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 federation_domain_whitelist setting can be used to restrict the homeservers communicated with over federation.
The product does not properly control the allocation and maintenance of a limited resource.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Synapse | Matrix | 0.99.0 (including) | 1.25.0 (excluding) |
| Matrix-synapse | Ubuntu | bionic | * |
| Matrix-synapse | Ubuntu | focal | * |
| Matrix-synapse | Ubuntu | groovy | * |
| Matrix-synapse | Ubuntu | trusty | * |
| Matrix-synapse | Ubuntu | upstream | * |
Mitigation of resource exhaustion attacks requires that the target system either:
The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.
The second solution is simply difficult to effectively institute – and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.