1 files changed, 3 insertions, 0 deletions
diff --git a/string.c b/string.c
index 1e4d867f9d..049b088a0d 100644
--- a/string.c
+++ b/string.c
@@ -1914,6 +1914,9 @@ rb_str_modify_expand(VALUE str, long expand)
     else if (expand > 0) {
 	long len = RSTRING_LEN(str);
 	long capa = len + expand;
+	if (expand >= LONG_MAX - len - termlen) {
+	    rb_raise(rb_eArgError, "string size too big");
+	}
 	if (!STR_EMBED_P(str)) {
 	    REALLOC_N(RSTRING(str)->as.heap.ptr, char, capa + termlen);
 	    RSTRING(str)->as.heap.aux.capa = capa;