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Advanced Malvertising Attack Hits German ISP T-Online

A large number of German users might have been hit by a recent malvertising campaign that leveraged some clever tricks to deliver malware without being detected.

According to endpoint threat protection company Invincea, one of the websites impacted by the attack was T-Online.de, the official website of Deutsche Telekom-owned T-Online, Germany’s biggest Internet service provider (ISP).

An analysis performed by Invincea revealed that the attackers used the malvertising operation to deliver the Tinba banking Trojan, a threat designed to steal financial information, and Bedep click fraud bots, which turn infected computers into zombie hosts that generate a profit for cybercriminals by clicking on ads without the victim’s knowledge.

The malicious ads were designed to take users to an exploit kit landing page that leveraged an Adobe Flash Player vulnerability to push malware onto victims’ machines. In order to avoid detection, the malicious actors used a technique known as just-in-time (JIT) malware assembly.

JIT malware assembly, a technique described by Invincea in July, helps cybercriminals evade detection by gradually pushing snippets of the malware code and combining them into the actual payload on the victim’s machine. The malware is assembled using PowerShell, Script Host and other legitimate Windows utilities that are in many cases whitelisted.

In the attack affecting T-Online, the cybercrooks used the Windows command interpreter Cmd.exe to achieve their goal, Invincea said.

Researchers at Malwarebytes have also analyzed this malvertising campaign. According to the antivirus company, T-Online.de was just one of several high-traffic websites hit by the attack. The list includes eBay.de, which has 131 million monthly visits, arcor.de, which has 7.6 million monthly hits, and swp.de, with 790,000 visits, according to SimilarWeb.

Malwarebytes said the attackers gained the trust of reputable ad networks, including Germany-based ad technology platform MP New Media, which reportedly addressed the problem after it was brought to its attention. The security firm noted that the Angler and Neutrino exploit kits have been used in this campaign.

According to Invincea, T-Online was targeted between October 16 and October 20. Deutsche Telekom representatives have confirmed for SecurityWeek that they learned of the incident on October 20 and immediately restricted the advertiser responsible for pushing the malicious ads.

“t-online.de generally is working together only with trustworthy partners: t-online doesn’t take ads from third parties unknown to the company but solely from hand-picked business partners in order to reduce the risk of compromised ads. Nevertheless malvertising of the advertisements still can happen due to the possibility of manipulating advertisements in the supply chain (before the advertisements are delivered to the visitors of a website),” Deutsche Telekom said in an emailed statement.

“Therefore it is of crucial importance to recognize the compromising of ads as soon as possible and take down the technical service providers that distribute these ads. t-online got to know about the compromising on Monday, October 20th and reacted immediately by shutting down the advertiser that delivered the ads,” the company added.

Malwarebytes has alerted the affected publishers and ad networks, but the security firm says it’s possible that the campaign might still be active via other services.

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Eduard Kovacs (@EduardKovacs) is a contributing editor at SecurityWeek. He worked as a high school IT teacher for two years before starting a career in journalism as Softpedia’s security news reporter. Eduard holds a bachelor’s degree in industrial informatics and a master’s degree in computer techniques applied in electrical engineering.