diff --git a/Lib/imaplib.py b/Lib/imaplib.py
index df35cb1..f803a5c 100644
--- a/Lib/imaplib.py
+++ b/Lib/imaplib.py
@@ -1023,6 +1023,8 @@
             raise self.abort('socket error: EOF')
 
         # Protocol mandates all lines terminated by CRLF
+        if not line.endswith(b'\r\n'):
+            raise self.abort('socket error: unterminated line')
 
         line = line[:-2]
         if __debug__: