Release Date:

Summary:

Password:

January 30, 2020 (updated February 5, 2020 with additional exploitation details)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 standardprompt 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 affected:

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

$ 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

CVE ID:

Details:

$ 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)

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

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 use of the 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.

Impact:

If pwfeedback is listed in the "Matching Defaults entries" output, the sudoers configuration is affected. In the following example, the sudoers configuration is vulnerable: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):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.There are two flaws that contribute to this vulnerability: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 isimpact 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.

Workaround:

Defaults pwfeedback

Defaults !pwfeedback

sudo -l

$ 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

Fix:

Credit:

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:To:After disabling pwfeedback in sudoers using the visudo command, the exampleoutput becomes: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.