Difference between revisions of "Unsafe command processing"

Latest revision as of 01:02, 12 May 2013

It is never safe or secure to process user input into system commands. The system() and exec() functions are mostly universal between programming languages along with the backtick (`) characters for command substitution. In some languages, the $() operator for command substitution is also valid.

Proof of concept

 >>> os.system("echo %s" % input("# "))

$ unsafe; ping -c1 google.com
PING google.com (74.125.224.168) 56(84) bytes of data.
64 bytes from lax02s01-in-f8.1e100.net (74.125.224.168): icmp_req=1 ttl=51 time=13.1 ms

Mitigation

Instead of using system, backticks, popen, exec or any other command executing function, use the language's native built in library. If it does not have one, then write one - but do not simply wrap system() and call this a library; this is bad form. All interpreted languages have a way to load shared libraries (*.so files) and interface with the functions provided by their export tables. These are what should be utilized when authoring libraries that seemingly need you to run a command. Instead, the C interface (CTypes) can be used.

Ctype or native examples:

• Python:

>>> from ctypes import *; cdll.LoadLibrary("libc.so.6").printf(c_char_p("abcdefgh\n")); # import printf() from libc.so.6
'abcdefgh'

• Perl:

• PHP:

• Ruby:

Auditing

Auditing command processing is simple: check for all uses of system(), exec(), backticks, popen, and any language specific function.

find -type f -regextype posix-awk -regex ".*\.(rb|php|py|pl|pm)"  -exec grep -EHnC2 "system\(\|[pP]open\(\|\`\|exec.*\(\|passthru\(" '{}' \;