Signal is no longer fully open source
-
A rather discreet announcement in November 2021:
"To prevent spam on Signal, we need to build this spam-battling logic in a separate server component. The interfaces to this code will be public, but the implementation will not be shared. We’re working hard to keep the amount of non-public code as small as we possibly can, and all other parts of the server software and all of its interactions with spam-fighting code remain available in the open. Keeping this small piece of software private helps us stay one step ahead of spammers and doesn’t change the fundamentals of Signal’s security model."Source: https://signal.org/blog/keeping-spam-off-signal/
Anyway the more general question is what code the Signal infrastructure servers are actually running, the published version (now incomplete) or a modified version (for intelligence purposes).