A recently discovered malspam campaign targeting customers of a Polish bank was found using forgeries of Google reCAPTCHA images to fake legitimacy.

The banking malware was delivered via phishing emails that purported to seekin confirmation of a recent banking transaction that in reality never occurred, according to Feb. 21 blog post published week by Sucuri.

Recipients who panicked at the notion of an unauthorized transaction may have then clicked on a link leading to a malicious PHP file.

If the malicious file's code determines that the infected machine has user-agents associated with Google web crawler, then the user simply receives a fake 404 error page. All other infected machines are delivered a fraudulent Google reCAPTCHA page.

"This page does a decent job at replicating the look of Google’s reCAPTCHA, but since it relies on static elements, the images will always be the same unless the malicious PHP file’s coding is changed," explains the blog post, authored by Sucuri analyst Luke Leal.

At this point, the PHP code again checks user agent status to determine if the infected machine is Android-based. If so, it downloads a malicious .apk file that the attackers can use to intercept incoming SMS messages (including those used for two-factor authentication), as well as record audio. Non-Android devices are instead infected with a malicious .zip download.