I've looked through many seemingly-related issues regarding a slave not being able to connect to a master. However, after hours of attempting different fixes (disabling security, attemtping to pass -jnlpCredentials of an existing jenkins user); nothing seems to work but downgrading to <2.7. The slave reports a 403 Forbidden error when attempting to access build-agent.jnlp
The following is what appears on the master log:
INFO: Accepted connection #8 from /10.128.218.31:33521
Mar 22, 2017 1:48:29 PM jenkins.slaves.DefaultJnlpSlaveReceiver afterProperties
WARNING: Rejecting connection to win2k8 from /10.128.218.32:33521 as a JNLP agent as the launcher class hudson.plugins.virtualbox.VirtualBoxComputerLauncher does not extend JNLPLauncher or does not implement DelegatingComputerLauncher with a delegation chain leading to a JNLPLauncher. Set system property jenkins.slaves.DefaultJnlpSlaveReceiver.disableStrictVerification=true to allowconnections until the plugin has been fixed.
Mar 22, 2017 1:49:22 PM hudson.slaves.RetentionStrategy$Demand check
INFO: Launching computer win2k8 as it has been in demand for 13 hr