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Bank-raid ZeuS malware waltzes around web with 'valid app signature'

Check your vaults. Did someone just lose their secret software signing key?

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A variant of the bank-account-raiding ZeuS Trojan is masquerading as a legit Windows app using a valid digital signature – and packs a rootkit to burrow deep into victims' PCs.

It appears miscreants have somehow gained access to the private signing key belonging to a Microsoft-registered third-party developer in Switzerland, and used it to cryptographically sign the malware's executable. This key should be kept a closely guarded secret.

The Windows operating system and antivirus tools can check the validity of a program's digital signature before trusting it, because an invalid signature indicates the code has been tampered with in transit, for instance. By generating a valid signature for the ZeuS Trojan from a trusted private key, crims can dress the software nasty up as a legit application.

It's a sign that crooks are adopting tactics used by sophisticated espionage software, such as Flame: signing binaries to slip code onto victims' machines is part and parcel of modern-day cyber-spying. The use of stolen certs by criminals is rare but not unprecedented.

Researchers at SSL-certificate flogger Comodo, using telemetry collected from users of its security software, detected 200 installs of this latest ZeuS variant.

ZeuS (aka Zbot) is typically distributed by hackers planting malicious code on legitimate websites that exploit browser bugs to install the nasty, or through email phishing by tricking netizens into running attachments.

The malware discovered by Comodo presented itself a web-page with the Internet Explorer logo on it, whereas it's really a malicious signed executable that drops an installer on the hard drive to run, which tries to download a rootkit from the web. This is decrypted into a driver and lined up to execute early on in the PC's boot sequence – meaning it can get to work hiding the Trojan from the rest of the operating system and applications, particularly tools that try to remove the Trojan.

When running, the software's goal is to intercept usernames and passwords, credit card numbers and other highly sensitive personal information submitted through website forms – particularly those on banking websites – and siphon the data off to crooks to exploit.

Richards Moulds, veep of product strategy at Thales e-Security, said that the digitally signed ZeuS binary undermines the system of trust that Windows and other operating systems rely on.

"Windows, iOS, Android, and Linux all use code signing to ensure that only legitimate, signed code is installed and executed," Moulds explained. "Code-signing provides the best mechanism for proving that code hasn’t been modified and therefore is a way of spotting malware infected software and rejecting it. If an attacker can sign their malicious code in a way that passes this validation process they are a huge step further in mounting an attack." ®

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