Buffer overflow when pwfeedback is set in sudoers

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.

Sudo versions 1.7.1 to 1.8.30 inclusive are affected but only if the pwfeedback option is enabled in sudoers. It was originally thought to not be exploitable in sudo versions 1.8.26 through 1.8.30 but that has been shown to not be the case.

A user with sudo privileges can check whether pwfeedback is enabled by running:

sudo -l

If pwfeedback is listed in the “Matching Defaults entries” output, the sudoers configuration is affected. In the following example, the 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:

(ALL : ALL) ALL

This vulnerability has been assigned CVE-2019-18634 in the Common Vulnerabilities and Exposures database.

Exploiting the bug does not require sudo permissions, merely that pwfeedback be enabled. The bug can be reproduced by passing a large input with embedded terminal kill characters to sudo from a pseudo-terminal that cannot be written to. For example, using the socat utility and assuming the terminal kill character is set to control-U (0x15):

$ socat pty,link=/tmp/pty,waitslave exec:"perl -e 'print((\"A\" x 100 . chr(0x15)) x 50)'" &
$ sudo -S -k id < /tmp/pty
Password: Segmentation fault (core dumped)

For sudo versions prior to 1.8.26, and on systems with uni-directional pipes, reproducing the bug is simpler. Here, the terminal kill character is set to the NUL character (0x00) since sudo is not reading from a terminal. This method is not effective in newer versions of sudo due to a change in EOF handling introduced in 1.8.26.

$ perl -e 'print(("A" x 100 . chr(0)) x 50)' | sudo -S -k id
Password: Segmentation fault (core dumped)

There are two flaws that contribute to this vulnerability:

• The pwfeedback option is not ignored, as it should be, when reading from something other than the user’s terminal, /dev/tty. The use of the -S option should effectively disable pwfeedback.

• The code that erases the line of asterisks does not properly reset the buffer position if there is a write error, but it does reset the remaining buffer length. As a result, the getln() function can write past the end of the buffer, leading to an overflow. If the user can cause sudo to receive a write error when it attempts to erase the line of asterisks, the bug can be triggered. Because the remaining buffer length is not reset correctly on write error when the line is erased, a buffer on the stack can be overflowed.

There is no impact unless pwfeedback has been enabled in the sudoers file.

If pwfeedback is enabled in sudoers, the stack overflow may allow unprivileged users to escalate to the root account. Because the attacker has complete control of the data used to overflow the buffer, there is a high likelihood of exploitability.

If the sudoers file has pwfeedback enabled, disabling it by pre-pending an exclamation point is sufficient to prevent exploitation of the bug. For example, change:

Defaults pwfeedback

To:

Defaults !pwfeedback

After disabling pwfeedback in sudoers using the visudo command, the example sudo -l output becomes:

$ sudo -l
Matching Defaults entries for millert on linux-build:

insults, mail_badpass, mailerpath=/usr/sbin/sendmail

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

(ALL : ALL) ALL

This advisory was originally released on January 30, 2020. It was revised on February 5, 2020 with additional exploitation details.

The bug is fixed in sudo 1.8.31.

Joe Vennix from Apple Information Security found and analyzed the bug.

William Bowling reported a way to exploit the bug in sudo 1.8.26 though 1.8.30.