Malware Strikes With Valid Digital Certificate

One of the foundational elements of ecommerce is the web of trust enabled by digital certificates. When you go to a web site, you can feel confident that it’s legitimate because it has a certificate from a recognized certificate authority that validates it. But the certificates themselves can be vulnerable. Case in point: Security firm Malwarebytes recently discovered some malware in the wild with a valid, signed digital certificate.

“One of our security researchers identified this piece of malware,” says Jerome Segura, senior security researcher at Malwarebytes. “It’s a typical Trojan with one peculiarity: It was signed, and unlike a lot of malware that uses signatures, this one was valid.”

The malware is a banking/password stealer that Segura says uses email to spread. It appears to be a PDF invoice with a valid certificate issued to a real Brazilian software company called “Buster Paper Comercial Ltda,” Segura says. The certificate was issued by SSL certificate authority DigiCert.

“I don’t think it’s stolen, per se,” Segura says. “It looks like what [the criminals] did is they looked at this company in Brazil, which is a software company, and essentially made a request in their name to DigiCert. From the point of view of the certificate authority, it looks normal. [The criminals] probably spoofed the email address to buy the certificate. It looks to me as if it’s too easy for anybody who does a bit of research to either impersonate a company or set up a fake web site as if it were a company and then buy a certificate.”

DigiCert has confirmed that it did issue the certificate but revoked it as soon as it learned of the misuse.

“DigiCert has conducted a thorough review of this matter and can confirm that the certificate was validated and issued in accordance with industry guidelines,” the company said in an official statement Monday evening. “At the time that the code signing certificate was issued, Buster Paper Comercial Ltda was a legally registered business as confirmed through the Brazilian Ministerio da Fazenda: Cadastro Sincronizado Nacional. DigiCerts Terms of Use clearly state that malware is not an accepted type of activity for which our certificates can be used. As soon as DigiCert learned of the misuse of the certificate, it was immediately revoked.”

When someone clicks on this particular piece of malware, Segura says, it opens what appears to be a PDF invoice. But it also creates a number of processes that connect to an enterprise cloud storage company.

“This is a sub-domain for a cloud storage company focusing on file sharing for the enterprise,” Segura says. “Well, in our case, it’s file storage for the criminals.”

The fake PDF downloads two very large files—WIDEAWAKE1.zip and WIDEAWAKE1.ecl. Segura notes that Malwarebytes has also reached out to the cloud storage company about the issue but have yet to receive a response.

Segura notes that ThreatExpert, provider of an automated threat analysis system, found a similar Trojan with a valid digital certificate last November. That Trojan’s certificate has since been revoked.

“What we have here is a total abuse of hosting services, digital certificates and repeat offenses from the same people,” Segura says. “Clearly if digital certificates can be abused so easily, we have a big problem on our hands.”

Digital Certificates Used for Spear Phishing Attacks

“Digital certificate theft can be used in targeted attacks as [for] spear phishing, for example,” Segura says. “As we know, one of the weakest links in the security chain is the end-user (and this is especially true in the enterprise world). An attacker can easily find out or guess what antivirus a company is running and craft a piece of malware that will not be detected by it. Because such attacks are very narrow, the sample will not be disseminated around the world, making its discovery less likely.”

Segura recommends that end-users still check for valid digital certificates before opening an attachment received via email (even if they know the sender). But he also recommends following two basic but “powerful” rules:

• Check the file extension and beware the multiple file extension trick (i.e., document.pdf.xls.exe)
• Never trust file icons; just because it looks like a Word document or PDF, that doesn’t mean it is

Thor Olavsrud covers IT Security, Big Data, Open Source, Microsoft Tools and Servers for CIO.com. Follow Thor on Twitter @ThorOlavsrud. Follow everything from CIO.com on Twitter @CIOonline and on Facebook. Email Thor at tolavsrud@cio.com