diff --git a/auth-krb5.c b/auth-krb5.c
index 6c62bdf..11c8562 100644
--- a/auth-krb5.c
+++ b/auth-krb5.c
@@ -54,6 +54,21 @@
extern ServerOptions options;
+int
+ssh_krb5_kuserok(krb5_context krb5_ctx, krb5_principal krb5_user, const char *client,
+ int k5login_exists)
+{
+ if (options.use_kuserok || !k5login_exists)
+ return krb5_kuserok(krb5_ctx, krb5_user, client);
+ else {
+ char kuser[65];
+
+ if (krb5_aname_to_localname(krb5_ctx, krb5_user, sizeof(kuser), kuser))
+ return 0;
+ return strcmp(kuser, client) == 0;
+ }
+}
+
static int
krb5_init(void *context)
{
@@ -157,8 +172,9 @@ auth_krb5_password(Authctxt *authctxt, const char *password)
if (problem)
goto out;
- if (!krb5_kuserok(authctxt->krb5_ctx, authctxt->krb5_user,
- authctxt->pw->pw_name)) {
+ /* Use !options.use_kuserok here to make ssh_krb5_kuserok() not
+ * depend on the existance of .k5login */
+ if (!ssh_krb5_kuserok(authctxt->krb5_ctx, authctxt->krb5_user, authctxt->pw->pw_name, !options.use_kuserok)) {
problem = -1;
goto out;
}
diff --git a/gss-serv-krb5.c b/gss-serv-krb5.c
index 60de320..0a4930e 100644
--- a/gss-serv-krb5.c
+++ b/gss-serv-krb5.c
@@ -67,6 +67,7 @@ static int ssh_gssapi_krb5_cmdok(krb5_principal, const char *, const char *,
int);
static krb5_context krb_context = NULL;
+extern int ssh_krb5_kuserok(krb5_context, krb5_principal, const char *, int);
/* Initialise the krb5 library, for the stuff that GSSAPI won't do */
@@ -92,6 +93,103 @@ ssh_gssapi_krb5_init(void)
* Returns true if the user is OK to log in, otherwise returns 0
*/
+/* The purpose of the function is to find out if a Kerberos principal is
+ * allowed to log in as the given local user. This is a general problem with
+ * Kerberized services because by design the Kerberos principals are
+ * completely independent from the local user names. This is one of the
+ * reasons why Kerberos is working well on different operating systems like
+ * Windows and UNIX/Linux. Nevertheless a relationship between a Kerberos
+ * principal and a local user name must be established because otherwise every
+ * access would be granted for every principal with a valid ticket.
+ *
+ * Since it is a general issue libkrb5 provides some functions for
+ * applications to find out about the relationship between the Kerberos
+ * principal and a local user name. They are krb5_kuserok() and
+ * krb5_aname_to_localname().
+ *
+ * krb5_kuserok() can be used to "Determine if a principal is authorized to
+ * log in as a local user" (from the MIT Kerberos documentation of this
+ * function). Which is exactly what we are looking for and should be the
+ * preferred choice. It accepts the Kerberos principal and a local user name
+ * and let libkrb5 or its plugins determine if they relate to each other or
+ * not.
+ *
+ * krb5_aname_to_localname() can use used to "Convert a principal name to a
+ * local name" (from the MIT Kerberos documentation of this function). It
+ * accepts a Kerberos principle and returns a local name and it is up to the
+ * application to do any additional checks. There are two issues using
+ * krb5_aname_to_localname(). First, since POSIX user names are case
+ * sensitive, the calling application in general has no other choice than
+ * doing a case-sensitive string comparison between the name returned by
+ * krb5_aname_to_localname() and the name used at the login prompt. When the
+ * users are provided by a case in-sensitive server, e.g. Active Directory,
+ * this might lead to login failures because the user typing the name at the
+ * login prompt might not be aware of the right case. Another issue might be
+ * caused if there are multiple alias names available for a single user. E.g.
+ * the canonical name of a user is user@group.department.example.com but there
+ * exists a shorter login name, e.g. user@example.com, to safe typing at the
+ * login prompt. Here krb5_aname_to_localname() can only return the canonical
+ * name, but if the short alias is used at the login prompt authentication
+ * will fail as well. All this can be avoided by using krb5_kuserok() and
+ * configuring krb5.conf or using a suitable plugin to meet the needs of the
+ * given environment.
+ *
+ * The Fedora and RHEL version of openssh contain two patches which modify the
+ * access control behavior:
+ * - openssh-6.6p1-kuserok.patch
+ * - openssh-6.6p1-force_krb.patch
+ *
+ * openssh-6.6p1-kuserok.patch adds a new option KerberosUseKuserok for
+ * sshd_config which controls if krb5_kuserok() is used to check if the
+ * principle is authorized or if krb5_aname_to_localname() should be used.
