From 0870c8db28be9eb457ee3d4f9a168959d9507efd Mon Sep 17 00:00:00 2001 From: Aris Adamantiadis Date: Tue, 12 Dec 2023 23:30:26 +0100 Subject: [PATCH] CVE-2023-48795: Server side mitigations Signed-off-by: Aris Adamantiadis Signed-off-by: Jakub Jelen Reviewed-by: Andreas Schneider Upstream-Status: Backport [https://gitlab.com/libssh/libssh-mirror/-/commit/0870c8db28be9eb457ee3d4f9a168959d9507efd] CVE: CVE-2023-48795 Signed-off-by: Vijay Anusuri --- include/libssh/kex.h | 1 + src/kex.c | 46 ++++++++++++++++++++++++++++++++++---------- src/server.c | 8 +++++++- 3 files changed, 44 insertions(+), 11 deletions(-) diff --git a/include/libssh/kex.h b/include/libssh/kex.h index a626d105..2b1a74d5 100644 --- a/include/libssh/kex.h +++ b/include/libssh/kex.h @@ -36,6 +36,7 @@ SSH_PACKET_CALLBACK(ssh_packet_kexinit); int ssh_send_kex(ssh_session session, int server_kex); void ssh_list_kex(struct ssh_kex_struct *kex); int ssh_set_client_kex(ssh_session session); +int ssh_kex_append_extensions(ssh_session session, struct ssh_kex_struct *pkex); int ssh_kex_select_methods(ssh_session session); int ssh_verify_existing_algo(enum ssh_kex_types_e algo, const char *name); char *ssh_keep_known_algos(enum ssh_kex_types_e algo, const char *list); diff --git a/src/kex.c b/src/kex.c index 2ed90235..b03e6484 100644 --- a/src/kex.c +++ b/src/kex.c @@ -766,11 +766,8 @@ int ssh_set_client_kex(ssh_session session) { struct ssh_kex_struct *client= &session->next_crypto->client_kex; const char *wanted; - char *kex = NULL; - char *kex_tmp = NULL; int ok; int i; - size_t kex_len, len; ok = ssh_get_random(client->cookie, 16, 0); if (!ok) { @@ -802,11 +799,33 @@ int ssh_set_client_kex(ssh_session session) return SSH_OK; } - /* Here we append ext-info-c and kex-strict-c-v00@openssh.com to the list of kex algorithms */ - kex = client->methods[SSH_KEX]; + ok = ssh_kex_append_extensions(session, client); + if (ok != SSH_OK){ + return ok; + } + + return SSH_OK; +} + +int ssh_kex_append_extensions(ssh_session session, struct ssh_kex_struct *pkex) +{ + char *kex = NULL; + char *kex_tmp = NULL; + size_t kex_len, len; + + /* Here we append ext-info-c and kex-strict-c-v00@openssh.com for client + * and kex-strict-s-v00@openssh.com for server to the list of kex algorithms + */ + kex = pkex->methods[SSH_KEX]; len = strlen(kex); - /* Comma, comma, nul byte */ - kex_len = len + 1 + strlen(KEX_EXTENSION_CLIENT) + 1 + strlen(KEX_STRICT_CLIENT ) + 1; + if (session->server) { + /* Comma, nul byte */ + kex_len = len + 1 + strlen(KEX_STRICT_SERVER) + 1; + } else { + /* Comma, comma, nul byte */ + kex_len = len + 1 + strlen(KEX_EXTENSION_CLIENT) + 1 + + strlen(KEX_STRICT_CLIENT) + 1; + } if (kex_len >= MAX_PACKET_LEN) { /* Overflow */ return SSH_ERROR; @@ -816,9 +835,16 @@ int ssh_set_client_kex(ssh_session session) ssh_set_error_oom(session); return SSH_ERROR; } - snprintf(kex_tmp + len, kex_len - len, ",%s,%s", KEX_EXTENSION_CLIENT, KEX_STRICT_CLIENT); - client->methods[SSH_KEX] = kex_tmp; - + if (session->server){ + snprintf(kex_tmp + len, kex_len - len, ",%s", KEX_STRICT_SERVER); + } else { + snprintf(kex_tmp + len, + kex_len - len, + ",%s,%s", + KEX_EXTENSION_CLIENT, + KEX_STRICT_CLIENT); + } + pkex->methods[SSH_KEX] = kex_tmp; return SSH_OK; } diff --git a/src/server.c b/src/server.c index bc98da4f..f3d24a7b 100644 --- a/src/server.c +++ b/src/server.c @@ -158,7 +158,13 @@ static int server_set_kex(ssh_session session) { } } - return 0; + /* Do not append the extensions during rekey */ + if (session->flags & SSH_SESSION_FLAG_AUTHENTICATED) { + return SSH_OK; + } + + rc = ssh_kex_append_extensions(session, server); + return rc; } int ssh_server_init_kex(ssh_session session) { -- 2.25.1