Share all photos in the library or folder: Select “All photos and albums” or “All folders.”
  • Share only certain photo albums or folders: Select “Selected albums” or “Selected folders,” then select the items you want to share from the list below. If you share selected albums, click Albums to select photo albums or People to select photos of specific individuals.
  • Share videos: Select “Include videos.”

1. Click OK when you’re finished selecting photo sharing options.

Turn off Home Sharing

1. On your Mac, choose Apple menu  > System Preferences, then click Sharing.
2. Select Media Sharing.
3. Deselect the Home Sharing checkbox.

Well, i am vaguely aware that this mediashare deamon is for sharing music locally. I think. I checked and found that it was blocked in my firewall. It's odd that i can't find much about it on the web. Only this discussion here.

Me too. Why did this suddenly start appearing with Catalina? I am pretty sure it's benign. But still.

Thank you. This answer solved the problem of the 'Do you want the application "mediasharinggd" to accept incoming network connections' dialogue showing up on startup on Catalina for me. :)

After upgrading to Catalina 10.15.2, a few popups appeared and disappeared so fast that I could neither allow nor deny them, until today, when one popup lasted long enough for me to read that it requested permission to allow "mediasharingd". I neither allowed nor denied it, and it disappeared again after lingering on my screen just long enough to be read. A Google search for "mediasharingd" showed this Apple support thread as the top hit, followed immediately by the second most popular hit, which linked to an Apple support document, as shown in the first screenshot (below). From my desktop menu bar, I had clicked the bitten black Apple icon at the top left of my screen, then selected its first option, "about this Mac". That led me to the Mac OS user guide or manual (see my second screenshot). Having bounced in and out of today's "history" on my desktops so many times, I am not sure which of the many selections I made first, but I felt it was important to avoid selecting anything within the popups. The Apple Support Mac OS Catalina user guide suggested system preferences, which led me to the blue icon showing the yellow pedestrian triangle for "sharing". Because there are other iMac computers and iPhones in my household LAN (Local Area Network), I want to access at least some of my Apple ID iCloud library from all of them. So I allowed media sharing within that system preference/sharing window. At some point, a popup appeared, and it looked identical to the popup that had lingered for several seconds earlier today, immediately after logging in to my iMac. Again, I avoided making any clicks or selections in that suspected duplicate popup. After allowing media sharing in my sharing peferences, and arrangong the two screenshots, I restarted my iMac, and no similar popups have recurred. I don't know for certain whether I have allowed some ghost app to access my iCloud library, either hidden in one of my household devices, or out there in the fog of the clouds. I simply tried to follow instructions.

P.S. I don't have a valid url for my screenshots, so I was not allowed to post them. Bear with me, and perhaps I can provide actual links that will lead you along the same path I followed.