matrix-js-sdk has insufficient MXC URI validation which allows client-side path traversal

Summary

matrix-js-sdk before 34.11.0 is vulnerable to client-side path traversal via crafted MXC URIs. A malicious room member can trigger clients based on the matrix-js-sdk to issue arbitrary authenticated GET requests to the client's homeserver.

Details

The Matrix specification demands homeservers to perform validation of the server-name and media-id components of MXC URIs with the intent to prevent path traversal. However, it is not mentioned that a similar check must also be performed on the client to prevent client-side path traversal. matrix-js-sdk fails to perform this validation.

Patches

Fixed in matrix-js-sdk 34.11.1.

Workarounds

None.

References

• https://spec.matrix.org/v1.12/client-server-api/#security-considerations-5
• https://blog.doyensec.com/2024/07/02/cspt2csrf.html