A new malware is targeting Android-based smartphones and turning them into a spam botnet, a British security firm says. Security company Sophos said spam messages promoting counterfeit Viagra and other pharmaceuticals are being sent from Google Android phones and tablets using Yahoo\'s mail service. The spam botnet appears to be affecting users who downloaded pirated copies of paid Android apps that which were infected with trojans, said Chester Wisniewski, a senior security adviser at Sophos. \"Android users should exercise caution when downloading applications for their devices and definitely avoid downloading pirated programs from unofficial sources,\" Wisniewski told PCworld. \"Google, Amazon and others may not be perfect at keeping malware off of their stores, but the risk increases dramatically outside of their ecosystems.\" Spam messages have been identified as coming from Argentina, Ukraine, Pakistan, Georgia and Russia.