diff --git a/ChangeLog b/ChangeLog
index fc56787c32..1e7382d71b 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,7 @@
+Thu Oct 15 00:26:07 2009  Tanaka Akira  <akr@fsij.org>
+
+	* ext/socket/init.c (rsock_init_sock): validate file descriptor.
+
 Wed Oct 14 13:24:14 2009  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* ruby.c (process_options): script name should not be shown in an
diff --git a/ext/socket/init.c b/ext/socket/init.c
index ed85e0b503..85cc3de4f9 100644
--- a/ext/socket/init.c
+++ b/ext/socket/init.c
@@ -43,6 +43,12 @@ VALUE
 rsock_init_sock(VALUE sock, int fd)
 {
     rb_io_t *fp;
+    struct stat sbuf;
+
+    if (fstat(fd, &sbuf) < 0)
+        rb_sys_fail(0);
+    if (!S_ISSOCK(sbuf.st_mode))
+        rb_raise(rb_eArgError, "not a socket file descriptor");
 
     MakeOpenFile(sock, fp);
     fp->fd = fd;