Synapse allows unsupported content types to lead to memory exhaustion

Impact

In Synapse before 1.120.1, multipart/form-data requests can in certain configurations transiently increase memory consumption beyond expected levels while processing the request, which can be used to amplify denial of service attacks.

Patches

Synapse 1.120.1 resolves the issue by denying requests with unsupported multipart/form-data content type.

Workarounds

Limiting request sizes or blocking the multipart/form-data content type before the requests reach Synapse, for example in a reverse proxy, alleviates the issue. Another approach that mitigates the attack is to use a low max_upload_size in Synapse.

References

• https://github.com/twisted/twisted/issues/4688#issuecomment-1167705518
• https://github.com/twisted/twisted/issues/4688#issuecomment-2385711609

For more information

If you have any questions or comments about this advisory, please email us at security at element.io.