+++ /dev/null
-From 1dc5659aa62848f0aeb5de7bd3839fecc7debefa Mon Sep 17 00:00:00 2001
-Date: Sat, 19 Jun 2021 15:39:15 +0100
-Subject: [PATCH] New option to reject 'trivial' success of userauth.
-
-Suggested by Manfred Kaiser, who also wrote most of this patch
-(although outlying parts, like documentation and SSH-1 support, are by
-me).
-
-This is a second line of defence against the kind of spoofing attacks
-in which a malicious or compromised SSH server rushes the client
-through the userauth phase of SSH without actually requiring any auth
-inputs (passwords or signatures or whatever), and then at the start of
-the connection phase it presents something like a spoof prompt,
-intended to be taken for part of userauth by the user but in fact with
-some more sinister purpose.
-
-Our existing line of defence against this is the trust sigil system,
-and as far as I know, that's still working. This option allows a bit of
-extra defence in depth: if you don't expect your SSH server to
-trivially accept authentication in the first place, then enabling this
-option will cause PuTTY to disconnect if it unexpectedly does so,
-without the user having to spot the presence or absence of a fiddly
-little sigil anywhere.
-
-Several types of authentication count as 'trivial'. The obvious one is
-the SSH-2 "none" method, which clients always try first so that the
-failure message will tell them what else they can try, and which a
-server can instead accept in order to authenticate you unconditionally.
-But there are two other ways to do it that we know of: one is to run
-keyboard-interactive authentication and send an empty INFO_REQUEST
-packet containing no actual prompts for the user, and another even
-weirder one is to send USERAUTH_SUCCESS in response to the user's
-preliminary *offer* of a public key (instead of sending the usual PK_OK
-to request an actual signature from the key).
-
-This new option detects all of those, by clearing the 'is_trivial_auth'
-flag only when we send some kind of substantive authentication response
-(be it a password, a k-i prompt response, a signature, or a GSSAPI
-token). So even if there's a further path through the userauth maze we
-haven't spotted, that somehow avoids sending anything substantive, this
-strategy should still pick it up.
-
-(cherry picked from commit 5f5c710cf3704737e24ffceb2c918e412a2a674f)
-
-[Retrieved from:
-https://git.tartarus.org/?p=simon/putty.git;a=commit;h=1dc5659aa62848f0aeb5de7bd3839fecc7debefa]
----
- cmdline.c | 8 ++++++++
- config.c | 4 ++++
- doc/config.but | 43 +++++++++++++++++++++++++++++++++++++++++++
- pscp.c | 2 ++
- psftp.c | 2 ++
- putty.h | 1 +
- settings.c | 2 ++
- ssh.c | 4 +++-
- ssh1login.c | 14 +++++++++++++-
- ssh2userauth.c | 20 ++++++++++++++++----
- sshppl.h | 2 +-
- unix/uxplink.c | 2 ++
- windows/winhelp.h | 1 +
- windows/winplink.c | 2 ++
- 14 files changed, 100 insertions(+), 7 deletions(-)
-
-diff --git a/cmdline.c b/cmdline.c
-index 7ea5cacc..62d65e19 100644
---- a/cmdline.c
-+++ b/cmdline.c
-@@ -598,6 +598,14 @@ int cmdline_process_param(const char *p, char *value,
- SAVEABLE(0);
- conf_set_bool(conf, CONF_tryagent, false);
- }
-+
-+ if (!strcmp(p, "-no-trivial-auth")) {
-+ RETURN(1);
-+ UNAVAILABLE_IN(TOOLTYPE_NONNETWORK);
-+ SAVEABLE(0);
-+ conf_set_bool(conf, CONF_ssh_no_trivial_userauth, true);
-+ }
-+
- if (!strcmp(p, "-share")) {
- RETURN(1);
- UNAVAILABLE_IN(TOOLTYPE_NONNETWORK);
-diff --git a/config.c b/config.c
-index ca808511..b8348530 100644
---- a/config.c
-+++ b/config.c
-@@ -2772,6 +2772,10 @@ void setup_config_box(struct controlbox *b, bool midsession,
- HELPCTX(ssh_auth_bypass),
- conf_checkbox_handler,
- I(CONF_ssh_no_userauth));
-+ ctrl_checkbox(s, "Disconnect if authentication succeeds trivially",
-+ 'n', HELPCTX(ssh_no_trivial_userauth),
-+ conf_checkbox_handler,
-+ I(CONF_ssh_no_trivial_userauth));
-
- s = ctrl_getset(b, "Connection/SSH/Auth", "methods",
- "Authentication methods");
-diff --git a/doc/config.but b/doc/config.but
-index a00ae476..77313282 100644
---- a/doc/config.but
-+++ b/doc/config.but
-@@ -2623,6 +2623,49 @@ interact with them.)
