diff --git a/ChangeLog b/ChangeLog
index c28b553cb2..4b036bc997 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Wed Mar 28 23:37:18 2018  Nobuyoshi Nakada  <nobu@ruby-lang.org>
+
+	pack.c: fix underflow
+
+	* pack.c (pack_unpack_internal): get rid of underflow.
+	  https://hackerone.com/reports/298246
+
 Wed Mar 28 23:35:28 2018  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	unixsocket.c: check NUL bytes
diff --git a/pack.c b/pack.c
index a5982c9c0f..2b51d83923 100644
--- a/pack.c
+++ b/pack.c
@@ -1203,7 +1203,7 @@ pack_unpack(VALUE str, VALUE fmt)
 	else if (ISDIGIT(*p)) {
 	    errno = 0;
 	    len = STRTOUL(p, (char**)&p, 10);
-	    if (errno) {
+	    if (len < 0 || errno) {
 		rb_raise(rb_eRangeError, "pack length too big");
 	    }
 	}
diff --git a/test/ruby/test_pack.rb b/test/ruby/test_pack.rb
index 18f71e4a2f..449911198c 100644
--- a/test/ruby/test_pack.rb
+++ b/test/ruby/test_pack.rb
@@ -480,6 +480,9 @@ class TestPack < Test::Unit::TestCase
     assert_equal([1, 2], "\x01\x00\x00\x02".unpack("C@3C"))
     assert_equal([nil], "\x00".unpack("@1C")) # is it OK?
     assert_raise(ArgumentError) { "\x00".unpack("@2C") }
+
+    pos = (1 << [nil].pack("p").bytesize * 8) - 100 # -100
+    assert_raise(RangeError) {"0123456789".unpack("@#{pos}C10")}
   end
 
   def test_pack_unpack_percent
diff --git a/version.h b/version.h
index 72af22fbaf..a5b8c99a98 100644
--- a/version.h
+++ b/version.h
@@ -1,6 +1,6 @@
 #define RUBY_VERSION "2.2.10"
 #define RUBY_RELEASE_DATE "2018-03-28"
-#define RUBY_PATCHLEVEL 485
+#define RUBY_PATCHLEVEL 486
 
 #define RUBY_RELEASE_YEAR 2018
 #define RUBY_RELEASE_MONTH 3