Typically, sites providing free/custom subdomains are providing
A records, whereas the ACME DNS-01 challenge requires adding a
TXT record. This would make what you suggest very unlikely.
If a site allows adding arbitrary
TXT records for subdomains and doesn't reserve the
_acme-challenge, then there's nothing in the protocol that would prevent abusing such a feature, and it would be a vulnerable service. However, that's a use case so uncommon that no-one would exclusively consider that when designing protocols like this.
Luckily, if this happens, Let's Encrypt certificates will expire in three months, and it's also possible to revoke the certificate even without access to its private key, just for cases like this:
If someone issued a certificate after compromising your host or your
DNS, you’ll want to revoke that certificate once you regain control.
In order to revoke the certificate, Let’s Encrypt will need to ensure
that you control the domain names in that certificate (otherwise
people could revoke each other’s certificates without permission)! To
validate this control, Let’s Encrypt uses the same methods it uses to
validate control for issuance: you can put a value in a DNS TXT
record or put a file on an HTTP server. - -
Once you’ve validated control of all the domain names in the
certificate you want to revoke, you can download the certificate
from crt.sh, then proceed to revoke the certificate as if you had
certbot revoke --cert-path /PATH/TO/downloaded-cert.pem