Cherry picked from upstream CPython commit b9509ba

diff --git a/Lib/mailcap.py b/Lib/mailcap.py
index bd0fc0981c..dcd4b449e8 100644
--- a/Lib/mailcap.py
+++ b/Lib/mailcap.py
@@ -2,6 +2,7 @@

 import os
 import warnings
+import re

 __all__ = ["getcaps","findmatch"]

@@ -13,6 +14,11 @@ def lineno_sort_key(entry):
     else:
         return 1, 0

+_find_unsafe = re.compile(r'[^\xa1-\U0010FFFF\w@+=:,./-]').search
+
+class UnsafeMailcapInput(Warning):
+    """Warning raised when refusing unsafe input"""
+

 # Part 1: top-level interface.

@@ -165,15 +171,22 @@ def findmatch(caps, MIMEtype, key='view', filename="/dev/null", plist=[]):
     entry to use.

     """
+    if _find_unsafe(filename):
+        msg = "Refusing to use mailcap with filename %r. Use a safe temporary filename." % (filename,)
+        warnings.warn(msg, UnsafeMailcapInput)
+        return None, None
     entries = lookup(caps, MIMEtype, key)
     # XXX This code should somehow check for the needsterminal flag.
     for e in entries:
         if 'test' in e:
             test = subst(e['test'], filename, plist)
+            if test is None:
+                continue
             if test and os.system(test) != 0:
                 continue
         command = subst(e[key], MIMEtype, filename, plist)
-        return command, e
+        if command is not None:
+            return command, e
     return None, None

 def lookup(caps, MIMEtype, key=None):
@@ -206,6 +219,10 @@ def subst(field, MIMEtype, filename, plist=[]):
             elif c == 's':
                 res = res + filename
             elif c == 't':
+                if _find_unsafe(MIMEtype):
+                    msg = "Refusing to substitute MIME type %r into a shell command." % (MIMEtype,)
+                    warnings.warn(msg, UnsafeMailcapInput)
+                    return None
                 res = res + MIMEtype
             elif c == '{':
                 start = i
@@ -213,7 +230,12 @@ def subst(field, MIMEtype, filename, plist=[]):
                     i = i+1
                 name = field[start:i]
                 i = i+1
-                res = res + findparam(name, plist)
+                param = findparam(name, plist)
+                if _find_unsafe(param):
+                    msg = "Refusing to substitute parameter %r (%s) into a shell command" % (param, name)
+                    warnings.warn(msg, UnsafeMailcapInput)
+                    return None
+                res = res + param
             # XXX To do:
             # %n == number of parts if type is multipart/*
             # %F == list of alternating type and filename for parts
diff --git a/Lib/test/test_mailcap.py b/Lib/test/test_mailcap.py
index c08423c670..920283d9a2 100644
--- a/Lib/test/test_mailcap.py
+++ b/Lib/test/test_mailcap.py
@@ -121,7 +121,8 @@ class HelperFunctionTest(unittest.TestCase):
             (["", "audio/*", "foo.txt"], ""),
             (["echo foo", "audio/*", "foo.txt"], "echo foo"),
             (["echo %s", "audio/*", "foo.txt"], "echo foo.txt"),
-            (["echo %t", "audio/*", "foo.txt"], "echo audio/*"),
+            (["echo %t", "audio/*", "foo.txt"], None),
+            (["echo %t", "audio/wav", "foo.txt"], "echo audio/wav"),
             (["echo \\%t", "audio/*", "foo.txt"], "echo %t"),
             (["echo foo", "audio/*", "foo.txt", plist], "echo foo"),
             (["echo %{total}", "audio/*", "foo.txt", plist], "echo 3")
@@ -205,7 +206,10 @@ class FindmatchTest(unittest.TestCase):
              ('"An audio fragment"', audio_basic_entry)),
             ([c, "audio/*"],
              {"filename": fname},
-             ("/usr/local/bin/showaudio audio/*", audio_entry)),
+             (None, None)),
+            ([c, "audio/wav"],
+             {"filename": fname},
+             ("/usr/local/bin/showaudio audio/wav", audio_entry)),
             ([c, "message/external-body"],
              {"plist": plist},
              ("showexternal /dev/null default john python.org     /tmp foo bar", message_entry))
