Differences between revisions 13 and 14
Revision 13 as of 2012-02-07 02:50:47
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Editor: dethrophes
Comment: cleaned up the examples a bit more.
Revision 14 as of 2012-02-07 04:26:56
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Editor: dethrophes
Comment: Better way to check valid variable name.
Deletions are marked like this. Additions are marked like this.
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    #[[ ${1:?Missing Variable Name} =~ ^[a-zA-Z][a-zA-Z0-9_]*$ ]] || return 3
    [ "${1:?Missing Variable Name}" = "${1//[!a-zA-Z0-9_]}" ] || return 3
    #(unset "${1:?Missing Variable Name}" ) || return 3
    case "${1:?Missing Variable Name}" in
      [!a-zA-Z_]* | *[!a-zA-Z_0-9]* ) return 3;;
    esac

Eval command and security issues

The eval command is extremely powerful and extremely easy to abuse.

It causes your code to be parsed twice instead of once; this means that, for example, if your code has variable references in it, the shell's parser will evaluate the contents of that variable. If the variable contains a shell command, the shell might run that command, whether you wanted it to or not. This can lead to unexpected results, especially when variables can be read from untrusted sources (like users or user-created files).

Examples of bad use of eval

"eval" is a common misspelling of "evil". The section of this FAQ dealing with spaces in file names used to include the following quote "helpful tool (which is probably not as safe as the \0 technique)", end quote.

    Syntax : nasty_find_all <path> <command> [maxdepth]

    # This code is evil and must never be used!
    export IFS=" "
    [ -z "$3" ] && set -- "$1" "$2" 1
    FILES=`find "$1" -maxdepth "$3" -type f -printf "\"%p\" "`
    #warning, evilness
    eval FILES=($FILES)
    for ((I=0; I < ${#FILES[@]}; I++))
    do
        eval "$2 \"${FILES[I]}\""
    done
    unset IFS

This script was supposed to recursively search for files and run a user-specified command on them, even if they had newlines and/or spaces in their names. The author thought that find -print0 | xargs -0 was unsuitable for some purposes such as multiple commands. It was followed by an instructional description of all the lines involved, which we'll skip.

To its defense, it worked:

$ ls -lR
.:
total 8
drwxr-xr-x  2 vidar users 4096 Nov 12 21:51 dir with spaces
-rwxr-xr-x  1 vidar users  248 Nov 12 21:50 nasty_find_all

./dir with spaces:
total 0
-rw-r--r--  1 vidar users 0 Nov 12 21:51 file?with newlines
$ ./nasty_find_all . echo 3
./nasty_find_all
./dir with spaces/file
with newlines
$

But consider this:

$ touch "\"); ls -l $'\x2F'; #"

You just created a file called  "); ls -l $'\x2F'; #

Now FILES will contain  ""); ls -l $'\x2F'; #. When we do eval FILES=($FILES), it becomes

FILES=(""); ls -l $'\x2F'; #"

Which becomes the two statements  FILES=("");  and  ls -l / . Congratulations, you just allowed execution of arbitrary commands.

$ touch "\"); ls -l $'\x2F'; #"
$ ./nasty_find_all . echo 3
total 1052
-rw-r--r--   1 root root 1018530 Apr  6  2005 System.map
drwxr-xr-x   2 root root    4096 Oct 26 22:05 bin
drwxr-xr-x   3 root root    4096 Oct 26 22:05 boot
drwxr-xr-x  17 root root   29500 Nov 12 20:52 dev
drwxr-xr-x  68 root root    4096 Nov 12 20:54 etc
drwxr-xr-x   9 root root    4096 Oct  5 11:37 home
drwxr-xr-x  10 root root    4096 Oct 26 22:05 lib
drwxr-xr-x   2 root root    4096 Nov  4 00:14 lost+found
drwxr-xr-x   6 root root    4096 Nov  4 18:22 mnt
drwxr-xr-x  11 root root    4096 Oct 26 22:05 opt
dr-xr-xr-x  82 root root       0 Nov  4 00:41 proc
drwx------  26 root root    4096 Oct 26 22:05 root
drwxr-xr-x   2 root root    4096 Nov  4 00:34 sbin
drwxr-xr-x   9 root root       0 Nov  4 00:41 sys
drwxrwxrwt   8 root root    4096 Nov 12 21:55 tmp
drwxr-xr-x  15 root root    4096 Oct 26 22:05 usr
drwxr-xr-x  13 root root    4096 Oct 26 22:05 var
./nasty_find_all
./dir with spaces/file
with newlines
./
$

It doesn't take much imagination to replace  ls -l  with  rm -rf  or worse.

