CVE-2020-15078: Authentication bypass with deferred authentication

Overview

OpenVPN 2.5.1 and earlier versions allows a remote attackers to bypass
authentication and access control channel data on servers configured
with deferred authentication, which can be used to potentially trigger
further information leaks.

Detailed description

This bug allows - under very specific circumstances - to trick a server
using delayed authentication (plugin or management) into returning a
PUSH_REPLY before the AUTH_FAILED message, which can possibly be used
to gather information about a VPN setup.

In combination with "--auth-gen-token" or a user-specific token auth
solution it can be possible to get access to a VPN with an
otherwise-invalid account.

Pre-Dependency:
CVE-2020-15078-0.patch: https://github.com/OpenVPN/openvpn/commit/14511010

CVE-Fix:
CVE-2020-15078-1.patch: https://github.com/OpenVPN/openvpn/commit/3aca477a
CVE-2020-15078-2.patch: https://github.com/OpenVPN/openvpn/commit/3d18e308
CVE-2020-15078-3.patch: https://github.com/OpenVPN/openvpn/commit/f7b3bf06

Closes: #987380

1 files changed, 276 insertions, 0 deletions

diff --git a/debian/patches/CVE-2020-15078-0.patch b/debian/patches/CVE-2020-15078-0.patch
new file mode 100644
index 0000000..d048994
--- /dev/null
+++ b/debian/patches/CVE-2020-15078-0.patch
@@ -0,0 +1,276 @@
+From 145110101b70599cb9adcf4ed071856daac9c8af Mon Sep 17 00:00:00 2001
+From: Arne Schwabe <arne@rfc2549.org>
+Date: Sun, 28 Mar 2021 14:02:40 +0200
+Subject: [PATCH] Move context_auth from context_2 to tls_multi and name it
+ multi_state
+
+context_2 and tls_multi have the same life cycle for TLS connections
+but so this move does not affect behaviour of the variable.
+
+OpenVPN TLS multi code has a grown a lot more complex and code that
+handles multi objects needs to know the state that the object is in.
+Since not all code has access to the context_2 struct, the code that
+does not have access is often not checking the state directly but
+checks other parts of multi that have been affected from a state
+change.
+
+This patch also renames it to multi_state as this variable represents
+the multi state machine status rather than just the state of the connect
+authentication (more upcoming patches will move other states
+into this variable).
+
+Patch V2: also rename context_auth to multi_state, explain a bit why this
+          change is done.
+Patch V3: Add comments for c2->multi NULL check forwarding. Fix compile
+          with ENABLE_ASYNC_PUSH.
+
+Signed-off-by: Arne Schwabe <arne@rfc2549.org>
+Acked-by: Antonio Quartulli <antonio@openvpn.net>
+Acked-by: Gert Doering <gert@greenie.muc.de>
+Message-Id: <20210418160111.1494779-1-arne@rfc2549.org>
+URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg22155.html
+Signed-off-by: Gert Doering <gert@greenie.muc.de>
+(backported from commit 0767d5b447044e4cdcfd198058aef1f85f63bbe6)
+---
+ src/openvpn/forward.c    | 10 ++++++----
+ src/openvpn/multi.c      | 28 ++++++++++++++--------------
+ src/openvpn/openvpn.h    | 14 --------------
+ src/openvpn/push.c       |  5 +++--
+ src/openvpn/ssl_common.h | 14 ++++++++++++++
+ 5 files changed, 37 insertions(+), 34 deletions(-)
+
+diff --git a/src/openvpn/forward.c b/src/openvpn/forward.c
+index 7ed8d0d75..fd7412f73 100644
+--- a/src/openvpn/forward.c
++++ b/src/openvpn/forward.c
+@@ -526,9 +526,10 @@ encrypt_sign(struct context *c, bool comp_frag)
+ 
+     /*
+      * Drop non-TLS outgoing packet if client-connect script/plugin
+-     * has not yet succeeded.
++     * has not yet succeeded. In non-TLS mode tls_multi is not defined
++     * and we always pass packets.
+      */
+-    if (c->c2.context_auth != CAS_SUCCEEDED)
++    if (c->c2.tls_multi && c->c2.tls_multi->multi_state != CAS_SUCCEEDED)
+     {
+         c->c2.buf.len = 0;
+     }
+@@ -973,9 +974,10 @@ process_incoming_link_part1(struct context *c, struct link_socket_info *lsi, boo
+ 
+         /*
+          * Drop non-TLS packet if client-connect script/plugin and cipher selection
+-         * has not yet succeeded.
++         * has not yet succeeded. In non-TLS mode tls_multi is not defined
++         * and we always pass packets.
+          */
+-        if (c->c2.context_auth != CAS_SUCCEEDED)