- This option only affects SSH-2 connections. SSH-1 connections always
- require an authentication step.
-
-+\S{config-ssh-notrivialauth} \q{Disconnect if authentication succeeds
-+trivially}
-+
-+This option causes PuTTY to abandon an SSH session and disconnect from
-+the server, if the server accepted authentication without ever having
-+asked for any kind of password or signature or token.
-+
-+This might be used as a security measure. There are some forms of
-+attack against an SSH client user which work by terminating the SSH
-+authentication stage early, and then doing something in the main part
-+of the SSH session which \e{looks} like part of the authentication,
-+but isn't really.
-+
-+For example, instead of demanding a signature from your public key,
-+for which PuTTY would ask for your key's passphrase, a compromised or
-+malicious server might allow you to log in with no signature or
-+password at all, and then print a message that \e{imitates} PuTTY's
-+request for your passphrase, in the hope that you would type it in.
-+(In fact, the passphrase for your public key should not be sent to any
-+server.)
-+
-+PuTTY's main defence against attacks of this type is the \q{trust
-+sigil} system: messages in the PuTTY window that are truly originated
-+by PuTTY itself are shown next to a small copy of the PuTTY icon,
-+which the server cannot fake when it tries to imitate the same message
-+using terminal output.
-+
-+However, if you think you might be at risk of this kind of thing
-+anyway (if you don't watch closely for the trust sigils, or if you
-+think you're at extra risk of one of your servers being malicious),
-+then you could enable this option as an extra defence. Then, if the
-+server tries any of these attacks involving letting you through the
-+authentication stage, PuTTY will disconnect from the server before it
-+can send a follow-up fake prompt or other type of attack.
-+
-+On the other hand, some servers \e{legitimately} let you through the
-+SSH authentication phase trivially, either because they are genuinely
-+public, or because the important authentication step happens during
-+the terminal session. (An example might be an SSH server that connects
-+you directly to the terminal login prompt of a legacy mainframe.) So
-+enabling this option might cause some kinds of session to stop
-+working. It's up to you.
-+
- \S{config-ssh-tryagent} \q{Attempt authentication using Pageant}
-
- If this option is enabled, then PuTTY will look for Pageant (the SSH
-diff --git a/pscp.c b/pscp.c
-index 0bfda92d..a11d1e18 100644
---- a/pscp.c
-+++ b/pscp.c
-@@ -2203,6 +2203,8 @@ static void usage(void)
- printf(" -i key private key file for user authentication\n");
- printf(" -noagent disable use of Pageant\n");
- printf(" -agent enable use of Pageant\n");
-+ printf(" -no-trivial-auth\n");
-+ printf(" disconnect if SSH authentication succeeds trivially\n");
- printf(" -hostkey keyid\n");
- printf(" manually specify a host key (may be repeated)\n");
- printf(" -batch disable all interactive prompts\n");
-diff --git a/psftp.c b/psftp.c
-index 40cb73f5..c4b5374b 100644
---- a/psftp.c
-+++ b/psftp.c
-@@ -2538,6 +2538,8 @@ static void usage(void)
- printf(" -i key private key file for user authentication\n");
- printf(" -noagent disable use of Pageant\n");
- printf(" -agent enable use of Pageant\n");
-+ printf(" -no-trivial-auth\n");
-+ printf(" disconnect if SSH authentication succeeds trivially\n");
