Corruption has been a major problem in Iraq for thousands of years. The three decades of Saddam Hussein's rule did nothing to clean things up. Army officers would report they had more troops than they actually did, and pocket the pay and benefits for the "phantom soldiers." Weapons scientists would propose complex programs for new devices, and then steal a lot of the money Saddam gave them. Saddam, and his secret police would use money, and favors, to keep people loyal, or at least less disloyal.
American troops quickly learned, once they were in Iraq for a while, that lots of people were for sale. And they soon discovered that they were not the only wealthy player in the neighborhood. The Baath Party, the organization Saddam used to run Iraq, was still in business. Baath had lots of money, and all of Saddam's secret policemen were out of work and looking for a job. Coalition forces soon began catching spies for Baath who had gotten jobs working for coalition forces. Sometimes the spies were caught in the act; taking pictures, notes, or computer files and such. Other spies were nailed because another Iraqi tipped off the Americans. Some of the spies were former secret policemen, and these guys were never very popular with Iraqis. This made it easier for other Iraqis to turn them in.
By last Summer, American counterintelligence officers knew they had a major problem on their hands with the well paid Baath spies all over the place. More rewards were offered for information on spies, and counterintelligence experts installed systems and procedures that made it more likely that spies would be caught. One very unpopular, to the Iraqis, measure was to keep all Iraqis out of the loop when it came to planning military, and even non-military, operations. Baath quickly found out that the American troops were no pushovers in a guerilla war, and that was a major reason Baath was spending so much to bribe Iraqis working for the Americans. Apparently, Baath was unable to seriously penetrate American security. But at the same time, they haven't given up. More spies are caught every month. In almost all cases, the culprit was doing it for a pile of cash (a minority were doing it because they "hated the invaders" or were al Qaeda followers.)