++        if (c->c2.tls_multi && c->c2.tls_multi->multi_state != CAS_SUCCEEDED)
+         {
+             c->c2.buf.len = 0;
+         }
+diff --git a/src/openvpn/multi.c b/src/openvpn/multi.c
+index 137381805..599ffd86d 100644
+--- a/src/openvpn/multi.c
++++ b/src/openvpn/multi.c
+@@ -678,7 +678,7 @@ multi_close_instance(struct multi_context *m,
+ #ifdef MANAGEMENT_DEF_AUTH
+     set_cc_config(mi, NULL);
+ #endif
+-    if (mi->context.c2.context_auth == CAS_SUCCEEDED)
++    if (mi->context.c2.tls_multi->multi_state == CAS_SUCCEEDED)
+     {
+         multi_client_disconnect_script(mi);
+     }
+@@ -788,7 +788,7 @@ multi_create_instance(struct multi_context *m, const struct mroute_addr *real)
+         goto err;
+     }
+ 
+-    mi->context.c2.context_auth = CAS_PENDING;
++    mi->context.c2.tls_multi->multi_state = CAS_PENDING;
+ 
+     if (hash_n_elements(m->hash) >= m->max_clients)
+     {
+@@ -2436,18 +2436,18 @@ multi_client_connect_late_setup(struct multi_context *m,
+     mi->reporting_addr_ipv6 = mi->context.c2.push_ifconfig_ipv6_local;
+ 
+     /* set context-level authentication flag */
+-    mi->context.c2.context_auth = CAS_SUCCEEDED;
++    mi->context.c2.tls_multi->multi_state = CAS_SUCCEEDED;
+ 
+     /* authentication complete, calculate dynamic client specific options */
+     if (!multi_client_set_protocol_options(&mi->context))
+     {
+-        mi->context.c2.context_auth = CAS_FAILED;
++        mi->context.c2.tls_multi->multi_state = CAS_FAILED;
+     }
+     /* Generate data channel keys only if setting protocol options
+      * has not failed */
+     else if (!multi_client_generate_tls_keys(&mi->context))
+     {
+-        mi->context.c2.context_auth = CAS_FAILED;
++        mi->context.c2.tls_multi->multi_state = CAS_FAILED;
+     }
+ 
+     /* send push reply if ready */
+@@ -2595,7 +2595,7 @@ multi_connection_established(struct multi_context *m, struct multi_instance *mi)
+ 
+     /* We are only called for the CAS_PENDING_x states, so we
+      * can ignore other states here */
+-    bool from_deferred = (mi->context.c2.context_auth != CAS_PENDING);
++    bool from_deferred = (mi->context.c2.tls_multi->multi_state != CAS_PENDING);
+ 
+     int *cur_handler_index = &mi->client_connect_defer_state.cur_handler_index;
+     unsigned int *option_types_found =
+@@ -2607,7 +2607,7 @@ multi_connection_established(struct multi_context *m, struct multi_instance *mi)
+         *cur_handler_index = 0;
+         *option_types_found = 0;
+         /* Initially we have no handler that has returned a result */
+-        mi->context.c2.context_auth = CAS_PENDING_DEFERRED;
++        mi->context.c2.tls_multi->multi_state = CAS_PENDING_DEFERRED;
+ 
+         multi_client_connect_early_setup(m, mi);
+     }
+@@ -2630,7 +2630,7 @@ multi_connection_established(struct multi_context *m, struct multi_instance *mi)
+                  * Remember that we already had at least one handler
+                  * returning a result should we go to into deferred state
+                  */
+-                mi->context.c2.context_auth = CAS_PENDING_DEFERRED_PARTIAL;
++                mi->context.c2.tls_multi->multi_state = CAS_PENDING_DEFERRED_PARTIAL;
+                 break;
+ 
+             case CC_RET_SKIPPED:
+@@ -2682,12 +2682,12 @@ multi_connection_established(struct multi_context *m, struct multi_instance *mi)
+     {
+         /* run the disconnect script if we had a connect script that
+          * did not fail */
+-        if (mi->context.c2.context_auth == CAS_PENDING_DEFERRED_PARTIAL)
++        if (mi->context.c2.tls_multi->multi_state == CAS_PENDING_DEFERRED_PARTIAL)
+         {
+             multi_client_disconnect_script(mi);
+         }
+ 
+-        mi->context.c2.context_auth = CAS_FAILED;
++        mi->context.c2.tls_multi->multi_state = CAS_FAILED;
+     }
+ 
+     /* increment number of current authenticated clients */
+@@ -2990,13 +2990,13 @@ multi_process_post(struct multi_context *m, struct multi_instance *mi, const uns
+         {
+             /* connection is "established" when SSL/TLS key negotiation succeeds
+              * and (if specified) auth user/pass succeeds */
+-            if (is_cas_pending(mi->context.c2.context_auth)
