Would-be phishers can buy, or obtain for free, phishing kits, which include the files necessary to duplicate a targeted Web site and scripts to steal information submitted by phishing victims. They're widely available online, but they're also untrustworthy.

In January, Netcraft security researcher Paul Mutton identified a phishing tool kit distributed by a group of Moroccan cybercriminals that had been compromised with a back door. Unbeknownst to its users, the phishing kit sent copies of stolen information to its creators.

Now it turns out that more than 40% of the live phishing kits found online (61 out of 150) have back doors designed to steal from the information thieves using them.

In a paper presented on Monday at the Usenix Conference in San Jose, Calif. -- "There Is No Free Phish: An Analysis Of 'Free' And Live Phishing Kits" -- security researchers Marco Cova, Christopher Kruegel, and Giovanni Vigna from the University of California, Santa Barbara, have found that the big phishers -- the authors of phishing kits -- feed on the little phishers who deploy phishing kits.

And there are a surprising number of phishing tool kits. From 21 different distribution sites, the authors of the paper identified 379 distinct phishing kits, 129 of which contained back doors.

The phishing kits targeted 49 different organizations, mainly banks and auction sites, but also e-mail providers and gaming portals. Among the kits downloaded from distribution sites, the five most common targets were Bank of America (21 kits), eBay (19), Wachovia (18), HSBC (18), and PayPal (15).

Most of the live backdoor phishing kits send hijacked information to e-mail drop accounts. Two of the kits stored hijacked information in a file on the phishing site server, and one sent the information to an outside server using a POST request.

And in an attempt to conceal the true nature of their software, phishing kit authors frequently obfuscate their code and include comments in their code designed to discourage modifications that might close their secret back door.

However, comments like "Don't need to change anything here" do more to invite suspicion than to allay it.

"In other cases, comments sound outright sarcastic," the paper said. "In one instance, the indexes of the array used in a permutation-based obfuscation read 'good for your scam.'"