The following flaw was found in Jenkins:

Jenkins allows plugins to annotate build logs, adding new content or changing the presentation of existing content while the build is running. Popular examples include the highlighting of sections by Ant Plugin, or the timestamp metadata from Timestamper. Malicious Jenkins users, or users with SCM access, could configure jobs or modify build scripts such that they print serialized console notes that perform cross-site scripting attacks on Jenkins users viewing the build logs.

To prevent this, console notes are now signed by Jenkins when created, and Jenkins will only deserialize correctly signed console notes. As a side effect, console notes created before updating to a release containing this fix will no longer be deserialized. To restore the previous (unsafe) behavior, set the system property hudson.console.ConsoleNote.INSECURE to true as described on Features controlled by system properties.

External References:

https://wiki.jenkins-ci.org/display/SECURITY/Jenkins+Security+Advisory+2017-02-01

Upstream patch:

https://github.com/jenkinsci/jenkins/commit/2527e66dd8f9dd23a2571f8bd8f2918edf6fc0cd