CVE 2014-3517

api/metadata/handler.py in OpenStack Compute (Nova) before 2013.2.4, 2014.x before 2014.1.2, and Juno before Juno-2, when proxying metadata requests through Neutron, makes it easier for remote attackers to guess instance ID signatures via a brute-force attack that relies on timing differences in responses to instance metadata requests.

See the CVE page on Mitre.org for more details.