Denial of service (via resource exhaustion) due to improper input validation on third-party identifier endpoints

Impact

Missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion.

Patches

The issue is fixed by #9321.

Workarounds

Depending on the needs and configuration of the homeserver a few options are available:

1. Using email as third-party identifiers be disabled by not configuring the email setting.

2. Using phone numbers as third-party identifiers can be disabled by ensuring that account_threepid_delegates.msisdn is not configured.

3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:

   • ^/_matrix/client/(r0|unstable)/register/email
   • ^/_matrix/client/(r0|unstable)/register/msisdn
   • ^/_matrix/client/(r0|unstable)/account/password
   • ^/_matrix/client/(r0|unstable)/account/3pid