Malicious PDF Leads to Discovery of Adobe Reader, Windows Zero-Days

Researchers at ESET recently came across a malicious PDF file set up to exploit two zero-day vulnerabilities affecting Adobe Reader and Microsoft Windows.

The malicious document leverages a privilege escalation flaw in Windows (CVE-2018-8120) and a remote code execution vulnerability in Adobe Reader (CVE-2018-4990). CVE-2018-8120 is one of the two zero-day vulnerabilities fixed by Microsoft with its May 2018 Patch Tuesday updates, while CVE-2018-4990 was addressed by Adobe on May 14 with the release of updates that fix nearly 50 other issues.

By combining the two flaws, attackers can execute arbitrary code with elevated privileges with minimal user interaction – specifically, opening the malicious PDF.

In order to make it more difficult for attackers to execute arbitrary code on a system running its Reader software, Adobe has implemented a sandbox. Exploiting only CVE-2018-4990 allows code execution within the sandbox, but combining it with the Windows privilege escalation flaw makes it possible to escape the sandbox and execute the code in kernel mode.

It’s worth noting that CVE-2018-8120 only affects Windows 7 and Windows Server 2008 – newer versions of the operating system include security features that prevent attacks.

ESET discovered the malicious PDF in a public malware repository (likely VirusTotal). However, the company has not shared any information on who may have found the flaws and who the attackers may have planned on targeting.

“Even though the sample does not contain a real malicious final payload, which may suggest that it was caught during its early development stages, the author(s) demonstrated a high level of skills in vulnerability discovery and exploit writing,” explained Anton Cherepanov, the ESET researcher credited by Microsoft and Adobe for reporting the flaws.

ESET has published a blog post containing technical information on both vulnerabilities.

At the time of writing, 18 of the 59 antivirus engines on VirusTotal detect the files discovered by ESET as a generic Trojan or exploit.

Using two zero-day exploits in a single file is not unheard of. Last year, the Russia-linked threat actor known as APT28, Pawn Storm, Fancy Bear, Sofacy, Sednit and Strontium leveraged an Office RCE flaw (CVE-2017-0262) and a Windows privilege escalation (CVE-2017-0263) to deliver malware.

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