diff --git a/Src/StdLib/Lib/mailcap.py b/Src/StdLib/Lib/mailcap.py
index 97e303522..4cbda7ac0 100644
--- a/Src/StdLib/Lib/mailcap.py
+++ b/Src/StdLib/Lib/mailcap.py
@@ -1,9 +1,18 @@
 """Mailcap file handling.  See RFC 1524."""
 
 import os
+import warnings
+import re
 
 __all__ = ["getcaps","findmatch"]
 
+# https://github.com/IronLanguages/ironpython3/issues/1274
+# _find_unsafe = re.compile(r'[^\xa1-\U0010FFFF\w@+=:,./-]').search
+_find_unsafe = re.compile(r'[^\w@+=:,./-]').search
+
+class UnsafeMailcapInput(Warning):
+    """Warning raised when refusing unsafe input"""
+
 # Part 1: top-level interface.
 
 def getcaps():
@@ -144,15 +153,22 @@ 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):
@@ -184,6 +200,10 @@ 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
@@ -191,7 +211,12 @@ 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/Src/StdLib/Lib/test/test_mailcap.py b/Src/StdLib/Lib/test/test_mailcap.py
index a4cd09c70..3e774a385 100644
--- a/Src/StdLib/Lib/test/test_mailcap.py
+++ b/Src/StdLib/Lib/test/test_mailcap.py
@@ -101,8 +101,9 @@ def test_subst(self):
             (["", "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"], "echo %t"),
+            (["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")
         ]
@@ -183,7 +184,10 @@ def test_findmatch(self):
              ('"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))