An attack campaign conducted by the Cobalt Gang used a specially crafted PDF document to evade detection by static analysis tools.

Palo Alto Networks’ Unit 42 threat intelligence team observed the operation near the end of October 2018. The analyzed example used an email containing the subject line “Confirmations on October 16, 2018” to target employees at several banking organizations.

Attached to the email was a PDF document that didn’t come with an exploit or malicious code. Instead, an embedded link within the PDF document redirected recipients to a legitimate Google location which, in turn, redirected the browser to a Microsoft Word document containing malicious macros.

How Does the Cobalt Gang

At the time of discovery, the PDF attack bypassed nearly all traditional antivirus software. It was able to do so because the Cobalt Gang added some empty pages and pages with text to make the document look more authentic. These characteristics prevented the PDF from raising red flags with most static analysis tools.

Using specially crafted PDF documents isn’t the only way that digital attackers can fly under the radar. For instance, plenty don’t even use exploits and instead turn to spear phishing emails that leverage social engineering techniques.

Those that do use exploits can conduct their attacks with the help of tools like ThreadKit, a document exploit builder kit. These utilities enable individuals with low levels of technical expertise to get into the world of digital crime without forcing threat actors to come up with potentially attributable custom build processes for their attack documents.

How to Protect Against This PDF Attack

Security professionals can defend against this latest attack campaign from the Cobalt Gang by analyzing flagged PDF documents for base64-encoded strings, JavaScript keywords and other content that might be indicative of malspam. They should also use a ranking formula to prioritize vulnerabilities by risk so that they can close security weaknesses before exploit documents have a chance to abuse them.

Source: Palo Alto Networks

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