CVE-2021-21274: Denial of service attack via .well-known lookups
Impact 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.
Patches Issue is resolved by #8950. A bug not affecting the security aspects of this was fixed in #9108.
Workarounds The federationdomainwhitelist setting can be used to restrict the homeservers communicated with over federation.
Other sources
Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, 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 in version 1.25.0. As a workaround the federationdomainwhitelist setting can be used to restrict the homeservers communicated with over federation.
Affected Software
Remediation
Patch Available
Event History
Frequently Asked Questions
What is CVE-2021-21274?
CVE-2021-21274 is a vulnerability in Synapse, a Matrix reference homeserver, that allows a malicious server to redirect requests to a large file.
What is Synapse?
Synapse is a Matrix reference homeserver written in Python that provides an ecosystem for open federated Instant Messaging and VoIP.
What is the severity of CVE-2021-21274?
CVE-2021-21274 has a severity of 6.5 (Medium).
How can CVE-2021-21274 be fixed?
CVE-2021-21274 can be fixed by upgrading to Synapse version 1.25.0 or later.
Are there any references for CVE-2021-21274?
Yes, you can find references for CVE-2021-21274 at the following links: [GitHub Commit](https://github.com/matrix-org/synapse/commit/ff5c4da1289cb5e097902b3e55b771be342c29d6), [GitHub Pull Request](https://github.com/matrix-org/synapse/pull/8950), [GitHub Release](https://github.com/matrix-org/synapse/releases/tag/v1.25.0).