diff --git a/mountd.c.orig b/mountd.c
--- a/mountd.c.orig
+++ b/mountd.c
@@ -3518,6 +3518,8 @@
 	struct group *gr;
 	gid_t groups[NGROUPS_MAX + 1];
 	int ngroups;
+	unsigned long name_ul;
+	char *end = NULL;
 
 	/*
 	 * Set up the unprivileged user.
@@ -3532,10 +3534,17 @@
 	names = namelist;
 	name = strsep_quote(&names, ":");
 	/* Bug?  name could be NULL here */
-	if (isdigit(*name) || *name == '-')
-		pw = getpwuid(atoi(name));
-	else
+	name_ul = strtoul(name, &end, 10);
+	if (end == NULL || *end != '\0' || end == name) {
 		pw = getpwnam(name);
+	} else {
+		pw = getpwuid((uid_t)name_ul);
+	}
+	if (pw == NULL) {
+		syslog(LOG_ERR, "unknown user: %s", name);
+		return;
+	}
+
 	/*
 	 * Credentials specified as those of a user.
 	 */
@@ -3573,24 +3582,34 @@
 	 * Explicit credential specified as a colon separated list:
 	 *	uid:gid:gid:...
 	 */
-	if (pw != NULL)
+	if (pw != NULL) {
 		cr->cr_uid = pw->pw_uid;
-	else if (isdigit(*name) || *name == '-')
-		cr->cr_uid = atoi(name);
-	else {
-		syslog(LOG_ERR, "unknown user: %s", name);
-		return;
+	} else {
+		name_ul = strtoul(name, &end, 10);
+		if (end == NULL || *end != '\0' || end == name) {
+			pw = getpwnam(name);
+		} else {
+			pw = getpwuid((uid_t)name_ul);
+		}
+		if (pw == NULL) {
+			syslog(LOG_ERR, "unknown user: %s", name);
+			return;
+		}
 	}
+
 	cr->cr_ngroups = 0;
 	while (names != NULL && *names != '\0' && cr->cr_ngroups < NGROUPS_MAX) {
 		name = strsep_quote(&names, ":");
-		if (isdigit(*name) || *name == '-') {
-			groups[cr->cr_ngroups++] = atoi(name);
+		name_ul = strtoul(name, &end, 10);
+		if (end == NULL || *end != '\0' || end == name) {
+			gr = getgrnam(name);
 		} else {
-			if ((gr = getgrnam(name)) == NULL) {
-				syslog(LOG_ERR, "unknown group: %s", name);
-				continue;
-			}
+			gr = getgrgid((gid_t)name_ul);
+		}
+		if (gr == NULL) {
+			syslog(LOG_ERR, "unknown group: %s", name);
+			continue;
+		} else {
 			groups[cr->cr_ngroups++] = gr->gr_gid;
 		}
 	}