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# 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 <gray@gnu.org.ua> 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