EAP-pwd server: Detect reflection attacks

When processing an EAP-pwd Commit frame, verify that the peer's scalar
and elliptic curve element differ from the one sent by the server. This
prevents reflection attacks where the adversary reflects the scalar and
element sent by the server. (CVE-2019-9497)

The vulnerability allows an adversary to complete the EAP-pwd handshake
as any user. However, the adversary does not learn the negotiated
session key, meaning the subsequent 4-way handshake would fail. As a
result, this cannot be abused to bypass authentication unless EAP-pwd is
used in non-WLAN cases without any following key exchange that would
require the attacker to learn the MSK.

Signed-off-by: Mathy Vanhoef <mathy.vanhoef@nyu.edu>

diff --git a/src/eap_server/eap_server_pwd.c b/src/eap_server/eap_server_pwd.c
index 74979da6ee5f8d7bf24a6c97c4952245da9f3816..16057e94ff48a038599a89ede0cc5aa22e2db78e 100644 (file)
--- a/src/eap_server/eap_server_pwd.c
+++ b/src/eap_server/eap_server_pwd.c
@@ -753,6 +753,15 @@ eap_pwd_process_commit_resp(struct eap_sm *sm, struct eap_pwd_data *data,
                }
        }
 
+       /* detect reflection attacks */
+       if (crypto_bignum_cmp(data->my_scalar, data->peer_scalar) == 0 ||
+           crypto_ec_point_cmp(data->grp->group, data->my_element,
+                               data->peer_element) == 0) {
+               wpa_printf(MSG_INFO,
+                          "EAP-PWD (server): detected reflection attack!");
+               goto fin;
+       }
+
        /* compute the shared key, k */
        if ((crypto_ec_point_mul(data->grp->group, data->grp->pwe,
                                 data->peer_scalar, K) < 0) ||