- printf(" -hostkey keyid\n");
- printf(" manually specify a host key (may be repeated)\n");
- printf(" -batch disable all interactive prompts\n");
-diff --git a/putty.h b/putty.h
-index ef6f7a37..7789f217 100644
---- a/putty.h
-+++ b/putty.h
-@@ -1428,6 +1428,7 @@ NORETURN void cleanup_exit(int);
- X(INT, NONE, sshprot) \
- X(BOOL, NONE, ssh2_des_cbc) /* "des-cbc" unrecommended SSH-2 cipher */ \
- X(BOOL, NONE, ssh_no_userauth) /* bypass "ssh-userauth" (SSH-2 only) */ \
-+ X(BOOL, NONE, ssh_no_trivial_userauth) /* disable trivial types of auth */ \
- X(BOOL, NONE, ssh_show_banner) /* show USERAUTH_BANNERs (SSH-2 only) */ \
- X(BOOL, NONE, try_tis_auth) \
- X(BOOL, NONE, try_ki_auth) \
-diff --git a/settings.c b/settings.c
-index 547af0dc..32a53c54 100644
---- a/settings.c
-+++ b/settings.c
-@@ -609,6 +609,7 @@ void save_open_settings(settings_w *sesskey, Conf *conf)
- #endif
- write_setting_s(sesskey, "RekeyBytes", conf_get_str(conf, CONF_ssh_rekey_data));
- write_setting_b(sesskey, "SshNoAuth", conf_get_bool(conf, CONF_ssh_no_userauth));
-+ write_setting_b(sesskey, "SshNoTrivialAuth", conf_get_bool(conf, CONF_ssh_no_trivial_userauth));
- write_setting_b(sesskey, "SshBanner", conf_get_bool(conf, CONF_ssh_show_banner));
- write_setting_b(sesskey, "AuthTIS", conf_get_bool(conf, CONF_try_tis_auth));
- write_setting_b(sesskey, "AuthKI", conf_get_bool(conf, CONF_try_ki_auth));
-@@ -1025,6 +1026,7 @@ void load_open_settings(settings_r *sesskey, Conf *conf)
- gpps(sesskey, "LogHost", "", conf, CONF_loghost);
- gppb(sesskey, "SSH2DES", false, conf, CONF_ssh2_des_cbc);
- gppb(sesskey, "SshNoAuth", false, conf, CONF_ssh_no_userauth);
-+ gppb(sesskey, "SshNoTrivialAuth", false, conf, CONF_ssh_no_trivial_userauth);
- gppb(sesskey, "SshBanner", true, conf, CONF_ssh_show_banner);
- gppb(sesskey, "AuthTIS", false, conf, CONF_try_tis_auth);
- gppb(sesskey, "AuthKI", true, conf, CONF_try_ki_auth);
-diff --git a/ssh.c b/ssh.c
-index 00a516ea..91e7e869 100644
---- a/ssh.c
-+++ b/ssh.c
-@@ -254,7 +254,9 @@ static void ssh_got_ssh_version(struct ssh_version_receiver *rcv,
- connection_layer, ssh->savedhost, ssh->fullhostname,
- conf_get_filename(ssh->conf, CONF_keyfile),
- conf_get_bool(ssh->conf, CONF_ssh_show_banner),
-- conf_get_bool(ssh->conf, CONF_tryagent), username,
-+ conf_get_bool(ssh->conf, CONF_tryagent),
-+ conf_get_bool(ssh->conf, CONF_ssh_no_trivial_userauth),
-+ username,
- conf_get_bool(ssh->conf, CONF_change_username),
- conf_get_bool(ssh->conf, CONF_try_ki_auth),
- #ifndef NO_GSSAPI
-diff --git a/ssh1login.c b/ssh1login.c
-index 8486cbf0..519838d7 100644
---- a/ssh1login.c
-+++ b/ssh1login.c
-@@ -27,7 +27,7 @@ struct ssh1_login_state {
-
- char *savedhost;
- int savedport;
-- bool try_agent_auth;
-+ bool try_agent_auth, is_trivial_auth;
-
- int remote_protoflags;
- int local_protoflags;
-@@ -105,6 +105,8 @@ PacketProtocolLayer *ssh1_login_new(
- s->savedhost = dupstr(host);
- s->savedport = port;
- s->successor_layer = successor_layer;
-+ s->is_trivial_auth = true;
-+
- return &s->ppl;
- }
-
-@@ -645,6 +647,7 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
- s->ppl.bpp, SSH1_CMSG_AUTH_RSA_RESPONSE);
- put_data(pkt, ret + 5, 16);
- pq_push(s->ppl.out_pq, pkt);
-+ s->is_trivial_auth = false;
- crMaybeWaitUntilV(
- (pktin = ssh1_login_pop(s))
- != NULL);
-@@ -814,6 +817,7 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
- s->ppl.bpp, SSH1_CMSG_AUTH_RSA_RESPONSE);
- put_data(pkt, buffer, 16);