One might think these circumstances are obscure, but one should not be tricked by this. All it takes is one malicious user, or perhaps more likely, a benign user who left the terminal unlocked when going to the bathroom, or wrote a funny PHP uploading script that doesn't sanity check file names, or who made the same mistake as oneself in allowing arbitrary code execution (now instead of being limited to the www-user, an attacker can use nasty_find_all to traverse chroot jails and/or gain additional privileges), or uses an IRC or IM client that's too liberal in the filenames it accepts for file transfers or conversation logs, etc.

Examples of good use of eval

The most common correct use of eval is reading variables from the output of a program which is specifically designed to be used this way. For example,

# On older systems, one must run this after resizing a window:
eval `resize`

# Less primitive: get a passphrase for an SSH private key.
# This is typically executed from a .xsession or .profile type of file.
# The variables produced by ssh-agent will be exported to all the processes in
# the user's session, so that an eventual ssh will inherit them.
eval `ssh-agent -s`

eval has other uses especially when creating variables out of the blue (indirect variable references). Here is an example of one way to parse command line options that do not take parameters:

# POSIX
#
# Create option variables dynamically. Try call:
#
#    sh -x example.sh --verbose --test --debug

for i in "$@"
do
    case "$i" in
       --test|--verbose|--debug)
            shift                   # Remove option from command line
            name=${i#--}            # Delete option prefix
            eval "$name='$name'"    # make *new* variable
            ;;
    esac
done

echo "verbose: $verbose"
echo "test: $test"
echo "debug: $debug"

So, why is this version acceptable? It's acceptable because we have restricted the eval command so that it will only be executed when the input is one of a finite set of known values. Therefore, it can't ever be abused by the user to cause arbitrary command execution -- any input with funny stuff in it wouldn't match one of the three predetermined possible inputs. This variant would not be acceptable:

# Dangerous code.  Do not use this!
for i in "$@"
do
    case "$i" in
       --test*|--verbose*|--debug*)
            shift                   # Remove option from command line
            name=${i#--}            # Delete option prefix
            eval "$name='$name'"    # make *new* variable
            ;;
    esac
done

All that's changed is that we attempted to make the previous "good" example (which doesn't do very much) useful in some way, by letting it take things like --test=foo. But look at what this enables:

$ ./foo --test='; ls -l /etc/passwd;x='
-rw-r--r-- 1 root root 943 2007-03-28 12:03 /etc/passwd

Once again: by permitting the eval command to be used on unfiltered user input, we've permitted arbitrary command execution.

Alternatives to eval

  • Could this not be done better with declare? eg:

     for i in "$@"
     do
        case "$i" in
           --test|--verbose|--debug)
                shift                   # Remove option from command line
                name=${i#--}            # Delete option prefix
                declare $name=Yes       # set default value
                ;;
           --test=*|--verbose=*|--debug=*)
                shift
                name=${i#--}
                value=${name#*=}        # value is whatever's after first word and =
                name=${name%%=*}        # restrict name to first word only (even if there's another = in the value)
                declare $name="$value"  # make *new* variable
                ;;
        esac
     done

    Note that --name for a default, and --name=value are the required formats.

    declare does seem to have some sort of parser magic in it, much like [[ does. Here's a test I performed with bash 3.1.17:

     griffon:~$ declare foo=x;date;x=Yes
     Sun Nov  4 09:36:08 EST 2007
     
     griffon:~$ name='foo=x;date;x'
     griffon:~$ declare $name=Yes
     griffon:~$ echo $foo
     x;date;x=Yes

    It appears that, at least in bash, declare is much safer than eval.

For a list of ways to reference or to populate variables indirectly without using eval, please see FAQ #6. (This section was written before #6 was, but I've left it here as a reference.)

Robust eval usage

Another approach can be to encapsulate dangerous code in a function. So for example instead of doing something like this.

    eval "${ArrayName}"'="${Value}"'

Now the above example is reasonably ok, but it still has a vulnerability. Notice what happens if we set ArrayName="echo rm -rf /tmp/dummyfolder/*; tvar" The answer is to create a function that gives you a certain amount of security it its use and allows for cleaner code.

