libsignal-service-rs is a Rust version of the libsignal-service-java library which implements the core functionality to communicate with Signal servers. Prior to commit 82d70f6720e762898f34ae76b0894b0297d9b2f8, any contact may forge a sync message, impersonating another device of the local user. The origin of sync messages is not checked. Patched libsignal-service can be found after commit 82d70f6720e762898f34ae76b0894b0297d9b2f8. The Metadata struct contains an additional was_encrypted field, which breaks the API, but should be easily resolvable. No known workarounds are available.
The product does not sufficiently verify the origin or authenticity of data, in a way that causes it to accept invalid data.