Security hardening: Jenkins LTS 2.107.1 switches XStream / Remoting blacklists to whitelists (JEP-200)

Published on 2018-03-15 by Oleg Nenashev

This is a post about a major change in Jenkins, which is available starting from Jenkins 2.102 and Jenkins LTS 2.107.1. This is a change with a serious risk of regressions in plugins. If you are a Jenkins administrator, please read this blogpost and upgrade guidelines BEFORE upgrading.

I would like to provide some heads-up about the JEP-200 change, which is included into the new Jenkins LTS 2.107.x baseline.

Background

For many years Jenkins used to specifically blacklist certain classes and packages according to known or suspected exploits. This approach has been proven unsustainable due to the risk of deserialization attacks via unknown classes from 3rd-party components, after the SECURITY-429/CVE-2017-1000353 fix in 2.46.2 it was decided to replace blacklists by more restrictive whitelists. In October 2017 Jesse Glick proposed a Jenkins Enhancement Proposal, which finally got accepted as JEP-200.

The change implies a risk of regressions in plugins serializing non-whitelisted Java-internal and 3rd-party classes, and that’s why it is so important to follow the upgrade guidelines for this release.

Current state

JEP-200 was first integrated in Jenkins 2.102 (released in January 2018), and it has got a lot of testing since that. See this blogpost for the original announcement.

Over the last two months we received more than 75 issues from users of Jenkins weekly releases. All these issues have been triaged, and we have released most of the fixes. More than 50 plugins were fixed in total, and many more plugins were updated in order to enable compatibility testing. A significant part of the discovered regressions were caused by real defects which were causing performance and stability issues in plugins. Thanks a lot to all the Jenkins contributors and plugin maintainers who helped deliver timely changes for this effort!

Over last 6 weeks Jenkins weekly releases had positive community ratings, the overall JEP-200 adoption reached ~12% of all Jenkins installations on March 01. All major plugins have been also tested directly or verified in the wild on weekly releases. So we are confident that the change is ready to be released in LTS.

On the other hand, we continue to receive JEP-200 regression reports. They are mostly caused by niche plugins which are not widely used in weekly releases, and unfortunately not all fixes have been released yet (see the Wiki page for up-to-date info). We anticipate more regressions to be reported after the LTS release and broader adoption.

In order to simplify the upgrade to the new LTS baseline, I have prepared some helpful materials together with Liam Newman and Jesse Glick. Below you can find the embedded slide deck and video, or scroll down to see the key information in the text form.

Video:

For Jenkins administrators

Upgrading to a core with JEP-200 requires a special update procedure, which is described below.

Upgrading Jenkins

  1. JEP-200 is not the only major change in 2.107.1, please read the full upgrade guide carefully

  2. If you have a way of testing the upgrade before applying it to production, do it

  3. Back up your instance so you have any easy way of rolling back

  4. Update all affected plugins. See this Wiki page for the list of affected plugins, fix statuses and workarounds

  5. Apply workarounds for non-released patches if needed (see below)

  6. Update to the new version of the Jenkins core

Using backups and staging servers is good advice before any upgrade but especially this one, given the relatively high risk of regression. Due to the nature of the changes, some plugins may refuse to load after the upgrade and cause your Jenkins service to fail to start.

After the upgrade

To the extent that advance testing of the impact of this change on popular plugins has been completed, most users (and even plugin developers) should not notice any difference. Still, it is highly advised to monitor your system after the upgrade, especially the following:

  • Jenkins System log (especially during the startup)

  • Job/Build logs

If you do encounter a log message referencing the https://jenkins.io/redirect/class-filter/ URL, most likely it is a JEP-200 regression. Example:

some.pkg.and.ClassName in file:/var/lib/jenkins/plugins/some-plugin-name/WEB-INF/lib/some-library-1.2.jar might be dangerous, so rejecting; see https://jenkins.io/redirect/class-filter/

If you see this kind of message, we highly recommend reporting it so that it can be investigated and probably fixed quickly.

Reporting JEP-200 issues

Starting from May 01, JEP-200 issues are triaged by plugin and core maintainers. JEP-200 maintainers are available for code reviews if needed, but they will not be reviewing cases in JIRA and searching for miscategorized issues on a daily basis. If you experience new JEP-200 regressions, please follow the guidelines below.

Please report any issues you encounter matching the above pattern in the Jenkins issue tracker, under the appropriate plugin component. Before reporting please check whether this issue has already been reported.

  1. Add the JEP-200 label

  2. Include the stacktrace you see in the log

  3. If possible, include complete steps to reproduce the problem from scratch

You can find examples of previously reported issues using this query.

Jenkins developers will evaluate issues and strive to offer a fix in the form of a core and/or plugin update. Right after the feature release there was be a special team triaging the reports. Starting from May 01 the issues will be triaged by plugin and core maintainers. See JEP-200 Maintenance plan for more info.

For more details and current status, see Plugins affected by fix for JEP-200.

Applying workarounds

Assuming you see no particular reason to think that the class in question has dangerous deserialization semantics, which is rare, it is possible to work around the problem in your own installation as a temporary expedient. Note the class name(s) mentioned in the JEP-200 log messages, and run Jenkins with the hudson.remoting.ClassFilter startup option, e.g.:

java -Dhudson.remoting.ClassFilter=some.pkg.and.ClassName,some.pkg.and.OtherClassName -jar jenkins.war ...

This workaround process may require several iterations, because classes whitelisted in the workaround may also include fields with types requiring whitelisting.

For plugin developers

If you are a plugin developer, please see the original JEP-200 announcement. That blog post provides guidelines for testing and fixing plugin compatibility after the JEP-200 changes. The presentation above also provides some information about what needs to be tested.

About the Author
Oleg Nenashev

Jenkins core maintainer and board member, open source software and open hardware advocate, TOC chair in the Continuous Delivery Foundation. Oleg started using Hudson for Hardware/Embedded projects in 2008 and became an active Jenkins contributor in 2012. Nowadays he maintains [Jenkinsfile Runner](https://github.com/jenkinsci/jenkinsfile-runner/), contributes to several Jenkins SIGs and outreach programs (Google Summer of Code, Hacktoberfest) and organizes Jenkins meetups in Switzerland and Russia. Oleg works on open source programs and [Keptn](https://keptn.sh/) at the [Dynatrace](https://dynatrace.com), Open Source Program Office.