+ * The reason to add this patch was that krb5_kuserok() by default checks if
+ * a .k5login file exits in the users home-directory. With this the user can
+ * give access to his account for any given principal which might be
+ * in violation with company policies and it would be useful if this can be
+ * rejected. Nevertheless the patch ignores the fact that krb5_kuserok() does
+ * no only check .k5login but other sources as well and checking .k5login can
+ * be disabled for all applications in krb5.conf as well. With this new
+ * option KerberosUseKuserok set to 'no' (and this is the default for RHEL7
+ * and Fedora 21) openssh can only use krb5_aname_to_localname() with the
+ * restrictions mentioned above.
+ *
+ * openssh-6.6p1-force_krb.patch adds a ksu like behaviour to ssh, i.e. when
+ * using GSSAPI authentication only commands configured in the .k5user can be
+ * executed. Here the wrong assumption that krb5_kuserok() only checks
+ * .k5login is made as well. In contrast ksu checks .k5login directly and
+ * does not use krb5_kuserok() which might be more useful for the given
+ * purpose. Additionally this patch is not synced with
+ * openssh-6.6p1-kuserok.patch.
+ *
+ * The current patch tries to restore the usage of krb5_kuserok() so that e.g.
+ * localauth plugins can be used. It does so by adding a forth parameter to
+ * ssh_krb5_kuserok() which indicates whether .k5login exists or not. If it
+ * does not exists krb5_kuserok() is called even if KerberosUseKuserok is set
+ * to 'no' because the intent of the option is to not check .k5login and if it
+ * does not exists krb5_kuserok() returns a result without checking .k5login.
+ * If .k5login does exists and KerberosUseKuserok is 'no' we fall back to
+ * krb5_aname_to_localname(). This is in my point of view an acceptable
+ * limitation and does not break the current behaviour.
+ *
+ * Additionally with this patch ssh_krb5_kuserok() is called in
+ * ssh_gssapi_krb5_cmdok() instead of only krb5_aname_to_localname() is
+ * neither .k5login nor .k5users exists to allow plugin evaluation via
+ * krb5_kuserok() as well.
+ *
+ * I tried to keep the patch as minimal as possible, nevertheless I see some
+ * areas for improvement which, if they make sense, have to be evaluated
+ * carefully because they might change existing behaviour and cause breaks
+ * during upgrade:
+ * - I wonder if disabling .k5login usage make sense in sshd or if it should
+ * be better disabled globally in krb5.conf
+ * - if really needed openssh-6.6p1-kuserok.patch should be fixed to really
+ * only disable checking .k5login and maybe .k5users
+ * - the ksu behaviour should be configurable and maybe check the .k5login and
+ * .k5users files directly like ksu itself does
+ * - to make krb5_aname_to_localname() more useful an option for sshd to use
+ * the canonical name (the one returned by getpwnam()) instead of the name
+ * given at the login prompt might be useful */
+
static int
ssh_gssapi_krb5_userok(ssh_gssapi_client *client, char *name)
{
@@ -116,7 +214,8 @@ ssh_gssapi_krb5_userok(ssh_gssapi_client *client, char *name)
/* NOTE: .k5login and .k5users must opened as root, not the user,
* because if they are on a krb5-protected filesystem, user credentials
* to access these files aren't available yet. */
- if (krb5_kuserok(krb_context, princ, name) && k5login_exists) {
+ if (ssh_krb5_kuserok(krb_context, princ, name, k5login_exists)
+ && k5login_exists) {
retval = 1;
logit("Authorized to %s, krb5 principal %s (krb5_kuserok)",
name, (char *)client->displayname.value);
@@ -171,9 +270,8 @@ ssh_gssapi_krb5_cmdok(krb5_principal principal, const char *name,
snprintf(file, sizeof(file), "%s/.k5users", pw->pw_dir);
/* If both .k5login and .k5users DNE, self-login is ok. */
if (!k5login_exists && (access(file, F_OK) == -1)) {
- return (krb5_aname_to_localname(krb_context, principal,
- sizeof(kuser), kuser) == 0) &&
