[CVE-2015-20107] mailcap.findmatch: document shell command Injection danger in filename parameter

[TheRegRunner]

BPO	24778
Nosy	@vstinner, @bitdancer
Files	screenshot.png The Quote Problem.py mailcap patch.zip: mailcap.py patches and diffs for python2.7 and python 3.5

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2015-08-02.08:25:07.171>
labels = ['type-security', '3.11', 'library', 'docs']
title = 'mailcap.findmatch: document shell command Injection danger in filename parameter'
updated_at = <Date 2022-04-06.15:30:37.106>
user = 'https://bugs.python.org/TheRegRunner'

bugs.python.org fields:

activity = <Date 2022-04-06.15:30:37.106>
actor = 'vstinner'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation', 'Library (Lib)']
creation = <Date 2015-08-02.08:25:07.171>
creator = 'TheRegRunner'
dependencies = []
files = ['40099', '40116', '40897']
hgrepos = []
issue_num = 24778
keywords = []
message_count = 14.0
messages = ['247857', '247861', '247944', '247946', '247951', '247979', '247992', '248058', '248061', '248062', '248070', '248074', '253689', '416878']
nosy_count = 4.0
nosy_names = ['vstinner', 'r.david.murray', 'docs@python', 'TheRegRunner']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'security'
url = 'https://bugs.python.org/issue24778'
versions = ['Python 3.11']

Linked PRs

• [3.9] gh-68966: fix versionchanged in docs #105298
• [3.8] gh-68966: fix versionchanged in docs #105299
• [3.7] gh-68966: fix versionchanged in docs #105300

[TheRegRunner]

if the filename contains Shell Commands they will be executed if they
are passed to os.system() as discribed in the docs.
Filename should be quoted with quote(filename) to fix the bug.

https://docs.python.org/2/library/mailcap.html

"mailcap.findmatch(/caps/, /MIMEtype/[, /key/[, /filename/[, /plist/]]])

Return a 2-tuple; the first element is a string containing the
command line to be executed
(which can be passed to\*os.system() \*),

......"

Exploid Demo wich runs xterm but should not :
=============================

import mailcap
d=mailcap.getcaps()
commandline,MIMEtype=mailcap.findmatch(d, "text/*", filename="'$(xterm);#.txt")
## commandline = "less ''$(xterm);#.txt'"
import os
os.system(commandline)
## xterm starts

=============================

By the way ... please do not use os.system() in your code, makes it unsafe.

Best regards
Bernd Dietzel
Germany

[TheRegRunner]

Maybe it would be a good idea to do so as run-mailcap does :

theregrunner@mint17 : ~ € run-mailcap --debug "';xterm;#'.txt"

• parsing parameter "';xterm;#'.txt"
• Reading mime.types file "/etc/mime.types"...
• extension "txt" maps to mime-type "text/plain"
• Reading mailcap file "/etc/mailcap"...
  Processing file "';xterm;#'.txt" of type "text/plain" (encoding=none)...
• checking mailcap entry "text/plain; less '%s'; needsterminal"
• program to execute: less '%s'
• filename contains shell meta-characters; aliased to '/tmp/fileV7f2MZ'
• executing: less '/tmp/fileV7f2MZ'
  theregrunner@mint17 : ~ €

[bitdancer]

In this case os.system is an appropriate API, because it mirrors the API of mailcap itself (that is, mailcap entries are shell commands).

I'm not convinced there is a security bug here. It seems to me that there are two cases: either the filename is determined by the program, in which case there is no security issue, or the filename comes from an external source, and the program will have had to *write it to the file system* before the mailcap command will do anything. So the security hole, if any, will have happened earlier in the process.

Now, one can argue that the quoting should be done in order to preserve the meaning of an arbitrary filename. Which would allay your concern even if I disagree that it is a real security bug :)

(I don't understand why run-mailcap uses an alias rather than correctly quoting the meta-characters.)

[TheRegRunner]

@david
Thanks for the comment :-)

I think if you read the Documentation
https://docs.python.org/2/library/mailcap.html
this may lead new programmers, wich may never heard of Shell Injections before, step by step directly to write insecure webbbrowsers and/or mail readers. At least there should be a warning in the docs !

You ask why run-mailcap do not use quotig, i believe because quoting is not an easy thing to do, i attached a demo ;-)

Thank you.

[TheRegRunner]

Exploid Demo wich works with quote() :

>>> commandline,MIMETYPE=mailcap.findmatch(d, 'text/*', filename=quote(';xterm;#.txt'))
>>> commandline
"less '';xterm;#.txt''"
>>> os.system(commandline)
### xterm starts

[bitdancer]

Hmm. I see. The problem is that our desire to quote conflicts with mailcap's attempts to quote.

I now agree with you that run-mailcap's approach is correct, but creating a temporary alias is out of scope for findmatch. That would need to be done by findmatch's caller.

I think we should add a documentation note about the problem and the solution. I don't see any reliable way to detect the problem and raise an error for the same reason that quoting doesn't work. (The aliasing can tolerate false positives; but, for backward compatibility reasons, an error detection function here cannot.)

It would be possible to add a helper for the aliasing to 3.6, but if someone wants to propose that they should open an new issue for the enhancement.

I'm

[TheRegRunner]

Yes changing the docs is a good idea.

I was thinking about a patch :

import os
####### patch
import random
try:
 from shlex import quote
except ImportError:
 from pipes import quote
#######

....... and so on ....

# Part 3: using the database.

def findmatch(caps, MIMEtype, key='view', filename="/dev/null", plist=[]):
 """Find a match for a mailcap entry.

Return a tuple containing the command line, and the mailcap entry
used; (None, None) if no match is found. This may invoke the
'test' command of several matching entries before deciding which
entry to use.

"""

 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 and os.system(test) != 0:
 continue
####### patch
 ps=''.join(random.choice('python') for i in range(100))
 x=e[key]
 while '%s' in x:
 x=x.replace('%s',ps) 
 command=subst(x, MIMEtype, filename, plist)
 while "'"+ps+"'" in command:
 command=command.replace("'"+ps+"'",quote(filename))
 while ps in command:
 command=command.replace(ps,quote(filename)) 
###### command = subst(e[key], MIMEtype, filename, plist)
 return command, e
 return None, None

[TheRegRunner]

# for the docs ... quoting of the filename when you call mailcap.findmatch()

f=";xterm;#.txt" # Shell Command Demo ... xterm will run if quote() fails

import mailcap
import random
try:
 from shlex import quote
except ImportError:
 from pipes import quote
d=mailcap.getcaps()
PY=''.join(random.choice('PYTHON') for i in range(100))
cmd,MIMEtype=mailcap.findmatch(d, 'text/plain', filename=PY)
while "'"+PY+"'" in cmd:
 cmd=cmd.replace("'"+PY+"'",quote(f))
while PY in cmd:
 cmd=cmd.replace(PY,quote(f))
print(cmd) 
# less ';xterm;#.txt'

[bitdancer]

I have no idea what your code samples are trying to accomplish, I'm afraid, but that's not the kind of documentation I'm advocating anyway.

[TheRegRunner]

What i do is the last doc is like this :

1. Replace the filename with a random name
2. Run mailcap.findmatch() with the random name
3. If exists, replace the quote characters ' before and behind the random name with nothing.
4. Now the random name has no quoting from mailcap itself
5. So now we can use our own quote() savely