diff --git a/src/lib/idmap/sss_idmap.c b/src/lib/idmap/sss_idmap.c
index 7ad05659a76..43dd5028fdb 100644
--- a/src/lib/idmap/sss_idmap.c
+++ b/src/lib/idmap/sss_idmap.c
@@ -201,7 +201,7 @@ const char *idmap_error_string(enum idmap_error_code err)
     }
 }
 
-bool is_domain_sid(const char *sid)
+bool is_str_sid(const char *sid, int count)
 {
     const char *p;
     long long a;
@@ -228,15 +228,25 @@ bool is_domain_sid(const char *sid)
             return false;
         }
         c++;
-    } while(c < 3 && *endptr != '\0');
+    } while(c < count && *endptr != '\0');
 
-    if (c != 3 || *endptr != '\0') {
+    if (c != count || *endptr != '\0') {
         return false;
     }
 
     return true;
 }
 
+bool is_principal_sid(const char *str)
+{
+    return is_str_sid(str, 4);
+}
+
+bool is_domain_sid(const char *str)
+{
+    return is_str_sid(str, 3);
+}
+
 enum idmap_error_code sss_idmap_init(idmap_alloc_func *alloc_func,
                                      void *alloc_pvt,
                                      idmap_free_func *free_func,
diff --git a/src/lib/idmap/sss_idmap.exports b/src/lib/idmap/sss_idmap.exports
index 840677794bd..813987ebf86 100644
--- a/src/lib/idmap/sss_idmap.exports
+++ b/src/lib/idmap/sss_idmap.exports
@@ -62,5 +62,6 @@ SSS_IDMAP_0.5 {
 
         sss_idmap_ctx_set_extra_slice_init;
         sss_idmap_add_auto_domain_ex;
+        is_principal_sid;
 
 } SSS_IDMAP_0.4;
diff --git a/src/lib/idmap/sss_idmap.h b/src/lib/idmap/sss_idmap.h
index 9c27a160004..a544e8625b0 100644
--- a/src/lib/idmap/sss_idmap.h
+++ b/src/lib/idmap/sss_idmap.h
@@ -706,6 +706,17 @@ const char *idmap_error_string(enum idmap_error_code err);
  */
 bool is_domain_sid(const char *str);
 
+/**
+ * @brief Check if given string can be used as principal SID
+ *
+ * @param[in] str   String to check
+ *
+ * @return
+ *  - true: String can be used as principal SID
+ *  - false: String can not be used as principal SID
+ */
+bool is_principal_sid(const char *str);
+
 /**
  * @brief Check if a domain is configured with algorithmic mapping
  *
diff --git a/src/providers/ldap/sdap.h b/src/providers/ldap/sdap.h
index d66ca156afe..6ddfc4c4613 100644
--- a/src/providers/ldap/sdap.h
+++ b/src/providers/ldap/sdap.h
@@ -467,6 +467,7 @@ struct sdap_domain {
      * the base DN which will not be based on the DNS domain of the LDAP
      * server. naming_context might be NULL even after connection to an LDAP
      * server. */
+    char *fspdn; /* Foreign security principal search base */
     char *naming_context;
 
     struct sdap_search_base **search_bases;
diff --git a/src/providers/ldap/sdap_async_nested_groups.c b/src/providers/ldap/sdap_async_nested_groups.c
index ef995697dea..9b4ab31c9ae 100644
--- a/src/providers/ldap/sdap_async_nested_groups.c
+++ b/src/providers/ldap/sdap_async_nested_groups.c
@@ -531,34 +531,50 @@ static bool
 sdap_nested_member_is_fsp(struct sdap_nested_group_ctx *group_ctx,
                             const char *dn)
 {
-    char *fspdn, *basedn;
-    int  fspdn_len, dn_len, len_diff;
-    int  reti;
-    bool ret = false;
-
-    reti = domain_to_basedn(group_ctx, group_ctx->domain->realm, &basedn);
-    if (reti != EOK) {
-        DEBUG(SSSDBG_CRIT_FAILURE, "Unable to obtain basedn\n");
-        return false;
-    }
+    char    *fspdn;
+    size_t  fspdn_len, dn_len;
+    int     reti, len_diff;
+    bool    ret = false;
+
+    if (group_ctx->opts->sdom->fspdn == NULL) {
+        char *basedn;
+
+        reti = domain_to_basedn(group_ctx, group_ctx->domain->realm, &basedn);
+        if (reti != EOK) {
+            DEBUG(SSSDBG_CRIT_FAILURE, "Unable to obtain basedn\n");
+            return false;
+        }
 
-    fspdn = talloc_asprintf(group_ctx, "%s,%s",
-                            "CN=ForeignSecurityPrincipals", basedn);
-    if (fspdn == NULL) {
-        DEBUG(SSSDBG_CRIT_FAILURE, "Unable to run talloc_asprintf\n");
-        return false;
+        fspdn = talloc_asprintf(group_ctx->opts->sdom, "%s,%s",
+                                "CN=ForeignSecurityPrincipals", basedn);
+        if (fspdn == NULL) {
+            DEBUG(SSSDBG_CRIT_FAILURE, "Unable to run talloc_asprintf\n");
+            return false;
+        }
+        talloc_free(basedn);
+        group_ctx->opts->sdom->fspdn = fspdn;
+    } else {
+        fspdn = group_ctx->opts->sdom->fspdn;
     }
 
-    talloc_free(basedn);
     fspdn_len = strlen(fspdn);
     dn_len = strlen(dn);
     len_diff = dn_len - fspdn_len;
-    if (len_diff < 0) {
-       talloc_free(fspdn);
+    if (len_diff < sizeof("CN=S-")) {
        return false;
     }
-    ret = strncasecmp(&dn[len_diff], fspdn, fspdn_len) == 0;
-    talloc_free(fspdn);
+    ret = (strncasecmp(&dn[len_diff], fspdn, fspdn_len) == 0);
+
+    if (ret) { /* looks like FSP, so just double check to be 100% sure */
+       char *fsp_str = talloc_strdup(group_ctx, dn);
+
+       if (fsp_str == NULL)
+          return false;
+       fsp_str[len_diff - 1] = '\0';  /* replace comma with NULL */
+       ret = is_principal_sid(&fsp_str[3]);
+       talloc_free(fsp_str);
+    }
+
     return ret;
 }