Overwritehost in config.php conflicts with SSO Redirect

Steps to reproduce

1. define overwritehost domain in config.php from nextcloud
2. configure IdP on different subdomain than nextcloud
3. the redirect to the IdP from the user_saml plugin uses the defined overwritehost domain and NOT the correct subdomain of the IdP

Expected behaviour

Normally when I click on the Button of the IdP/SSO Service (option for multiple backends, the same with just the SSO Service if I deactivate that option) I should be redirected to the IdP Service.

Actual behaviour

Instead the plugin uses the overwritehost domain from the nextcloud settings. When I correct the domain in the adress bar and hit enter, the redirect continues normally and everything works fine. Sadly I can not test the configuration without setting the overwritehost variable because then my setup (reverse proxy etc.) is not working.

Server configuration

Operating system: Linux 4.15.0-91-generic #92-Ubuntu SMP Fri Feb 28 11:09:48 UTC 2020 x86_64

Web server: Apache/2.4.29 (Ubuntu) (fpm-fcgi)

Database: pgsql PostgreSQL 10.12 (Ubuntu 10.12-0ubuntu0.18.04.1) on x86_64-pc-linux-gnu, compiled by gcc (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0, 64-bit

PHP version: 7.2.24-0ubuntu0.18.04.3
Modules loaded: Core, date, libxml, openssl, pcre, zlib, filter, hash, Reflection, SPL, sodium, session, standard, cgi-fcgi, json, PDO, xml, bz2, calendar, ctype, curl, dom, mbstring, fileinfo, ftp, gd, gettext, gmp, iconv, imap, intl, redis, ldap, exif, pdo_pgsql, pgsql, Phar, posix, readline, shmop, SimpleXML, smbclient, soap, sockets, sysvmsg, sysvsem, sysvshm, tokenizer, wddx, xmlreader, xmlwriter, xsl, zip, libsmbclient, Zend OPcache

Nextcloud version: 18.0.3

Where did you install Nextcloud from:

List of activated apps:

Enabled:
 - accessibility: 1.4.0
 - apporder: 0.9.0
 - calendar: 2.0.2
 - checksum: 0.4.4
 - cloud_federation_api: 1.1.0
 - comments: 1.8.0
 - contacts: 3.2.0
 - dav: 1.14.0
 - dicomviewer: 1.2.1
 - drawio: 0.9.5
 - event_update_notification: 1.0.1
 - external: 3.5.0
 - extract: 1.2.3
 - federatedfilesharing: 1.8.0
 - federation: 1.8.0
 - files: 1.13.1
 - files_external: 1.9.0
 - files_pdfviewer: 1.7.0
 - files_rightclick: 0.15.2
 - files_sharing: 1.10.1
 - files_trashbin: 1.8.0
 - files_versions: 1.11.0
 - files_videoplayer: 1.7.0
 - firstrunwizard: 2.7.0
 - impersonate: 1.5.0
 - issuetemplate: 0.6.0
 - logreader: 2.3.0
 - lookup_server_connector: 1.6.0
 - metadata: 0.11.1
 - music: 0.13.2
 - nextcloud_announcements: 1.7.0
 - notes: 3.2.0
 - notifications: 2.6.0
 - oauth2: 1.6.0
 - password_policy: 1.8.0
 - photos: 1.0.0
 - polls: 1.3.0
 - privacy: 1.2.0
 - provisioning_api: 1.8.0
 - recommendations: 0.6.0
 - serverinfo: 1.8.0
 - settings: 1.0.0
 - sharebymail: 1.8.0
 - spreed: 8.0.5
 - support: 1.1.0
 - survey_client: 1.6.0
 - systemtags: 1.8.0
 - tasks: 0.12.1
 - text: 2.0.0
 - theming: 1.9.0
 - twofactor_backupcodes: 1.7.0
 - updatenotification: 1.8.0
 - user_ldap: 1.8.0
 - user_saml: 3.0.1
 - viewer: 1.2.0
 - workflowengine: 2.0.0
Disabled:
 - activity
 - admin_audit
 - encryption
 - mail

Nextcloud configuration:

    "passwordsalt": "***REMOVED SENSITIVE VALUE***",
    "secret": "***REMOVED SENSITIVE VALUE***",
    "trusted_domains": [
        "localhost",
        "192.168.47.129",
        "XXXX.cloud",
        "nextcloud"
    ],
    "datadirectory": "***REMOVED SENSITIVE VALUE***",
    "dbtype": "pgsql",
    "version": "18.0.3.00",
    "dbname": "***REMOVED SENSITIVE VALUE***",
    "dbhost": "***REMOVED SENSITIVE VALUE***",
    "dbport": "",
    "dbtableprefix": "oc_",
    "dbuser": "***REMOVED SENSITIVE VALUE***",
    "dbpassword": "***REMOVED SENSITIVE VALUE***",
    "installed": true,
    "instanceid": "***REMOVED SENSITIVE VALUE***",
    "mail_smtpmode": "smtp",
    "remember_login_cookie_lifetime": "1800",
    "log_rotate_size": "10485760",
    "simpleSignUpLink.shown": "false",
    "memcache.local": "\\OC\\Memcache\\Redis",
    "filelocking.enabled": true,
    "memcache.distributed": "\\OC\\Memcache\\Redis",
    "memcache.locking": "\\OC\\Memcache\\Redis",
    "redis": {
        "host": "***REMOVED SENSITIVE VALUE***",
        "port": 0,
        "timeout": 0.5,
        "dbindex": 0,
        "password": "***REMOVED SENSITIVE VALUE***"
    },
    "htaccess.RewriteBase": "\/",
    "maintenance": false,
    "overwrite.cli.url": "https:\/\/XXXX.cloud",
    "ldapIgnoreNamingRules": false,
    "ldapProviderFactory": "OCA\\User_LDAP\\LDAPProviderFactory",
    "overwritehost": "XXXX.cloud",
    "overwriteprotocol": "https",
    "mail_from_address": "***REMOVED SENSITIVE VALUE***",
    "mail_sendmailmode": "smtp",
    "mail_domain": "***REMOVED SENSITIVE VALUE***",
    "mail_smtphost": "***REMOVED SENSITIVE VALUE***",
    "mail_smtpport": "25",
    "loglevel": 2,
    "app_install_overwrite": [
        "polls",
        "dicomviewer",
        "drawio",
    ]
}

Client configuration

Browser:
All Browsers (Firefox, Edge, Chrome)

Operating system:
Windows 10

Start by reproducing the SSO redirect with overwritehost configured in config.php and the IdP on a different subdomain; inspect how the user_saml plugin constructs that redirect. Done means the redirect preserves the IdP subdomain while reverse-proxy behavior continues to work.