  # check_valid_var_name VariableName
  function check_valid_var_name {
    case "${1:?Missing Variable Name}" in
      [!a-zA-Z_]* | *[!a-zA-Z_0-9]* ) return 3;;
    esac
  }
  # set_variable VariableName [<Variable Value>]
  function set_variable {
    check_valid_var_name "${1:?Missing Variable Name}" || return $?
    eval "${1}"'="${2:-}"'
  }
  set_variable "laksdpaso" "dasädöas# #-c,c pos 9302 1´ " 
  set_variable "echo rm -rf /tmp/dummyfolder/*; tvar" "dasädöas# #-c,c pos 9302 1´ " 
  # return Error

For reference some other examples

  # get_array_element VariableName ArrayName ArrayElement
  function get_array_element {
    check_valid_var_name "${1:?Missing Variable Name}" || return $?
    check_valid_var_name "${2:?Missing Array Name}" || return $?
    eval "${1}"'="${'"${2}"'["${3:?Missing Array Index}"]}"'
  }
  # set_array_element ArrayName ArrayElement [<Variable Value>]
  function set_array_element {
    check_valid_var_name "${1:?Missing Array Name}" || return $?
    local -i ArrayIndex="${2:?Missing Array Index}"
    eval "${1}"'["${2:?Missing Array Index}"]="${3:-}"'
  }
  # unset_array_element ArrayName ArrayElement
  function unset_array_element {
    unset "${1}[${2}]"
  }
  # unset_array_element VarName ArrayName
  function get_array_element_cnt {
    check_valid_var_name "${1:?Missing Variable Name}" || return $?
    check_valid_var_name "${2:?Missing Array Name}" || return $?
    eval "${1}"'="${#'"${2}"'[@]}"'
  }
  # push_element ArrayName <New Element 1> [<New Element 2> ...]
  function push_element {
    check_valid_var_name "${1:?Missing Array Name}" || return $?
    local ArrayName="${1}"
    local LastElement
    eval 'LastElement="${#'"${ArrayName}"'[@]}"'
    while shift && [ $# -gt 0 ] ; do
      eval "${ArrayName}"'["${LastElement}"]="${1}"'
      let LastElement+=1
    done
  }
  # pop_element ArrayName <Destination Variable Name 1> [<Destination Variable Name 2> ...]
  function pop_element {
    check_valid_var_name "${1:?Missing Array Name}" || return $?
    local ArrayName="${1}"
    local LastElement
    eval 'LastElement="${#'"${ArrayName}"'[@]}"'
    while shift && [[ $# -gt 0 && ${LastElement} -gt 0 ]] ; do
      let LastElement-=1
      check_valid_var_name "${1:?Missing Variable Name}" || return $?
      eval "${1}"'="${'"${ArrayName}"'["${LastElement}"]}"'
      unset "${ArrayName}[${LastElement}]" 
    done
    [[ $# -eq 0 ]] || return 8
  }
  # shift_element ArrayName [<Destination Variable Name>]
  function shift_element {
    check_valid_var_name "${1:?Missing Array Name}" || return $?
    local ArrayName="${1}"
    local CurElement=0 LastElement
    eval 'LastElement="${#'"${ArrayName}"'[@]}"'
    while shift && [[ $# -gt 0 && ${LastElement} -gt ${CurElement} ]] ; do
      check_valid_var_name "${1:?Missing Variable Name}" || return $?
      eval "${1}"'="${'"${ArrayName}"'["${CurElement}"]}"'
      let CurElement+=1
    done
    eval "${ArrayName}"'=("${'"${ArrayName}"'[@]:${CurElement}}")'
    [[ $# -eq 0 ]] || return 8
  }
  # unshift_element ArrayName <New Element 1> [<New Element 2> ...]
  function unshift_element {
    check_valid_var_name "${1:?Missing Array Name}" || return $?
    [ $# -gt 1 ] || return 0
    eval "${1}"'=("${@:2}" "${'"${1}"'[@]}" )'
  }

Note the shift_element and unsift_element have poor performance and as such shouldn't be used. The rest have acceptable performance and I use them regularly.


CategoryShell

BashFAQ/048 (last edited 2014-06-25 22:49:04 by ormaaj)