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How to build and install cryptographically signed gems– and other security concerns.

Security practices are being actively discussed. Check back often.

General

Installing a gem allows that gem’s code to run in the context of your application. Clearly this has security implications: installing a malicious gem on a server could ultimately result in that server being completely penetrated by the gem’s author. Because of this, the security of gem code is a topic of active discussion within the Ruby community.

RubyGems has had the ability to cryptographically sign gems since version 0.8.11. This signing works by using the gem cert command to create a key pair, and then packaging signing data inside the gem itself. The gem install command optionally lets you set a security policy, and you can verify the signing key for a gem before you install it.

However, this method of securing gems is not widely used. It requires a number of manual steps on the part of the developer, and there is no well-established chain of trust for gem signing keys. Discussion of new signing models such as X509 and OpenPGP is going on in the rubygems-trust wiki, the RubyGems-Developers list and in IRC. The goal is to improve (or replace) the signing system so that it is easy for authors and transparent for users.

Using Gems

Install with a trust policy.

  • gem install gemname -P HighSecurity: All dependent gems must be signed and verified.

  • gem install gemname -P MediumSecurity: All signed dependent gems must be verified.

  • bundle --trust-policy MediumSecurity: Same as above, except Bundler only recognizes the long --trust-policy flag, not the short -P.

  • Caveat: Gem certificates are trusted globally, such that adding a cert.pem for one gem automatically trusts all gems signed by that cert.

Verify the checksum, if available

gem fetch gemname -v version
ruby -rdigest/sha2 -e "puts Digest::SHA512.new.hexdigest(File.read('gemname-version.gem'))"

Know the risks of being pwned, as described by Benjamin Smith’s Hacking with Gems talk

Building Gems

Sign with: gem cert

1) Create self-signed gem cert

cd ~/.ssh
gem cert --build your@email.com
chmod 600 gem-p*
  • use the email address you specify in your gemspecs

2) Configure gemspec with cert

Add cert public key to your repository

cd /path/to/your/gem
mkdir certs
cp ~/.ssh/gem-public_cert.pem certs/yourhandle.pem
git add certs/yourhandle.pem

Add cert paths to your gemspec

s.cert_chain  = ['certs/yourhandle.pem']
s.signing_key = File.expand_path("~/.ssh/gem-private_key.pem") if $0 =~ /gem\z/

3) Add your own cert to your approved list, just like anyone else

gem cert --add certs/yourhandle.pem

4) Build gem and test that you can install it

gem build gemname.gemspec
gem install gemname-version.gem -P HighSecurity
# or -P MediumSecurity if your gem depends on unsigned gems

5) Example text for installation documentation

MetricFu is cryptographically signed. To be sure the gem you install hasn’t been tampered with:

Add my public key (if you haven’t already) as a trusted certificate

gem cert --add <(curl -Ls https://raw.github.com/metricfu/metric_fu/master/certs/bf4.pem)

gem install metric_fu -P MediumSecurity

The MediumSecurity trust profile will verify signed gems, but allow the installation of unsigned dependencies.

This is necessary because not all of MetricFu’s dependencies are signed, so we cannot use HighSecurity.

Include checksum of released gems in your repository

require 'digest/sha2'
built_gem_path = 'pkg/gemname-version.gem'
checksum = Digest::SHA512.new.hexdigest(File.read(built_gem_path))
checksum_path = 'checksum/gemname-version.gem.sha512'
File.open(checksum_path, 'w' ) {|f| f.write(checksum) }
# add and commit 'checksum_path'

For details, see discussion with Yorick Peterse.

Reporting Security vulnerabilities

Reporting a security vulnerability with someone else’s gem

If you spot a security vulnerability in someone else’s gem, then you first step should be to check whether this is a known vulnerability. One way is by searching for an advisory on RubySec.

If this looks like a newly discovered vulnerability, then you should contact the author(s) privately (i.e., not via a pull request or issue on a public project) explaining the issue, how it can be exploited, and ideally offering an indication of how it might be fixed.

Reporting a security vulnerability with your own gem

First, request a CVE identifier by emailing one of these places or from GitHub by creating a Security Advisory. This identifier will make it easy to uniquely identify the vulnerability when talking about it.

Second, work out what people who depend on your gem should do to resolve the vulnerability. This may involve releasing a patched version of your gem that you can recommend they upgrade to.

Finally, you need to tell people about the vulnerability. Currently there is no single place to broadcast this information but some good places to start might be to:

  • Send an email to several lists including ruby-security-ann@googlegroups.com, rubysec-announce@googlegroups.com, and oss-security@lists.openwall.com outlining the vulnerability, which versions of your gem it affects, and what actions those depending on the gem should take. Make sure to use a subject that includes the gem name, some short summary of the vulnerability, and the CVE ID if you have one.

  • Add it to ruby-advisory-db. You can do this by following the CONTRIBUTING guidelines and submitting a pull request.

Credits

Several sources were used for content for this guide:

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