Bug 1236116 (CVE-2015-3900)

Created rubygems tracking bugs for this issue:

Affects: fedora-all [bug 1236117]

(In reply to Vasyl Kaigorodov from comment #0)
> Upstream patch: https://github.com/rubygems/rubygems/commit/6bbee35

That patch is incorrect / incomplete, as noted in comments for it.  This additional commit improves the original fix:

https://github.com/rubygems/rubygems/commit/5c7bfb5

So, for sake of sanity.
Original upstream patch got CVE-2015-3900 assigned:
https://github.com/rubygems/rubygems/commit/6bbee35

Later it was found that the original patch introduces another vulnerability, which got CVE-2015-4020:
https://github.com/rubygems/rubygems/commit/5c7bfb5

Analysis (from http://blog.rubygems.org/2015/05/14/CVE-2015-3900.html):

"""
RubyGems provides the ability of a domain to direct clients to a separate host that is used to fetch gems and make API calls against. This mechanism is implemented via DNS, specificly a SRV record _rubygems._tcp under the original requested domain.

  > dig _rubygems._tcp.rubygems.org SRV

  ;; ANSWER SECTION:
  _rubygems._tcp.rubygems.org. 600 IN	SRV	0 1 80 api.rubygems.org.

RubyGems did not validate the hostname returned in the SRV record before sending requests to it.

This left clients open to a DNS hijack attack, whereby an attacker could return a SRV of their choosing and get the client to use it. For example:  

  > dig _rubygems._tcp.rubygems.org SRV

  ;; ANSWER SECTION:
  _rubygems._tcp.rubygems.org. 600 IN	SRV	0 1 80 gems.nottobetrusted.wtf
"""

Fix for this issue verifies that the returned host is on the same domain as the requested domain. This fix included:

    if /#{host}\z/ =~ target

which is incomplete, because suffix match is sufficient, e.g. if the domain requested was rubygems.org, attacker served record of attackersrubygems.org would pass this check (CVE-2015-4020). The complete fix for both now includes:

    if /\.#{Regexp.quote(host)}\z/ =~ target


Rubygems code is also bundled into ruby. The vulnerable code (function api_endpoint in lib/rubygems/remote_fetcher.rb) is present in ruby-2.0 and above.

This code is invoked like this:

lib/rubygems/resolver/best_set.rb BestSet#pick_sets() cals
lib/rubygems/source.rb Source#dependency_resolver_set() calls 
lib/rubygems/source.rb Source#api_uri() calls 
lib/rubygems/remote_fetcher.rb RemoteFetcher#api_endpoint()

However, code that invokes vulnerable function was not added to ruby before 2.1.0, hence versions of ruby below 2.1 are not affected by this vulnerability, as the vulnerable code is included but never called.

The incomplete fix issue CVE-2015-4020 is tracked via bug 1250109.

Trustwave SpiderLabs (original reporter) security advisory for this issue:

https://www.trustwave.com/Resources/Security-Advisories/Advisories/TWSL2015-007/?fid=6356

Trustwave SpiderLabs blog post for CVE-2015-3900 and CVE-2015-4020:

https://www.trustwave.com/Resources/SpiderLabs-Blog/Attacking-Ruby-Gem-Security-with-CVE-2015-3900/

rubygems-2.4.8-100.fc23 has been pushed to the Fedora 23 stable repository.  If problems still persist, please make note of it in this bug report.

rubygems-2.4.8-100.fc22 has been pushed to the Fedora 22 stable repository.  If problems still persist, please make note of it in this bug report.

rubygems-2.2.5-100.fc21 has been pushed to the Fedora 21 stable repository.  If problems still persist, please make note of it in this bug report.

This issue has been addressed in the following products:

  Red Hat Software Collections for Red Hat Enterprise Linux 7.1 EUS
  Red Hat Software Collections for Red Hat Enterprise Linux 7
  Red Hat Software Collections for Red Hat Enterprise Linux 6.6 EUS
  Red Hat Software Collections for Red Hat Enterprise Linux 6
  Red Hat Software Collections for Red Hat Enterprise Linux 6.5 EUS

Via RHSA-2015:1657 https://rhn.redhat.com/errata/RHSA-2015-1657.html