Differences between revisions 24 and 26 (spanning 2 versions)
Revision 24 as of 2013-07-18 15:19:30
Size: 9485
Editor: Lhunath
Comment: get rid of the cruddy "robust eval" section and add real eval robusting.
Revision 26 as of 2013-07-29 14:15:10
Size: 9914
Editor: Lhunath
Comment: eval usage and non-usage
Deletions are marked like this. Additions are marked like this.
Line 199: Line 199:

For a list of ways to reference or to populate variables indirectly without using `eval`, please see [[BashFAQ/006|FAQ #6]]. (This section was written before #6 was, but I've left it here as a reference.)
Line 204: Line 201:
To make `eval` safe, you need to turn your literal data into bash literals. The easiest way to do this is by using `printf`'s `%q`:

{{{
eval "$(printf '%q=$value' "$var")"
}}}

This turns the data in `var` into a bash literal, meaning it can no longer be used for anything that is not literal (such as running arbitrary code). It is now safe for injection into, in this case, an assignment. NOTE: `$value` is not made literal by %q, but IT IS SINGLE QUOTED. You should ALWAYS single-quote the format string to printf: we don't want to allow `$value` to expand at this time: we want to end up with a string: `varcontent=$value`, which `eval` will then evaluate. If you're sloppy and double-quote the printf format string, you've allowed arbitrary code execution through `value`'s content!
This bears repeating: If at all possible, even if it requires you to completely rethink or rewrite your solution, avoid using `eval` at all cost.

If for some reason you still need to dynamically build bash code and evaluate it, make certain you take these precautions:

 1. Always '''quote''' the `eval` expression: `eval 'a=b'`
 2. Always '''single-quote''' code and expand your data into it using `printf`'s `%q`: `eval "$(printf 'myvar=%q' "$value")"`
 3. Do NOT use dynamic variable names. Even with careful `%q` usage, this can be exploited.

Why take heed? Here's how your scripts can be exploited if they fail to take the above advice:

 * If you don't single-quote your code, you run the risk of expanding data into it that isn't `%q`'ed. Which means free executable reign for that data:
{{{
name='Bob; echo I am arbitrary code'; eval "user=$name"
}}}
 * Even if you `%q` input data before treating it as a variable name, illegal variable names in assignments cause bash to search `PATH` for a command:
{{{
echo 'echo I am arbitrary code' > /usr/local/bin/a[1]=b; chmod +x /usr/local/bin/a[1]=b; var='a[1]' value=b; eval "$(printf '%q=%q' "$var" "$value")"
}}}

For a list of ways to reference or to populate variables indirectly '''without''' using `eval`, please see [[BashFAQ/006]].

For a list of ways to reference or to populate variables indirectly '''with''' `eval`, please see [[BashFAQ/006#eval]].

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 "$3" -type f -printf "\"%p\" "`
    # warning, BAD code
    eval FILES=($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.

Note that this is still frowned upon: It is a slippery slope and some later maintenance can easily turn this code into something dangerous. Eg. You want to add a feature that allows a bunch of different --test-xyz's to be passed. You change --test to --test-*, without going through the trouble of checking the implementation of the rest of the script. You test your use case case and it all works. Unfortunately, you've just introduced arbitrary command execution:

$ ./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.

AVOID PASSING DATA TO EVAL AT ALL COST, even if your code seems to handle all the edge cases today.

If you have thought really hard and asked #bash for an alternative way but there isn't any, skip ahead to "Robust eval usage".

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.

     attoparsec:~$ echo $BASH_VERSION 
     4.2.24(1)-release
     attoparsec:~$ danger='( $(printf "%s!\n" DANGER >&2) )'
     attoparsec:~$ declare safe=${danger}
     attoparsec:~$ declare -a unsafe
     attoparsec:~$ declare unsafe=${danger}
     DANGER!
     attoparsec:~$ 
    Regular variables may be safe with declare, but array variables are not.

Robust eval usage

This bears repeating: If at all possible, even if it requires you to completely rethink or rewrite your solution, avoid using eval at all cost.

If for some reason you still need to dynamically build bash code and evaluate it, make certain you take these precautions:

  1. Always quote the eval expression: eval 'a=b'

  2. Always single-quote code and expand your data into it using printf's %q: eval "$(printf 'myvar=%q' "$value")"

  3. Do NOT use dynamic variable names. Even with careful %q usage, this can be exploited.

Why take heed? Here's how your scripts can be exploited if they fail to take the above advice:

  • If you don't single-quote your code, you run the risk of expanding data into it that isn't %q'ed. Which means free executable reign for that data:

name='Bob; echo I am arbitrary code'; eval "user=$name"
  • Even if you %q input data before treating it as a variable name, illegal variable names in assignments cause bash to search PATH for a command:

echo 'echo I am arbitrary code' > /usr/local/bin/a[1]=b; chmod +x /usr/local/bin/a[1]=b; var='a[1]' value=b; eval "$(printf '%q=%q' "$var" "$value")"

For a list of ways to reference or to populate variables indirectly without using eval, please see BashFAQ/006.

For a list of ways to reference or to populate variables indirectly with eval, please see BashFAQ/006#eval.


CategoryShell

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