- (strcmp(kuser, luser) == 0);
+ return ssh_krb5_kuserok(krb_context, principal, luser,
+ k5login_exists);
}
if ((fp = fopen(file, "r")) == NULL) {
int saved_errno = errno;
diff --git a/servconf.c b/servconf.c
index 68fb9ef..904c869 100644
--- a/servconf.c
+++ b/servconf.c
@@ -157,6 +157,7 @@ initialize_server_options(ServerOptions *options)
options->ip_qos_interactive = -1;
options->ip_qos_bulk = -1;
options->version_addendum = NULL;
+ options->use_kuserok = -1;
}
void
@@ -312,6 +313,8 @@ fill_default_server_options(ServerOptions *options)
options->version_addendum = xstrdup("");
if (options->show_patchlevel == -1)
options->show_patchlevel = 0;
+ if (options->use_kuserok == -1)
+ options->use_kuserok = 1;
/* Turn privilege separation on by default */
if (use_privsep == -1)
@@ -338,7 +341,7 @@ typedef enum {
sPermitRootLogin, sLogFacility, sLogLevel,
sRhostsRSAAuthentication, sRSAAuthentication,
sKerberosAuthentication, sKerberosOrLocalPasswd, sKerberosTicketCleanup,
- sKerberosGetAFSToken,
+ sKerberosGetAFSToken, sKerberosUseKuserok,
sKerberosTgtPassing, sChallengeResponseAuthentication,
sPasswordAuthentication, sKbdInteractiveAuthentication,
sListenAddress, sAddressFamily,
@@ -410,11 +413,13 @@ static struct {
#else
{ "kerberosgetafstoken", sUnsupported, SSHCFG_GLOBAL },
#endif
+ { "kerberosusekuserok", sKerberosUseKuserok, SSHCFG_ALL },
#else
{ "kerberosauthentication", sUnsupported, SSHCFG_ALL },
{ "kerberosorlocalpasswd", sUnsupported, SSHCFG_GLOBAL },
{ "kerberosticketcleanup", sUnsupported, SSHCFG_GLOBAL },
{ "kerberosgetafstoken", sUnsupported, SSHCFG_GLOBAL },
+ { "kerberosusekuserok", sUnsupported, SSHCFG_ALL },
#endif
{ "kerberostgtpassing", sUnsupported, SSHCFG_GLOBAL },
{ "afstokenpassing", sUnsupported, SSHCFG_GLOBAL },
@@ -1526,6 +1531,10 @@ process_server_config_line(ServerOptions *options, char *line,
*activep = value;
break;
+ case sKerberosUseKuserok:
+ intptr = &options->use_kuserok;
+ goto parse_flag;
+
case sPermitOpen:
arg = strdelim(&cp);
if (!arg || *arg == '\0')
@@ -1811,6 +1820,7 @@ copy_set_server_options(ServerOptions *dst, ServerOptions *src, int preauth)
M_CP_INTOPT(max_authtries);
M_CP_INTOPT(ip_qos_interactive);
M_CP_INTOPT(ip_qos_bulk);
+ M_CP_INTOPT(use_kuserok);
M_CP_INTOPT(rekey_limit);
M_CP_INTOPT(rekey_interval);
@@ -2062,6 +2072,7 @@ dump_config(ServerOptions *o)
dump_cfg_fmtint(sUseDNS, o->use_dns);
dump_cfg_fmtint(sAllowTcpForwarding, o->allow_tcp_forwarding);
dump_cfg_fmtint(sUsePrivilegeSeparation, use_privsep);
+ dump_cfg_fmtint(sKerberosUseKuserok, o->use_kuserok);
/* string arguments */
dump_cfg_string(sPidFile, o->pid_file);
diff --git a/servconf.h b/servconf.h
index 37cfa9b..5117dfa 100644
--- a/servconf.h
+++ b/servconf.h
@@ -173,6 +173,7 @@ typedef struct {
int num_permitted_opens;
+ int use_kuserok;
char *chroot_directory;
char *revoked_keys_file;
char *trusted_user_ca_keys;
diff --git a/sshd_config b/sshd_config
index adfd7b1..e772ed5 100644
--- a/sshd_config
+++ b/sshd_config
@@ -87,6 +87,7 @@ ChallengeResponseAuthentication no
#KerberosOrLocalPasswd yes
#KerberosTicketCleanup yes
#KerberosGetAFSToken no
+#KerberosUseKuserok yes
# GSSAPI options
GSSAPIAuthentication yes
diff --git a/sshd_config.5 b/sshd_config.5
index 1fb002d..e0e5fff 100644
--- a/sshd_config.5
+++ b/sshd_config.5
@@ -697,6 +697,10 @@ Specifies whether to automatically destroy the user's ticket cache
file on logout.
The default is
.Dq yes .
+.It Cm KerberosUseKuserok
+Specifies whether to look at .k5login file for user's aliases.
+The default is
+.Dq yes .
.It Cm KexAlgorithms
Specifies the available KEX (Key Exchange) algorithms.
Multiple algorithms must be comma-separated.
@@ -862,6 +866,7 @@ Available keywords are
.Cm HostbasedUsesNameFromPacketOnly ,
.Cm KbdInteractiveAuthentication ,
.Cm KerberosAuthentication ,
+.Cm KerberosUseKuserok ,
.Cm MaxAuthTries ,
.Cm MaxSessions ,
.Cm PasswordAuthentication ,