Update February 9, 2019: Source: New TLS encryption-busting attack also impacts the newer TLS 1.3
"Seven researchers from all over the world found --yet again-- another way to break RSA PKCS#1 v1.5, the most common RSA configuration used to encrypt TLS connections nowadays. Besides TLS, this new Bleichenbacher attack also works against Google's new QUIC encryption protocol as well."
"Attacks can be used to deny clients access to any application of choice and cause servers to waste resources!" As of now, not particularly dangerous. Everything noted below only can cause denial of service (connection) and broken connections, thats about it.
QUIC uses 0-rtt (session resumption / tickets) which is potentially vulnerable to replay attacks, as noted here; so does tls; tls 0-rtt can be disabled in firefox, though I'm not sure if there are options to disable 0-rtt for quic in browsers at this time. After a little further digging even quic 1-rtt (connection handshake) is/was (2015 as of the time of the article) vulnerable to MITM as well, Server Config Replay Attacks, Source-Address Token Replay Attacks and Packet Manipulation Attacks:
We target the Chromium implementation of QUIC7 in our attacks, as this
is the canonical implementation. Our attacks were developed in python
using the scapy library. We summarize our attacks, their properties,
and impacts in Table 1. Replay Attacks. Server Config Replay Attack.
To conduct this attack, an attacker must first collect a copy of the
target server’s scfg. This can be done either by actively establishing
a connection to the server or by passively listening for a client to
attempt a connection. In either case, the server’s scfg can be readily
collected from a full, 1-RTT QUIC connection handshake. Once the
attacker has scfg, he waits for the target client to attempt to start
a connection. When the attacker sees a c hello message from the
client, he can respond with a spoofed srejectmessage using the
collected scfg and randomly generated stk and sno values:
Mind you the article was writ back in 2015 and the protocol has likely evolved since then.