In a large-scale sensor network individual sensors can be compromised to inject bogus sensing reports. While SEF can filter out the outfield false reports, it is incapable of detecting the in-field compromised nodes, which may collect sufficient number of keyed message authentication codes (MAC). An in-field attack proof mechanism is presented in this paper. The MAC delivery mechanism makes the MACs follow the direction of increasing signal strength, and the skipping out mechanism helps the MACs walk out of the compromised nodes. As the report is forwarded, each node along the way verifies the correctness of the MACs probabilistically and drops those with invalid MACs. As the in-field compromised node is prevented from gathering enough MACs, the report generated by it can be detected and dropped en-route. Analysis and simulation show that IAP can drop bogus reports injected by an in-field compromised node in many cases.