From e776858bce32d473bd7a69c616ad7f6c2f979dfc Mon Sep 17 00:00:00 2001 From: Benjamin Kaduk Date: Mon, 29 Mar 2021 21:27:49 -0700 Subject: Don't send key_share for PSK-only key exchange TLS 1.3 allows for the "psk_ke" and "psk_dhe_ke" key-exchange modes. Only the latter mode introduces a new ephemeral (Diffie-Hellman) key exchange, with the PSK being the only key material used in the former case. It's a compliance requirement of RFC 8446 that the server MUST NOT send a KeyShareEntry when using the "psk_ke" mode, but prior to this commit we would send a key-share based solely on whether the client sent one. This bug goes unnoticed in our internal test suite since openssl communicating with openssl can never negotiate the PSK-only key-exchange mode. However, we should still be compliant with the spec, so check whether the DHE mode was offered and don't send a key-share if it wasn't. Reviewed-by: Tomas Mraz (Merged from https://github.com/openssl/openssl/pull/14749) --- ssl/statem/extensions_srvr.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/ssl/statem/extensions_srvr.c b/ssl/statem/extensions_srvr.c index 8462a67c1a..b2d7ff8f39 100644 --- a/ssl/statem/extensions_srvr.c +++ b/ssl/statem/extensions_srvr.c @@ -1614,6 +1614,13 @@ EXT_RETURN tls_construct_stoc_key_share(SSL *s, WPACKET *pkt, } return EXT_RETURN_NOT_SENT; } + if (s->hit && (s->ext.psk_kex_mode & TLSEXT_KEX_MODE_FLAG_KE_DHE) == 0) { + /* + * PSK ('hit') and explicitly not doing DHE (if the client sent the + * DHE option we always take it); don't send key share. + */ + return EXT_RETURN_NOT_SENT; + } if (!WPACKET_put_bytes_u16(pkt, TLSEXT_TYPE_key_share) || !WPACKET_start_sub_packet_u16(pkt) -- cgit v1.2.1