4 files changed, 43 insertions, 0 deletions
diff --git a/src/providers/krb5/krb5_auth.c b/src/providers/krb5/krb5_auth.c
index c98535b1d..72f0711eb 100644
--- a/src/providers/krb5/krb5_auth.c
+++ b/src/providers/krb5/krb5_auth.c
@@ -427,6 +427,13 @@ struct tevent_req *krb5_auth_send(TALLOC_CTX *mem_ctx,
                 DEBUG(1, ("krb5_get_simple_upn failed.\n"));
                 goto done;
             }
+        } else {
+            ret = compare_principal_realm(kr->upn, realm,
+                                          &kr->upn_from_different_realm);
+            if (ret != 0) {
+                DEBUG(SSSDBG_OP_FAILURE, ("compare_principal_realm failed.\n"));
+                goto done;
+            }
         }
 
         kr->homedir = ldb_msg_find_attr_as_string(res->msgs[0], SYSDB_HOMEDIR,
diff --git a/src/providers/krb5/krb5_auth.h b/src/providers/krb5/krb5_auth.h
index cc079ba93..a23b8b47d 100644
--- a/src/providers/krb5/krb5_auth.h
+++ b/src/providers/krb5/krb5_auth.h
@@ -54,6 +54,7 @@ struct krb5child_req {
     bool active_ccache_present;
     bool valid_tgt_present;
     bool run_as_user;
+    bool upn_from_different_realm;
 };
 
 errno_t krb5_setup(TALLOC_CTX *mem_ctx, struct pam_data *pd,
diff --git a/src/providers/krb5/krb5_common.c b/src/providers/krb5/krb5_common.c
index 006dac1ce..45f126f7b 100644
--- a/src/providers/krb5/krb5_common.c
+++ b/src/providers/krb5/krb5_common.c
@@ -881,3 +881,34 @@ errno_t krb5_get_simple_upn(TALLOC_CTX *mem_ctx, struct krb5_ctx *krb5_ctx,
     *_upn = upn;
     return EOK;
 }
+
+errno_t compare_principal_realm(const char *upn, const char *realm,
+                                bool *different_realm)
+{
+    size_t upn_len;
+    size_t realm_len;
+    char *at_sign;
+
+    if (upn == NULL || realm == NULL || different_realm == NULL) {
+        return EINVAL;
+    }
+
+    upn_len = strlen(upn);
+    realm_len = strlen(realm);
+    at_sign = strchr(upn, '@');
+
+    /* if coming from the same realm the upn must be at least the size of the
+     * realm plus 1 for the '@' char. */
+    if (upn_len == 0 || realm_len == 0 || upn_len <= realm_len + 1 ||
+        at_sign == NULL) {
+        return EINVAL;
+    }
+
+    if (strcmp(realm, at_sign + 1) == 0) {
+        *different_realm = false;
+    } else {
+        *different_realm = true;
+    }
+
+    return EOK;
+}
diff --git a/src/providers/krb5/krb5_common.h b/src/providers/krb5/krb5_common.h
index 51bd26773..bc63bf983 100644
--- a/src/providers/krb5/krb5_common.h
+++ b/src/providers/krb5/krb5_common.h
@@ -177,6 +177,10 @@ errno_t remove_krb5_info_files(TALLOC_CTX *mem_ctx, const char *realm);
 errno_t krb5_get_simple_upn(TALLOC_CTX *mem_ctx, struct krb5_ctx *krb5_ctx,
                             const char *username, const char **_upn);
 
+errno_t compare_principal_realm(const char *upn, const char *realm,
+                                bool *different_realm);
+
+
 int sssm_krb5_auth_init(struct be_ctx *bectx,
                         struct bet_ops **ops,
                         void **pvt_auth_data);