++            if (is_cas_pending(mi->context.c2.tls_multi->multi_state)
+                 && CONNECTION_ESTABLISHED(&mi->context))
+             {
+                 multi_connection_established(m, mi);
+             }
+ #if defined(ENABLE_ASYNC_PUSH) && defined(ENABLE_DEF_AUTH)
+-            if (is_cas_pending(mi->context.c2.context_auth)
++            if (is_cas_pending(mi->context.c2.tls_multi->multi_state)
+                 && mi->client_connect_defer_state.deferred_ret_file)
+             {
+                 add_inotify_file_watch(m, mi, m->top.c2.inotify_fd,
+@@ -3953,7 +3953,7 @@ management_client_auth(void *arg,
+         {
+             if (auth)
+             {
+-                if (is_cas_pending(mi->context.c2.context_auth))
++                if (is_cas_pending(mi->context.c2.tls_multi->multi_state))
+                 {
+                     set_cc_config(mi, cc_config);
+                     cc_config_owned = false;
+@@ -3965,7 +3965,7 @@ management_client_auth(void *arg,
+                 {
+                     msg(D_MULTI_LOW, "MULTI: connection rejected: %s, CLI:%s", reason, np(client_reason));
+                 }
+-                if (!is_cas_pending(mi->context.c2.context_auth))
++                if (!is_cas_pending(mi->context.c2.tls_multi->multi_state))
+                 {
+                     send_auth_failed(&mi->context, client_reason); /* mid-session reauth failed */
+                     multi_schedule_context_wakeup(m, mi);
+diff --git a/src/openvpn/openvpn.h b/src/openvpn/openvpn.h
+index a7b597749..d131ac59e 100644
+--- a/src/openvpn/openvpn.h
++++ b/src/openvpn/openvpn.h
+@@ -211,17 +211,6 @@ struct context_1
+ };
+ 
+ 
+-/* client authentication state, CAS_SUCCEEDED must be 0 since
+- * non multi code path still checks this variable but does not initialise it
+- * so the code depends on zero initialisation */
+-enum client_connect_status {
+-    CAS_SUCCEEDED=0,
+-    CAS_PENDING,
+-    CAS_PENDING_DEFERRED,
+-    CAS_PENDING_DEFERRED_PARTIAL,   /**< at least handler succeeded, no result yet*/
+-    CAS_FAILED,
+-};
+-
+ static inline bool
+ is_cas_pending(enum client_connect_status cas)
+ {
+@@ -458,9 +447,6 @@ struct context_2
+     int push_ifconfig_ipv6_netbits;
+     struct in6_addr push_ifconfig_ipv6_remote;
+ 
+-
+-    enum client_connect_status context_auth;
+-
+     struct event_timeout push_request_interval;
+     int n_sent_push_requests;
+     bool did_pre_pull_restore;
+diff --git a/src/openvpn/push.c b/src/openvpn/push.c
+index e0d2eeaf2..c47f4c8b6 100644
+--- a/src/openvpn/push.c
++++ b/src/openvpn/push.c
+@@ -733,13 +733,14 @@ process_incoming_push_request(struct context *c)
+ {
+     int ret = PUSH_MSG_ERROR;
+ 
+-    if (tls_authentication_status(c->c2.tls_multi, 0) == TLS_AUTHENTICATION_FAILED || c->c2.context_auth == CAS_FAILED)
++    if (tls_authentication_status(c->c2.tls_multi, 0) == TLS_AUTHENTICATION_FAILED
++        || c->c2.tls_multi->multi_state == CAS_FAILED)
+     {
+         const char *client_reason = tls_client_reason(c->c2.tls_multi);
+         send_auth_failed(c, client_reason);
+         ret = PUSH_MSG_AUTH_FAILURE;
+     }
+-    else if (c->c2.context_auth == CAS_SUCCEEDED)
++    else if (c->c2.tls_multi->multi_state == CAS_SUCCEEDED)
+     {
+         time_t now;
+ 
+diff --git a/src/openvpn/ssl_common.h b/src/openvpn/ssl_common.h
+index a703f97cd..06c32ac1d 100644
+--- a/src/openvpn/ssl_common.h
++++ b/src/openvpn/ssl_common.h
+@@ -479,6 +479,19 @@ struct tls_session
+  */
+ #define KEY_SCAN_SIZE 3
+ 
++
++/* client authentication state, CAS_SUCCEEDED must be 0 since
++ * non multi code path still checks this variable but does not initialise it
++ * so the code depends on zero initialisation */
++enum client_connect_status {
++    CAS_SUCCEEDED=0,
++    CAS_PENDING,
++    CAS_PENDING_DEFERRED,
++    CAS_PENDING_DEFERRED_PARTIAL,   /**< at least handler succeeded, no result yet*/
++    CAS_FAILED,
++};
++
++
+ /**
+  * Security parameter state for a single VPN tunnel.
+  * @ingroup control_processor
+@@ -519,6 +532,7 @@ struct tls_multi
+ 
+     int n_sessions;             /**< Number of sessions negotiated thus
+                                  *   far. */
++    enum client_connect_status multi_state;
+ 
+     /*
+      * Number of errors.