Difference between revisions of "Unsafe command processing"
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| − | It is never safe or secure to process user input into system commands. The '''system()''' and '''exec()''' functions are mostly universal between | + | It is never safe or secure to process user input into system commands. The '''system()''' and '''exec()''' functions are mostly universal between [[programming language]]s along with the backtick ('''`''') characters for ''command substitution''. In some languages, the '''$()''' operator for command substitution is also valid. |
=== Proof of concept === | === Proof of concept === | ||
| Line 8: | Line 8: | ||
=== Mitigation === | === 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. | + | 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: | Ctype or native examples: | ||
| − | * Python: | + | * [[Python]]: |
{{code|text=<source lang="python">>>> from ctypes import *; cdll.LoadLibrary("libc.so.6").printf(c_char_p("abcdefgh\n")); # import printf() from libc.so.6 | {{code|text=<source lang="python">>>> from ctypes import *; cdll.LoadLibrary("libc.so.6").printf(c_char_p("abcdefgh\n")); # import printf() from libc.so.6 | ||
'abcdefgh'</source>}} | 'abcdefgh'</source>}} | ||
| − | * Perl: | + | * [[Perl]]: |
| − | * PHP: | + | * [[PHP]]: |
| − | * Ruby: | + | * [[Ruby]]: |
=== Auditing === | === Auditing === | ||
Auditing command processing is simple: check for all uses of system(), exec(), backticks, popen, and any language specific function. | Auditing command processing is simple: check for all uses of system(), exec(), backticks, popen, and any language specific function. | ||
| − | {{code|text=<source lang="bash">find -type f -regextype posix-awk -regex ".*\.(rb|php|py|pl|pm)" -exec grep -EHnC2 "system\(\|[pP]open\(\|\`\|exec.*\(\|passthru\(" '{}' \; | + | {{code|text=<source lang="bash">find -type f -regextype posix-awk -regex ".*\.(rb|php|py|pl|pm)" -exec grep -EHnC2 "system\(\|[pP]open\(\|\`\|exec.*\(\|passthru\(" '{}' \;</source>}} |
[[Category:Secure programming]] | [[Category:Secure programming]] | ||
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:
>>> 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\(" '{}' \; |
