GNU Mailutils 3.7 - Privilege Escalation

# Exploit Title: GNU Mailutils 3.7 - Local Privilege Escalation
# Date: 2019-11-06
# Exploit Author: Mike Gualtieri
# Vendor Homepage: https://mailutils.org/
# Software Link: https://ftp.gnu.org/gnu/mailutils/mailutils-3.7.tar.gz
# Version: 2.0 <= 3.7
# Tested on: Gentoo
# CVE : CVE-2019-18862

Title   : GNU Mailutils / Maidag Local Privilege Escalation
Author  : Mike Gualtieri :: https://www.mike-gualtieri.com
Date    : 2019-11-06
Updated : 2019-11-20

Vendor Affected:   GNU Mailutils :: https://mailutils.org/
Versions Affected: 2.0 - 3.7
CVE Designator:    CVE-2019-18862


1. Overview

The --url parameter included in the GNU Mailutils maidag utility (versions 2.0
through 3.7) can abused to write to arbitrary files on the host operating
system.  By default, maidag is set to execute with setuid root permissions,
which can lead to local privilege escalation through code/command execution by
writing to the system's crontab or by writing to other root owned files on the
operating system.



2. Detail

As described by the project's homepage, "GNU Mailutils is a swiss army knife of 
electronic mail handling. It offers a rich set of utilities and daemons for
processing e-mail".

Maidag, a mail delivery agent utility included in the suite, is by default
marked to execute with setuid (suid) root permissions.

The --url parameter of maidag can be abused to write to arbitrary files on the 
operating system.  Abusing this option while the binary is marked with suid 
permissions allows a low privileged user to write to arbitrary files on the 
system as root.  Writing to the crontab, for example, may lead to a root shell.

The flaw itself appears to date back to the 2008-10-19 commit, when the --url 
parameter was introduced to maidag.

	11637b0f - New maidag mode: --url
	https://git.savannah.gnu.org/cgit/mailutils.git/commit/?id=11637b0f262db62b4dc466cefb9315098a1a995a

	maidag/Makefile.am:
    	chmod 4755 $(DESTDIR)$(sbindir)/$$i;\


The following payload will execute arbitrary commands as root and works with 
versions of maidag, through version 3.7.

	maidag  --url /etc/crontab < /tmp/crontab.in

	The file /tmp/crontab.in would contain a payload like the following.  

	line 1:
	line 2: */1  *  * * *  root    /tmp/payload.sh

	Please note: For the input to be accepted by maidag, the first line of the
    file must be blank or be commented.

	In the above example, the file /tmp/payload.sh would include arbitrary 
    commands to execute as root.


Older versions of GNU Mailutils (2.2 and previous) require a different syntax:

	maidag --url 'mbox://user@localhost //etc/crontab' < /tmp/crontab.in



3. Solution

A fix for the flaw has been made in GNU Mailutils 3.8, which removes the maidag 
utility, and includes three new utilities that replace its functionality.  
Details about the new features can be found in the project's release notes:

	https://git.savannah.gnu.org/cgit/mailutils.git/tree/NEWS

Another workaround for those unable to upgrade, is to remove the suid bit on 
/usr/sbin/maidag (e.g. `chmod u-s /usr/sbin/maidag`).

It should be noted that some Linux distributions already remove the suid bit
from maidag by default, nullifying this privilege escalation flaw.

Another patch has been made available by Sergey Poznyakoff and posted to the
GNU Mailutils mailing list, which removes the setuid bit for maidag in all but
required cases.  The patch is intended for users who can not yet upgrade to
mailutils 3.8.  The patch has also been made available here:
https://www.mike-gualtieri.com/files/maidag-dropsetuid.patch



4. Additional Comments

This vulnerability disclosure was submitted to MITRE Corporation for inclusion
in the Common Vulnerabilities and Exposures (CVE) database.  The designator
CVE-2019-18862 has been assigned.

https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-18862
https://nvd.nist.gov/vuln/detail/CVE-2019-18862

The NIST National Vulnerability Database (NVD) has assigned the following
ratings:

CVSS 3.x Severity and Metrics: Base Score: 7.8 HIGH
CVSS 2.0 Severity and Metrics: Base Score: 4.6 MEDIUM

This disclosure will be updated as new information becomes available.  



5. History

2019-10-09 Informed Sergey Poznyakoff <[email protected]> of security issue

2019-10-10 Reply from Sergey acknowledging the issue

2019-10-12 Fix available in the GNU Mailutils git repository:
           739c6ee5 - Split maidag into three single-purpose tools
           https://git.savannah.gnu.org/cgit/mailutils.git/commit/?id=739c6ee525a4f7bb76b8fe2bd75e81a122764ced

2019-11-06 GNU Mailutils Version 3.8 released to close the issue

2019-11-06 Submission of this vulnerability disclosure to MITRE Corporate to
           obtain a CVE designator

2019-11-07 Patch offered by Sergey for those unable to upgrade to version 3.8

2019-11-11 CVE-2019-18862 assigned to flaw

2019-11-20 Vulnerability disclosure made publicly available