Bug 1796944 (CVE-2019-18634) - CVE-2019-18634 sudo: Stack based buffer overflow when pwfeedback is enabled
Summary: CVE-2019-18634 sudo: Stack based buffer overflow when pwfeedback is enabled
Alias: CVE-2019-18634
Product: Security Response
Classification: Other
Component: vulnerability
Version: unspecified
Hardware: All
OS: Linux
Target Milestone: ---
Assignee: Red Hat Product Security
QA Contact:
Depends On: 1796945 1798074 1798075 1798091 1798092 1798093 1798094 1798095 1798531 1799018
Blocks: 1796946
TreeView+ depends on / blocked
Reported: 2020-01-31 15:03 UTC by Pedro Sampaio
Modified: 2023-09-07 21:40 UTC (History)
11 users (show)

Fixed In Version: sudo 1.8.31
Doc Type: If docs needed, set a value
Doc Text:
A flaw was found in the Sudo application when the ’pwfeedback' option is set to true on the sudoers file. An authenticated user can use this vulnerability to trigger a stack-based buffer overflow under certain conditions even without Sudo privileges. The buffer overflow may allow an attacker to expose or corrupt memory information, crash the Sudo application, or possibly inject code to be run as a root user.
Clone Of:
Last Closed: 2020-02-14 14:09:35 UTC

Attachments (Terms of Use)

System ID Private Priority Status Summary Last Updated
Red Hat Knowledge Base (Solution) 5246581 0 None None None 2020-07-24 05:49:37 UTC
Red Hat Product Errata RHBA-2020:0587 0 None None None 2020-02-24 23:21:04 UTC
Red Hat Product Errata RHBA-2020:0821 0 None None None 2020-03-16 13:53:32 UTC
Red Hat Product Errata RHBA-2020:0822 0 None None None 2020-03-16 14:02:14 UTC
Red Hat Product Errata RHBA-2020:0906 0 None None None 2020-03-19 18:06:54 UTC
Red Hat Product Errata RHBA-2020:1421 0 None None None 2020-04-09 09:30:27 UTC
Red Hat Product Errata RHSA-2020:0487 0 None None None 2020-02-13 09:34:08 UTC
Red Hat Product Errata RHSA-2020:0509 0 None None None 2020-02-13 19:08:50 UTC
Red Hat Product Errata RHSA-2020:0540 0 None None None 2020-02-18 15:45:26 UTC
Red Hat Product Errata RHSA-2020:0726 0 None None None 2020-03-05 13:20:02 UTC

Description Pedro Sampaio 2020-01-31 15:03:05 UTC
In Sudo before 1.8.31, if pwfeedback is enabled in /etc/sudoers, users can trigger a stack-based buffer overflow in the privileged sudo process. (pwfeedback is a default setting in Linux Mint and elementary OS; however, it is NOT the default for upstream and many other packages, and would exist only if enabled by an administrator.) The attacker needs to deliver a long string to the stdin of getln() in tgetpass.c.

Upstream patch:




Comment 1 Pedro Sampaio 2020-01-31 15:03:29 UTC
Created sudo tracking bugs for this issue:

Affects: fedora-all [bug 1796945]

Comment 2 Marco Benatto 2020-02-04 14:15:08 UTC
External References:


Comment 10 Marco Benatto 2020-02-04 18:03:49 UTC
There's a bug on sudo package when pwfeedback feature is enabled and password is read from a pipe.
This happens due to the fact getln() fails to reset the pointer position from the read buffer when a erase character is read. This failure allows an attacker to craft a password input in order to cause sudo to crash, expose data, corrupt data and possibly inject code to be executed.

Comment 13 Marco Benatto 2020-02-05 14:47:36 UTC
Created sudo tracking bugs for this issue:

Affects: fedora-all [bug 1798531]

Comment 17 Doran Moppert 2020-02-06 00:02:45 UTC

Please follow the steps bellow as mitigation:

1. Check the default properties set for sudo by running:

    $ sudo -l
    [sudo] password for user:
    Matching Defaults entries for users on localhost:
        !visiblepw, pwfeedback, always_set_home, match_group_by_gid, always_query_group_plugin, env_reset, env_keep="COLORS DISPLAY HOSTNAME HISTSIZE KDEDIR LS_COLORS",

2. If `pwfeedback` is enabled as shown above, edit your `/etc/sudoers` file, changing the line:

    Defaults pwfeedback

    Defaults !pwfeedback
    This will disable visual feedback on password typing, making sure the attack is not possible anymore.

Comment 21 Marco Benatto 2020-02-06 19:42:08 UTC
A deeper analysis shows the sudo version shipped with RHEL 5 is not affected.
More recent versions include a break; statement when write(2) fails to write the backspace character to the output on getln() function and password feedback is enabled. The main issue with that is the fact write(2) will always fail to write to unidirectional pipes thus when facing a VERASE character sudo won't reset back the buffer position pointer in such scenarios, which will subsequently lead to the buffer overflow.

This can be observed on sudo version 1.8.0 and above. The commit which introduced the vulnerability seems to be:
commit 420db23714ffcab87caa67bae0e3de9f42222cf9                          
Author: Todd C. Miller <Todd.Miller>                       
Date:   Tue Aug 3 11:17:56 2010 -0400                                    
    Quiet gcc warnings on glibc systems that use warn_unused_result for  
    write(2) and others.                                                 
Red Hat Enterprise 5 ships sudo v.1.7.2p1 which still doesn't have this specific behavior. Even if write(2) fails due to unidirectional pipe it still decrease the current pointer position correctly when the termios's VERASE character is found in the password input.

Comment 24 Huzaifa S. Sidhpurwala 2020-02-12 04:33:27 UTC

This flaw can only be exploited if the option `pwfeedback` is enabled in sudo configuration. This option is not enabled by default in any version of Red Hat Enterprise Linux.
The sudo packages distributed with Red Hat Enterprise Linux versions are compiled using gcc's stack-protector feature. The "Stack Smashing Protection" may help mitigate code execution attacks for this flaw.
Red Hat Enterprise Linux 5 is not affected as it doesn't include the commit which introduced the vulnerability.

Comment 25 errata-xmlrpc 2020-02-13 09:34:04 UTC
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 8

Via RHSA-2020:0487 https://access.redhat.com/errata/RHSA-2020:0487

Comment 26 errata-xmlrpc 2020-02-13 19:08:49 UTC
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 8.0 Update Services for SAP Solutions

Via RHSA-2020:0509 https://access.redhat.com/errata/RHSA-2020:0509

Comment 27 Product Security DevOps Team 2020-02-14 14:09:35 UTC
This bug is now closed. Further updates for individual products will be reflected on the CVE page(s):


Comment 28 errata-xmlrpc 2020-02-18 15:45:25 UTC
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 7

Via RHSA-2020:0540 https://access.redhat.com/errata/RHSA-2020:0540

Comment 29 Scott Dowdle 2020-02-19 23:41:07 UTC
Per this portal reference (https://access.redhat.com/security/cve/cve-2019-18634), RHEL 6 is listed as affected.  Will there be an update made available for RHEL 6?  Or was the severity of this issue deemed not high enough to warrant a critical security update?

Comment 30 errata-xmlrpc 2020-03-05 13:20:01 UTC
This issue has been addressed in the following products:

  Red Hat Enterprise Linux 6

Via RHSA-2020:0726 https://access.redhat.com/errata/RHSA-2020:0726

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