There are services that claim to generate codes for iTunes gift cards, and forums that explain how to use prepaid Visa cards to get free App Store purchases.

The scale of the problem is difficult to gauge without Apple’s cooperation, though there is widespread anecdotal evidence, even on Apple’s own site. On one Apple support forum, a thread titled “iTunes store account hacked,” there are some 1,370 replies, starting in November 2010 and extending to Thursday. Last week, more than 100 people on Twitter who said they were iTunes users complained about stolen funds.

Last month, Daniel Saewitz, a 20-year-old Syracuse University student, was charged $81 for purchases related to a Chinese iPhone game. He alerted Apple and changed his iTunes password. But 24 hours later, he said, his account was hacked again. In an e-mail, Apple said it was refunding Mr. Saewitz’s money, but added that it was making an exception to its usual rules.

For developers, the scams can cause big headaches, eating up resources and damaging their reputations. Several game makers in China, where many of the hacks appear to originate, said they had lost hundreds of thousands of dollars because of fraud.

Hoolai Game, a Beijing-based developer that introduced an iPhone app last year, looked at its monthly payments from Apple and found that they were roughly 20 to 50 percent less than the sum of the daily reports it gets from the company. Hoolai and others say they believe these missing payments are fraudulent transactions that are wiped out by Apple.

More troubling for developers is that consumers whose accounts have been improperly charged often blame the game makers. The reviews in the App Store for Kingdom Conquest, from the Japanese game giant Sega, include dozens from incensed users who accuse Sega of robbing them. Sega, which first noticed a burst of fraudulent transactions last summer, is still working on the problem, according to Ben Harborne, a brand manager at the company.