Race condition in the object-group ACL feature in Cisco IOS 15.5(2)T and earlier allows remote attackers to bypass intended access restrictions via crafted network traffic that triggers improper handling of the timing of process switching and Cisco Express Forwarding (CEF) switching, aka Bug ID CSCun21071.
Concurrent Execution using Shared Resource with Improper Synchronization The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.