A malicious homeserver could redirect requests to their .well-known file to a large file. This can lead to a denial of service attack where homeservers will consume significantly more resources when requesting the .well-known file of a malicious homeserver.
This affects any server which accepts federation requests from untrusted servers.
Issue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108.
The federation_domain_whitelist
setting can be used to restrict the homeservers communicated with over federation.
github.com/matrix-org/synapse
github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6
github.com/matrix-org/synapse/pull/8950
github.com/matrix-org/synapse/releases/tag/v1.25.0
github.com/matrix-org/synapse/security/advisories/GHSA-2hwx-mjrm-v3g8
lists.fedoraproject.org/archives/list/[email protected]/message/TNNAJOZNMVMXM6AS7RFFKB4QLUJ4IFEY
nvd.nist.gov/vuln/detail/CVE-2021-21274