[Bug]: broken recursive shares

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Due to some broken recursive shares, one can have infinite recursion causing exception when browsing the folder with the broken share and excessive memory consumption.

Steps to reproduce

1. create an user group with at least 2 users
2. create a group folder associated to this group FolderForGroup
3. from one of the user in the group, create a folder inside the group folder InsideFolder
4. share the inside folder with the user within the group (via a direct share to an user)
5. use an user who has received the inside folder share and rename it to have those folders in its root folder (FolderForGroup and InsideFolderRenamed)
6. move the InsideFolder within FolderForGroup in the root folder
7. share InsideFolder to the same group that is owning the group folder
8. try to access files from the user who created InsideFolder

Expected behavior

Share would not recurse and the user receiving a share should not be allowed to share it with the user having originally shared it

Installation method

None

Nextcloud Server version

27

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

No response

List of activated Apps

No response

Nextcloud Signing status

No response

Nextcloud Logs

No response

Additional info

No response

Start by reproducing the numbered sharing sequence on Nextcloud Server 27 and capture the exception and memory growth when browsing the affected folder. Trace the share validation and recursive folder traversal involved in group-folder and direct-user shares. Done means the recursive share is rejected or safely bounded, the original sharer cannot receive the recursive share, and the reproduction no longer causes an exception or excessive memory use.