- pq_push(s->ppl.out_pq, pkt);
-+ s->is_trivial_auth = false;
-
- mp_free(challenge);
- mp_free(response);
-@@ -1105,6 +1109,7 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
- put_stringz(pkt, prompt_get_result_ref(s->cur_prompt->prompts[0]));
- pq_push(s->ppl.out_pq, pkt);
- }
-+ s->is_trivial_auth = false;
- ppl_logevent("Sent password");
- free_prompts(s->cur_prompt);
- s->cur_prompt = NULL;
-@@ -1121,6 +1126,13 @@ static void ssh1_login_process_queue(PacketProtocolLayer *ppl)
- }
- }
-
-+ if (conf_get_bool(s->conf, CONF_ssh_no_trivial_userauth) &&
-+ s->is_trivial_auth) {
-+ ssh_proto_error(s->ppl.ssh, "Authentication was trivial! "
-+ "Abandoning session as specified in configuration.");
-+ return;
-+ }
-+
- ppl_logevent("Authentication successful");
-
- if (conf_get_bool(s->conf, CONF_compression)) {
-diff --git a/ssh2userauth.c b/ssh2userauth.c
-index 01243baf..451d5abe 100644
---- a/ssh2userauth.c
-+++ b/ssh2userauth.c
-@@ -28,7 +28,7 @@ struct ssh2_userauth_state {
-
- PacketProtocolLayer *transport_layer, *successor_layer;
- Filename *keyfile;
-- bool show_banner, tryagent, change_username;
-+ bool show_banner, tryagent, notrivialauth, change_username;
- char *hostname, *fullhostname;
- char *default_username;
- bool try_ki_auth, try_gssapi_auth, try_gssapi_kex_auth, gssapi_fwd;
-@@ -82,6 +82,7 @@ struct ssh2_userauth_state {
- int len;
- PktOut *pktout;
- bool want_user_input;
-+ bool is_trivial_auth;
-
- agent_pending_query *auth_agent_query;
- bufchain banner;
-@@ -134,7 +135,7 @@ static const PacketProtocolLayerVtable ssh2_userauth_vtable = {
- PacketProtocolLayer *ssh2_userauth_new(
- PacketProtocolLayer *successor_layer,
- const char *hostname, const char *fullhostname,
-- Filename *keyfile, bool show_banner, bool tryagent,
-+ Filename *keyfile, bool show_banner, bool tryagent, bool notrivialauth,
- const char *default_username, bool change_username,
- bool try_ki_auth, bool try_gssapi_auth, bool try_gssapi_kex_auth,
- bool gssapi_fwd, struct ssh_connection_shared_gss_state *shgss)
-@@ -149,6 +150,7 @@ PacketProtocolLayer *ssh2_userauth_new(
- s->keyfile = filename_copy(keyfile);
- s->show_banner = show_banner;
- s->tryagent = tryagent;
-+ s->notrivialauth = notrivialauth;
- s->default_username = dupstr(default_username);
- s->change_username = change_username;
- s->try_ki_auth = try_ki_auth;
-@@ -157,6 +159,7 @@ PacketProtocolLayer *ssh2_userauth_new(
- s->gssapi_fwd = gssapi_fwd;
- s->shgss = shgss;
- s->last_methods_string = strbuf_new();
-+ s->is_trivial_auth = true;
- bufchain_init(&s->banner);
- bufchain_sink_init(&s->banner_bs, &s->banner);
-
-@@ -818,6 +821,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
- sigblob);
- pq_push(s->ppl.out_pq, s->pktout);
- s->type = AUTH_TYPE_PUBLICKEY;
-+ s->is_trivial_auth = false;
- } else {
- ppl_logevent("Pageant refused signing request");
- ppl_printf("Pageant failed to "
-@@ -1038,6 +1042,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
- ssh_key_free(key->key);
- sfree(key->comment);
- sfree(key);
-+ s->is_trivial_auth = false;
- }
-
- #ifndef NO_GSSAPI
-@@ -1169,6 +1174,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
- * no longer says CONTINUE_NEEDED
- */
- if (s->gss_sndtok.length != 0) {
-+ s->is_trivial_auth = false;
- s->pktout =
- ssh_bpp_new_pktout(
- s->ppl.bpp, SSH2_MSG_USERAUTH_GSSAPI_TOKEN);
-@@ -1288,7 +1294,6 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
- * Loop while the server continues to send INFO_REQUESTs.
