[3.10] gh-68966: Make mailcap refuse to match unsafe filenames/types/params (GH-91993) (GH-93543)

* gh-68966: Make mailcap refuse to match unsafe filenames/types/params (GH-91993)
(cherry picked from commit b9509ba7a9)
* Add a What's New entry for 3.10.8.

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

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