Necurs is the world’s largest spam botnet, it is constituted of millions of compromised computers worldwide.
Necurs was unavailable for a prolonged period at the beginning of 2017 and resumed its activity in April 2017. The Necurs botnet was used in the past months to push some malware, including Locky, Jaff, GlobeImposter, Dridex , Scarab and the Trickbot.
Now the botnet made the headlines again, the author implemented a new evasion technique. Crooks are sending out an email to a potential victim containing an archive file that once unzipped will present a file with the extension of .URL.
The .URL extension is associated with Windows shortcut file that opens an URL into a browser, in the campaign observed by the experts it points to a remote script file that downloads and executes a final payload.
In this last campaign, Necurs is delivering the Quant Loader to the victims to download another malware.
“As security vendors are wise to Necurs’s traditional infection chain (a script, a macro, or archives containing certain file formats), the malware has started using an internet shortcut or .URL file to bypass detection.” reads the analysis published by Trend Micro.
“In an attempt to evade spam detection through its attachments, Necurs used archives that included .ZIP files to disguise the script downloader, which was later enclosed in another .ZIP to hide itself.”
The infection chain allows avoiding anti-malware solutions that scan emails, looking for malicious links, boobytrapped attachments or other malicious patterns associated with an attack.
Once uncovered the new campaign, the security firms will update existing detection rules to rapidly discovers attacks and neutralize the spam campaign.
Malware researchers added that crooks are using the standard folder icon to hide .URL files to deceive victims on their malicious nature.
Once the victim has clicked on the archive it extracts a file that appears to the victims as a new folder on their PC. When the victims click on the folder to explore its content they will start the infection chain.
Notice that aside from the icons disguised as folders, the filenames were also crafted to resemble typical folder names such as IMG-20180404-9AC4DD, SCN-20180404-268CC1, and PIC-20180404-ADEEEE shown in Figure 2, to name a few.” states Trend Micro.
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Pedro Tavares is a professional in the field of information security working as an Ethical Hacker/Pentester, Malware Researcher and also a Security Evangelist. He is also a founding member at CSIRT.UBI and Editor-in-Chief of the security computer blog seguranca-informatica.pt.
In recent years he has invested in the field of information security, exploring and analyzing a wide range of topics, such as pentesting (Kali Linux), malware, exploitation, hacking, IoT and security in Active Directory networks. He is also Freelance Writer (Infosec. Resources Institute and Cyber Defense Magazine) and developer of the 0xSI_f33d – a feed that compiles phishing and malware campaigns targeting Portuguese citizens.
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