diff --git a/shadowsocks/obfsplugin/auth.py b/shadowsocks/obfsplugin/auth.py
index b787b33..943caa1 100755
--- a/shadowsocks/obfsplugin/auth.py
+++ b/shadowsocks/obfsplugin/auth.py
@@ -1319,12 +1319,12 @@ class auth_aes128_sha1(auth_base):
         sendback = False
 
         if not self.has_recv_header:
-            if len(self.recv_buf) < 7:
-                return (b'', False)
-            mac_key = self.server_info.recv_iv + self.server_info.key
-            sha1data = hmac.new(mac_key, self.recv_buf[:1], self.hashfunc).digest()[:6]
-            if sha1data != self.recv_buf[1:7]:
-                return self.not_match_return(self.recv_buf)
+            if len(self.recv_buf) >= 7 or len(self.recv_buf) in [2, 3]:
+                recv_len = min(len(self.recv_buf), 7)
+                mac_key = self.server_info.recv_iv + self.server_info.key
+                sha1data = hmac.new(mac_key, self.recv_buf[:1], self.hashfunc).digest()[:recv_len - 1]
+                if sha1data != self.recv_buf[1:recv_len]:
+                    return self.not_match_return(self.recv_buf)
 
             if len(self.recv_buf) < 31:
                 return (b'', False)
diff --git a/shadowsocks/obfsplugin/http_simple.py b/shadowsocks/obfsplugin/http_simple.py
index 6549f77..19e2a75 100644
--- a/shadowsocks/obfsplugin/http_simple.py
+++ b/shadowsocks/obfsplugin/http_simple.py
@@ -168,6 +168,11 @@ class http_simple(plain.plain):
             return (b'E'*2048, False, False)
         return (buf, True, False)
 
+    def error_return(self, buf):
+        self.has_sent_header = True
+        self.has_recv_header = True
+        return (b'E'*2048, False, False)
+
     def server_decode(self, buf):
         if self.has_recv_header:
             return (buf, True, False)
@@ -199,10 +204,10 @@ class http_simple(plain.plain):
                 if host not in hosts:
                     return self.not_match_return(buf)
             if len(ret_buf) < 4:
-                return self.not_match_return(buf)
+                return self.error_return(buf)
             if len(datas) > 1:
                 ret_buf += datas[1]
-            if len(ret_buf) >= 7:
+            if len(ret_buf) >= 13:
                 self.has_recv_header = True
                 return (ret_buf, True, False)
             return self.not_match_return(buf)