Attackers Deliver Latest Flash Exploit via Malicious Documents

FireEye has shared some technical details on the Flash Player zero-day that was patched last week by Adobe and revealed that attackers have been exploiting the vulnerability via specially crafted Microsoft Office documents.

The flaw in question, a type confusion tracked as CVE-2016-4117, was discovered by FireEye researchers on May 8 while analyzing an attack. Adobe released a patch four days after being notified of its existence.

“Attackers had embedded the Flash exploit inside a Microsoft Office document, which they then hosted on their web server, and used a Dynamic DNS (DDNS) domain to reference the document and payload. With this configuration, the attackers could disseminate their exploit via URL or email attachment,” FireEye’s Genwei Jiang, the researcher credited for reporting the flaw to Adobe, wrote in a blog post.

The exploit chain is triggered when the targeted user opens the malicious document. The document renders an embedded Flash file and runs the exploit only after ensuring that the Flash Player version present on the system is not older than 21.0.0.196.

The exploit is designed to run embedded shellcode that downloads a second shellcode from the attacker’s server and executes it. This second shellcode is responsible for downloading the actual malware and displaying a decoy document to avoid raising suspicion. The malware then connects to its command and control (C&C) server and waits for instructions from the attackers.

While FireEye has not specified which threat actor exploited CVE-2016-4117, Trend Micro researchers pointed out that the vulnerability shares similarities with a zero-day exploited in October 2015 by the Russia-linked cyber espionage group Pawn Storm (aka APT28, Sednit, Fancy Bear, Sofacy and Tsar Team).

The Flash Player vulnerability exploited last year by Pawn Storm, tracked as CVE-2015-7645, is also a type confusion in the IExternalizable interface, it’s triggered in the same manner, and the disassembled code is very similar.

“The similarity of these vulnerabilities indicates that there may be more type fusion risk vulnerabilities in the native code of the Flash Player‘s ActionScript Virtual Machine (AVM). The same vulnerable design and/or code may have been reused elsewhere,” noted Trend Micro threat analyst Moony Li.

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