[upofadown]

There has to be some reverse channel to do an oracle. Timing? That might not be a thing for messaging. Signal apparently also uses CBC with the same type of padding. So the same shade could be thrown in that direction if someone really wanted to do so.

I would be happier if there were fewer vague assertions in these sorts of writeups...

[tptacek]

I'm not sure what part of Signal you're referring to, but the Signal Protocol generally uses AEAD constructions. That aside: the kind of padding is not the issue; every serious system that uses CBC uses PKCS7 padding. The issue is the lack of authenticated ciphertext, which is what enables the attack. The authenticated scheme composing CBC and HMAC in an EtM arrangement is not susceptible to padding oracle attacks. There are other error and behavior oracles for other padding schemes, and for different block cipher modes.