[Bug]: Federated shares fails for user in format `user@email.com@cloud.nc.com`

⚠️ 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 (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• Nextcloud Server is running on 64bit capable CPU, PHP and OS.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

Federated shares for user in format email+domain fails.

Steps to reproduce

1. Share a folder by klink
2. On that page, clic on "Add to your Nextcloud" and input user@email.com@cloud.nc.com
3. Error 400

Expected behavior

Format user@email.com@cloud.nc.com is accepted.

Installation method

Community Manual installation with Archive

Nextcloud Server version

26

Operating system

Debian/Ubuntu

PHP engine version

PHP 8.1

Web server

Apache (supported)

Database engine version

MariaDB

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

Fresh Nextcloud Server install

Are you using the Nextcloud Server Encryption module?

Encryption is Disabled

What user-backends are you using?

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

Configuration report

{
    "system": {
        "passwordsalt": "***REMOVED SENSITIVE VALUE***",
        "secret": "***REMOVED SENSITIVE VALUE***",
        "trusted_domains": [
            "***REMOVED SENSITIVE VALUE***"
        ],
        "datadirectory": "***REMOVED SENSITIVE VALUE***",
        "dbtype": "mysql",
        "version": "26.0.1.1",
        "overwrite.cli.url": "***REMOVED SENSITIVE VALUE***",
        "dbname": "***REMOVED SENSITIVE VALUE***",
        "dbhost": "***REMOVED SENSITIVE VALUE***",
        "dbtableprefix": "oc_",
        "mysql.utf8mb4": true,
        "dbuser": "***REMOVED SENSITIVE VALUE***",
        "dbpassword": "***REMOVED SENSITIVE VALUE***",
        "installed": true,
        "skeletondirectory": "",
        "default_phone_region": "FR",
        "activity_expire_days": 1,
        "auth.bruteforce.protection.enabled": true,
        "htaccess.RewriteBase": "\/",
        "integrity.check.disabled": false,
        "knowledgebaseenabled": false,
        "logtimezone": "Europe\/Paris",
        "maintenance": false,
        "memcache.local": "\\OC\\Memcache\\Redis",
        "memcache.locking": "\\OC\\Memcache\\Redis",
        "appstoreenabled": true,
        "overwriteprotocol": "https",
        "preview_max_scale_factor": 1,
        "trashbin_retention_obligation": "auto, 7",
        "versions_retention_obligation": "auto, 7",
        "updater.release.channel": "stable",
        "mail_smtpauthtype": "LOGIN",
        "mail_smtpmode": "smtp",
        "mail_smtpsecure": "tls",
        "mail_sendmailmode": "smtp",
        "mail_from_address": "***REMOVED SENSITIVE VALUE***",
        "mail_domain": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpauth": 1,
        "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
        "mail_smtpport": "587",
        "mail_smtpname": "***REMOVED SENSITIVE VALUE***",
        "mail_smtppassword": "***REMOVED SENSITIVE VALUE***",
        "enabledPreviewProviders": [
            "OC\\Preview\\PNG",
            "OC\\Preview\\JPEG",
            "OC\\Preview\\GIF",
            "OC\\Preview\\HEIC",
            "OC\\Preview\\MP4",
            "OC\\Preview\\Movie"
        ],
        "instanceid": "***REMOVED SENSITIVE VALUE***",
        "overwritehost": "***REMOVED SENSITIVE VALUE***",
        "default_language": "fr",
        "default_locale": "fr_FR",
        "loglevel": 3,
        "shareapi_allow_share_dialog_user_enumeration": "no",
        "tempdirectory": "\/tmp",
        "trusted_proxies": "***REMOVED SENSITIVE VALUE***",
        "redis": {
            "host": "***REMOVED SENSITIVE VALUE***",
            "port": 0
        },
        "preview_max_memory": 256,
        "preview_max_x": 1024,
        "preview_max_y": 1024,
        "updater.secret": "***REMOVED SENSITIVE VALUE***",
        "theme": "",
        "filelocking.enabled": true,
        "updatedirectory": "\/tmp",
        "simpleSignUpLink.shown": false,
        "preview_concurrency_new": 2,
        "maintenance_window_start": 1,
        "updatechecker": false,
        "upgrade.disable-web": true,
        "bulkupload.enabled": false,
        "session_lifetime": 3600,
        "remember_login_cookie_lifetime": 0,
        "auto_logout": true
    }
}

List of activated Apps

Enabled:
  - bruteforcesettings: 2.5.0
  - calendar: 4.3.3
  - cloud_federation_api: 1.8.0
  - contacts: 5.2.0
  - contactsinteraction: 1.6.0
  - dav: 1.24.0
  - deck: 1.8.5
  - drop_account: 2.2.0
  - federatedfilesharing: 1.15.0
  - files: 1.20.1
  - files_automatedtagging: 1.15.3
  - files_pdfviewer: 2.6.0
  - files_rightclick: 1.4.0
  - files_sharing: 1.17.0
  - files_trashbin: 1.15.0
  - forms: 3.2.0
  - impersonate: 1.12.0
  - logreader: 2.10.0
  - lookup_server_connector: 1.13.0
  - notes: 4.7.2
  - notifications: 2.13.1
  - notify_push: 0.6.2
  - oauth2: 1.13.0
  - onlyoffice: 7.8.0
  - password_policy: 1.15.0
  - photos: 2.0.1
  - provisioning_api: 1.15.0
  - quota_warning: 1.16.0
  - registration: 2.1.0
  - related_resources: 1.0.4
  - settings: 1.7.0
  - sharebymail: 1.15.0
  - tasks: 0.14.5
  - theming: 2.0.1
  - twofactor_backupcodes: 1.14.0
  - twofactor_totp: 7.0.0
  - user_retention: 1.10.0
  - viewer: 1.9.0
  - workflowengine: 2.7.0

Nextcloud Signing status

N/A

Nextcloud Logs

No response

Additional info

No response

Start by tracing the federated sharing “Add to your Nextcloud” entry point for the user@email.com@cloud.nc.com value. Reproduce the HTTP 400 on the reported Nextcloud 26 setup and verify that this address format is accepted without breaking other federated-share addresses.