diff --git a/usr.sbin/mountd/mountd.c b/usr.sbin/mountd/mountd.c
--- a/usr.sbin/mountd/mountd.c
+++ b/usr.sbin/mountd/mountd.c
@@ -3613,13 +3613,15 @@
 	char *names;
 	struct passwd *pw;
 	struct group *gr;
-	gid_t groups[NGROUPS_MAX + 1];
-	int ngroups;
+	long ngroups_max;
+	int alloc_ngroups;
 	unsigned long name_ul;
 	char *end = NULL;
 
 	/* Start with 'cr_smallgrps' embedded storage. */
 	cr->cr_groups = cr->cr_smallgrps;
+	alloc_ngroups = SMALLNGROUPS;
+	_Static_assert(SMALLNGROUPS > 0, "SMALLNGROUPS must be > 0");
 
 	/*
 	 * Get the user's password table entry.
@@ -3640,25 +3642,52 @@
 		cr->cr_uid = name_ul;
 	}
 
+	/* How many groups do we support? */
+	ngroups_max = sysconf(_SC_NGROUPS_MAX);
+	if (ngroups_max == -1)
+		ngroups_max = NGROUPS_MAX;
+	/* Add space for the effective GID. */
+	++ngroups_max;
+
 	/*
 	 * Credentials specified as those of a user.
 	 */
 	if (names == NULL) {
+		u_int retries = 5;
+
 		if (pw == NULL) {
 			syslog(LOG_ERR, "unknown user: %s", name);
 			goto fallback;
 		}
 
-		ngroups = NGROUPS_MAX + 1;
-		if (getgrouplist(pw->pw_name, pw->pw_gid, groups, &ngroups)) {
-			syslog(LOG_ERR, "too many groups");
-			ngroups = NGROUPS_MAX + 1;
+		for (;;) {
+			if (retries == 0) {
+				syslog(LOG_ERR, "can't get stable group list "
+				    "for user: %s", name);
+				if (cr->cr_groups != cr->cr_smallgrps)
+					free(cr->cr_groups);
+				goto fallback;
+			}
+			--retries;
+			cr->cr_ngroups = alloc_ngroups;
+			if (getgrouplist(pw->pw_name, pw->pw_gid,
+			    cr->cr_groups, &cr->cr_ngroups) == 0)
+				break;
+			if (cr->cr_ngroups > ngroups_max) {
+				syslog(LOG_ERR, "too many groups");
+				if (alloc_ngroups == ngroups_max) {
+					cr->cr_ngroups = ngroups_max;
+					break;
+				}
+				alloc_ngroups = ngroups_max;
+			} else
+				alloc_ngroups = cr->cr_ngroups;
+			cr->cr_groups = realloc(
+			    cr->cr_groups == cr->cr_smallgrps ? NULL :
+			    cr->cr_groups, alloc_ngroups);
+			if (cr->cr_groups == NULL)
+				out_of_mem();
 		}
-
-		if (ngroups > SMALLNGROUPS)
-			cr->cr_groups = malloc(ngroups * sizeof(gid_t));
-		cr->cr_ngroups = ngroups;
-		memcpy(cr->cr_groups, groups, ngroups * sizeof(gid_t));
 		return;
 	}
 
@@ -3681,11 +3710,31 @@
 		} else {
 			group = name_ul;
 		}
-		if (cr->cr_ngroups == NGROUPS_MAX) {
+		if (cr->cr_ngroups == ngroups_max) {
 			syslog(LOG_ERR, "too many groups");
 			break;
 		}
-		groups[cr->cr_ngroups++] = group;
+		++cr->cr_ngroups;
+		/* Realloc if necessary. */
+		if (cr->cr_ngroups > alloc_ngroups) {
+			if (cr->cr_groups == cr->cr_smallgrps) {
+				alloc_ngroups = 2 * SMALLNGROUPS;
+				cr->cr_groups = malloc(alloc_ngroups);
+				if (cr->cr_groups == NULL)
+					out_of_mem();
+				memcpy(cr->cr_groups, cr->cr_smallgrps,
+				    sizeof(cr->cr_smallgrps));
+			} else {
+				alloc_ngroups = alloc_ngroups >= PAGE_SIZE ?
+				    alloc_ngroups + PAGE_SIZE :
+				    2 * alloc_ngroups;
+				cr->cr_groups = realloc(cr->cr_groups,
+				    alloc_ngroups);
+				if (cr->cr_groups == NULL)
+					out_of_mem();
+			}
+		}
+		cr->cr_groups[cr->cr_ngroups - 1] = group;
 	}
 	if (cr->cr_ngroups == 0) {
 		/*
@@ -3696,7 +3745,7 @@
 		 */
 		char *buf = NULL;
 		/* Arbitrary initial size, should cover most practical cases. */
-		size_t buf_size, next_size = 64;
+		size_t buf_size, next_size = 128;
 
 		for (;;) {
 			struct group *res, grp;
@@ -3731,9 +3780,6 @@
 		 */
 		cr->cr_ngroups = 1;
 	}
-	if (cr->cr_ngroups > SMALLNGROUPS)
-		cr->cr_groups = malloc(cr->cr_ngroups * sizeof(gid_t));
-	memcpy(cr->cr_groups, groups, cr->cr_ngroups * sizeof(gid_t));
 	return;
 
 fallback:
@@ -3741,6 +3787,7 @@
 	 * Set up the unprivileged user.
 	 */
 	cr->cr_uid = UID_NOBODY;
+	cr->cr_groups = cr->cr_smallgrps;
 	cr->cr_groups[0] = GID_NOGROUP;
 	cr->cr_ngroups = 1;
 }