- djm@cvs.openbsd.org 2008/07/02 12:36:39

     [auth2-none.c auth2.c]
     Make protocol 2 MaxAuthTries behaviour a little more sensible:
     Check whether client has exceeded MaxAuthTries before running
     an authentication method and skip it if they have, previously it
     would always allow one try (for "none" auth).
     Preincrement failure count before post-auth test - previously this
     checked and postincremented, also to allow one "none" try.
     Together, these two changes always count the "none" auth method
     which could be skipped by a malicious client (e.g. an SSH worm)
     to get an extra attempt at a real auth method. They also make
     MaxAuthTries=0 a useful way to block users entirely (esp. in a
     sshd_config Match block).
     Also, move sending of any preauth banner from "none" auth method
     to the first call to input_userauth_request(), so worms that skip
     the "none" method get to see it too.

1 files changed, 74 insertions, 4 deletions

diff --git a/auth2.c b/auth2.c
index 03d7f09dc..31f01f9fb 100644
--- a/auth2.c
+++ b/auth2.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: auth2.c,v 1.116 2007/09/29 00:25:51 dtucker Exp $ */
+/* $OpenBSD: auth2.c,v 1.117 2008/07/02 12:36:39 djm Exp $ */
 /*
  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
  *
@@ -26,10 +26,14 @@
 #include "includes.h"
 
 #include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/uio.h>
 
+#include <fcntl.h>
 #include <pwd.h>
 #include <stdarg.h>
 #include <string.h>
+#include <unistd.h>
 
 #include "xmalloc.h"
 #include "ssh2.h"
@@ -88,10 +92,74 @@ static void input_userauth_request(int, u_int32_t, void *);
 static Authmethod *authmethod_lookup(const char *);
 static char *authmethods_get(void);
 
+char *
+auth2_read_banner(void)
+{
+        struct stat st;
+        char *banner = NULL;
+        size_t len, n;
+        int fd;
+
+        if ((fd = open(options.banner, O_RDONLY)) == -1)
+                return (NULL);
+        if (fstat(fd, &st) == -1) {
+                close(fd);
+                return (NULL);
+        }
+        if (st.st_size > 1*1024*1024) {
+                close(fd);
+                return (NULL);
+        }
+
+        len = (size_t)st.st_size;               /* truncate */
+        banner = xmalloc(len + 1);
+        n = atomicio(read, fd, banner, len);
+        close(fd);
+
+        if (n != len) {
+                xfree(banner);
+                return (NULL);
+        }
+        banner[n] = '\0';
+
+        return (banner);
+}
+
+void
+userauth_send_banner(const char *msg)
+{
+        if (datafellows & SSH_BUG_BANNER)
+                return;
+
+        packet_start(SSH2_MSG_USERAUTH_BANNER);
+        packet_put_cstring(msg);
+        packet_put_cstring("");         /* language, unused */
+        packet_send();
+        debug("%s: sent", __func__);
+}
+
+static void
+userauth_banner(void)
+{
+        char *banner = NULL;
+
+        if (options.banner == NULL ||
+            strcasecmp(options.banner, "none") == 0 ||
+            (datafellows & SSH_BUG_BANNER) != 0)
+                return;
+
+        if ((banner = PRIVSEP(auth2_read_banner())) == NULL)
+                goto done;
+        userauth_send_banner(banner);
+
+done:
+        if (banner)
+                xfree(banner);
+}
+
 /*
  * loop until authctxt->success == TRUE
  */
-
 void
 do_authentication2(Authctxt *authctxt)
 {
@@ -179,6 +247,7 @@ input_userauth_request(int type, u_int32_t seq, void *ctxt)
                 authctxt->style = style ? xstrdup(style) : NULL;
                 if (use_privsep)
                         mm_inform_authserv(service, style);
+                userauth_banner();
         } else if (strcmp(user, authctxt->user) != 0 ||
             strcmp(service, authctxt->service) != 0) {
                 packet_disconnect("Change of username or service not allowed: "
@@ -197,7 +266,7 @@ input_userauth_request(int type, u_int32_t seq, void *ctxt)
 
         /* try to authenticate user */
         m = authmethod_lookup(method);
-        if (m != NULL) {
+        if (m != NULL && authctxt->failures < options.max_authtries) {
                 debug2("input_userauth_request: try method %s", method);
                 authenticated = m->userauth(authctxt);
         }
@@ -264,7 +333,7 @@ userauth_finish(Authctxt *authctxt, int authenticated, char *method)
                 /* now we can break out */
                 authctxt->success = 1;
         } else {
-                if (authctxt->failures++ > options.max_authtries) {
+                if (++authctxt->failures > options.max_authtries) {
 #ifdef SSH_AUDIT_EVENTS
                         PRIVSEP(audit_event(SSH_LOGIN_EXCEED_MAXTRIES));
 #endif
@@ -320,3 +389,4 @@ authmethod_lookup(const char *name)
             name ? name : "NULL");
         return NULL;
 }
+