Em seg., 29 de abr. de 2024 às 14:56, Heikki Linnakangas <hlinnaka@iki.fi> escreveu: > On 29/04/2024 20:10, Ranier Vilela wrote: > > Hi, > > > > With TLS 1.3 and others there is possibly a security flaw using ALPN [1]. > > > > It seems to me that the ALPN protocol can be bypassed if the client does > > not correctly inform the ClientHello header. > > > > So, the suggestion is to check the ClientHello header in the server and > > terminate the TLS handshake early. > > Sounds to me like it's working as designed. ALPN in general is optional; > if the client doesn't request it, then you proceed without it. We do > require ALPN for direct SSL connections though. We can, because direct > SSL connections is a new feature in Postgres. But we cannot require it > for the connections negotiated with SSLRequest, or we break > compatibility with old clients that don't use ALPN. > Ok. But what if I have a server configured for TLS 1.3 and that requires ALPN to allow access? What about a client configured without ALPN requiring connection? > > There is a check in direct SSL mode that ALPN was used > (ProcessSSLStartup in backend_startup.c): > > > if (!port->alpn_used) > > { > > ereport(COMMERROR, > > (errcode(ERRCODE_PROTOCOL_VIOLATION), > > errmsg("received direct SSL connection > request without ALPN protocol negotiation extension"))); > > goto reject; > > } > > That happens immediately after the SSL connection has been established. > > Hmm. I guess it would be better to abort the connection earlier, without > completing the TLS handshake. Otherwise the client might send the first > message in wrong protocol to the PostgreSQL server. That's not a > security issue for the PostgreSQL server: the server disconnects without > reading the message. And I don't see any way for an ALPACA attack when > the server ignores the client's message. Nevertheless, from the point of > view of keeping the attack surface as small as possible, aborting > earlier seems better. > So the ClientHello callback is the correct way to determine the end. best regards, Ranier Vilela