diff --git a/src/rp_kernel_on_cpu.c b/src/rp_kernel_on_cpu.c
index 4b1ddd356..dbc42d2f5 100644
--- a/src/rp_kernel_on_cpu.c
+++ b/src/rp_kernel_on_cpu.c
@@ -152,6 +152,8 @@ static int mangle_prepend (MAYBE_UNUSED const u8 p0, MAYBE_UNUSED const u8 p1, u
 {
   const int out_len = len + 1;
 
+  if (out_len >= RP_PASSWORD_SIZE) return (len);
+
   for (int pos = len - 1; pos >= 0; pos--)
   {
     buf[pos + 1] = buf[pos];