Merged revisions 87873 via svnmerge from

svn+ssh://pythondev@svn.python.org/python/branches/py3k

........
  r87873 | r.david.murray | 2011-01-08 21:35:24 -0500 (Sat, 08 Jan 2011) | 12 lines

  #5871: protect against header injection attacks.

  This makes Header.encode throw a HeaderParseError if it winds up
  formatting a header such that a continuation line has no leading
  whitespace and looks like a header.  Since Header accepts values
  containing newlines and preserves them (and this is by design), without
  this fix any program that took user input (say, a subject in a web form)
  and passed it to the email package as a header was vulnerable to header
  injection attacks.  (As far as we know this has never been exploited.)

  Thanks to Jakub Wilk for reporting this vulnerability.
........

Lib/email/header.py

@@ -46,6 +46,10 @@ ecre = re.compile(r'''
 # For use with .match()
 fcre = re.compile(r'[\041-\176]+:$')
 
+# Find a header embeded in a putative header value.  Used to check for
+# header injection attack.
+_embeded_header = re.compile(r'\n[^ \t]+:')
+
 
 
 # Helpers
@@ -305,7 +309,11 @@ class Header:
                 if len(lines) > 1:
                     formatter.newline()
             formatter.add_transition()
-        return str(formatter)
+        value = str(formatter)
+        if _embeded_header.search(value):
+            raise HeaderParseError("header value appears to contain "
+                "an embedded header: {!r}".format(value))
+        return value
 
     def _normalize(self):
         # Step 1: Normalize the chunks so that all runs of identical charsets