- */
- while (pktin->type == SSH2_MSG_USERAUTH_INFO_REQUEST) {
--
- ptrlen name, inst;
- strbuf *sb;
-
-@@ -1308,6 +1313,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
- */
- s->num_prompts = get_uint32(pktin);
- for (uint32_t i = 0; i < s->num_prompts; i++) {
-+ s->is_trivial_auth = false;
- ptrlen prompt = get_string(pktin);
- bool echo = get_bool(pktin);
-
-@@ -1472,7 +1478,7 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
- pq_push_front(s->ppl.in_pq, pktin);
-
- } else if (s->can_passwd) {
--
-+ s->is_trivial_auth = false;
- /*
- * Plain old password authentication.
- */
-@@ -1731,6 +1737,12 @@ static void ssh2_userauth_process_queue(PacketProtocolLayer *ppl)
- }
-
- userauth_success:
-+ if (s->notrivialauth && s->is_trivial_auth) {
-+ ssh_proto_error(s->ppl.ssh, "Authentication was trivial! "
-+ "Abandoning session as specified in configuration.");
-+ return;
-+ }
-+
- /*
- * We've just received USERAUTH_SUCCESS, and we haven't sent
- * any packets since. Signal the transport layer to consider
-diff --git a/sshppl.h b/sshppl.h
-index b339d67c..2c1dbf42 100644
---- a/sshppl.h
-+++ b/sshppl.h
-@@ -116,7 +116,7 @@ PacketProtocolLayer *ssh2_transport_new(
- PacketProtocolLayer *ssh2_userauth_new(
- PacketProtocolLayer *successor_layer,
- const char *hostname, const char *fullhostname,
-- Filename *keyfile, bool show_banner, bool tryagent,
-+ Filename *keyfile, bool show_banner, bool tryagent, bool notrivialauth,
- const char *default_username, bool change_username,
- bool try_ki_auth,
- bool try_gssapi_auth, bool try_gssapi_kex_auth,
-diff --git a/unix/uxplink.c b/unix/uxplink.c
-index 3e2a9b6b..240783f4 100644
---- a/unix/uxplink.c
-+++ b/unix/uxplink.c
-@@ -527,6 +527,8 @@ static void usage(void)
- printf(" -i key private key file for user authentication\n");
- printf(" -noagent disable use of Pageant\n");
- printf(" -agent enable use of Pageant\n");
-+ printf(" -no-trivial-auth\n");
-+ printf(" disconnect if SSH authentication succeeds trivially\n");
- printf(" -noshare disable use of connection sharing\n");
- printf(" -share enable use of connection sharing\n");
- printf(" -hostkey keyid\n");
-diff --git a/windows/winhelp.h b/windows/winhelp.h
-index ae5a7a7f..9011df45 100644
---- a/windows/winhelp.h
-+++ b/windows/winhelp.h
-@@ -111,6 +111,7 @@
- #define WINHELP_CTX_ssh_kex_repeat "config-ssh-kex-rekey"
- #define WINHELP_CTX_ssh_kex_manual_hostkeys "config-ssh-kex-manual-hostkeys"
- #define WINHELP_CTX_ssh_auth_bypass "config-ssh-noauth"
-+#define WINHELP_CTX_ssh_no_trivial_userauth "config-ssh-notrivialauth"
- #define WINHELP_CTX_ssh_auth_banner "config-ssh-banner"
- #define WINHELP_CTX_ssh_auth_privkey "config-ssh-privkey"
- #define WINHELP_CTX_ssh_auth_agentfwd "config-ssh-agentfwd"
-diff --git a/windows/winplink.c b/windows/winplink.c
-index 9bda0712..58d43e6d 100644
---- a/windows/winplink.c
-+++ b/windows/winplink.c
-@@ -149,6 +149,8 @@ static void usage(void)
- printf(" -i key private key file for user authentication\n");
- printf(" -noagent disable use of Pageant\n");
- printf(" -agent enable use of Pageant\n");
-+ printf(" -no-trivial-auth\n");
-+ printf(" disconnect if SSH authentication succeeds trivially\n");
- printf(" -noshare disable use of connection sharing\n");
- printf(" -share enable use of connection sharing\n");
- printf(" -hostkey keyid\n");
---
-2.20.1
-
projects / buildroot-mgba.git / commitdiff