diff --git a/ChangeLog b/ChangeLog
index 07002c5c10..6e0e2ac4b5 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,16 @@
+Fri Feb 18 20:02:29 2011  Shugo Maeda  <shugo@ruby-lang.org>
+
+	* test/ruby/test_exception.rb (TestException::test_to_s_taintness_propagation):
+	  Test for below.
+
+Fri Feb 18 19:58:34 2011  URABE Shyouhei  <shyouhei@ruby-lang.org>
+
+	* error.c (exc_to_s): untainted strings can be tainted via
+	  Exception#to_s, which enables attackers to overwrite sane strings.
+	  Reported by: Yusuke Endoh <mame at tsg.ne.jp>.
+
+	* error.c (name_err_to_s): ditto.
+
 Wed Jan 19 17:38:03 2011  NAKAMURA Usaku  <usa@ruby-lang.org>
 
 	* win32/win32.c (init_stdhandle): backport mistake of r29382.
diff --git a/error.c b/error.c
index a0186182f6..b55a68daca 100644
--- a/error.c
+++ b/error.c
@@ -403,7 +403,6 @@ exc_to_s(exc)
     VALUE mesg = rb_attr_get(exc, rb_intern("mesg"));
 
     if (NIL_P(mesg)) return rb_class_name(CLASS_OF(exc));
-    if (OBJ_TAINTED(exc)) OBJ_TAINT(mesg);
     return mesg;
 }
 
@@ -667,10 +666,9 @@ name_err_to_s(exc)
     if (NIL_P(mesg)) return rb_class_name(CLASS_OF(exc));
     StringValue(str);
     if (str != mesg) {
-	rb_iv_set(exc, "mesg", mesg = str);
+	OBJ_INFECT(str, mesg);
     }
-    if (OBJ_TAINTED(exc)) OBJ_TAINT(mesg);
-    return mesg;
+    return str;
 }
 
 /*
diff --git a/test/ruby/test_exception.rb b/test/ruby/test_exception.rb
index 4c27c52f3c..c5f409117c 100644
--- a/test/ruby/test_exception.rb
+++ b/test/ruby/test_exception.rb
@@ -184,4 +184,26 @@ class TestException < Test::Unit::TestCase
       assert(false)
     end
   end
+
+  def test_to_s_taintness_propagation
+    for exc in [Exception, NameError]
+      m = "abcdefg"
+      e = exc.new(m)
+      e.taint
+      s = e.to_s
+      assert_equal(false, m.tainted?,
+                   "#{exc}#to_s should not propagate taintness")
+      assert_equal(false, s.tainted?,
+                   "#{exc}#to_s should not propagate taintness")
+    end
+    
+    o = Object.new
+    def o.to_str
+      "foo"
+    end
+    o.taint
+    e = NameError.new(o)
+    s = e.to_s
+    assert_equal(true, s.tainted?)
+  end
 end