diff --git a/SECURITY.md b/SECURITY.md
index 03e436819..2f0ae5b28 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -3,3 +3,16 @@
 ## Reporting a Vulnerability
 
 If you have any security concern, contact <matz@ruby.or.jp>.
+
+## Scope
+
+We consider following issues as vulnerabilities:
+
+* Remote code execution
+* Crash caused by a valid Ruby script
+
+We *don't* consider following issues as vulnerabilities:
+
+* Runtime C undefined behavior (including integer overflow)
+* Crash caused by misused API
+* Crash caused by tweaked compiled binary